Solartron7061 - Page 1
Forum section: electronics / repair

author: Dek
author_rank: Regular Contributor
timestamp: March 06, 2020, 10:45:48 pm
content: My repair project arrived today.The good bit :- the meter powers up.The notso good bits :- baaaaaad corrosion around the battery and GLUG DEATHFirst thing to do is to deal with the corrosion and any damage, fit a new battery.......Dek

author: Gyro
author_rank: Super Contributor
timestamp: March 06, 2020, 10:59:44 pm
content: At least it's separate from the analogue board.

author: Dek
author_rank: Regular Contributor
timestamp: March 10, 2020, 07:40:45 pm
content: Had a go at the 7061.Pulled the PCB from the meter, took the battery out and gave the affected area a good clean with dilute distilled vinegar and DI water. Rinsed with IPA and installed new battery.Powered the meter and the "Death of Glug" still hangs over the lab :-(Checked PSU rails look OK.Checked the + and - Glugs at TP202,203,204 and 206 - look OKRead the manual a bit more and carried out Glug Death check 5.3.This indicated problem with IC 315 or IC313Checking the cascading 74HC174 flip flops, OP 5 was found to be stuck and a likely culprit.I'll exchange this IC and see if that clears the dark cloud of Glug Death overshadowing the lab.Dek

author: grizewald
author_rank: Frequent Contributor
timestamp: March 13, 2020, 11:54:22 am
content: Good luck with your repair!This is the first one I have seen with the memory expansion option fitted. What does surprise me is the lack of ventilation holes in the cover over the oscillator and high frequency counter/divider section of the digital board.Here's what mine looks like:

author: Dek
author_rank: Regular Contributor
timestamp: March 13, 2020, 06:15:49 pm
content: Quote from: grizewald on March 13, 2020, 11:54:22 amI'll be able to play a good game of spot the difference :-)Mine's still not working.I'm trying to work out what the Glug Dead signal from IC315 should look like - This goes to the HD63B21PIA (could be that bit on the port might be 'doo-dared')The FIRQ and OSPCLK outputs from IC312b are working (1.5khz pulses) From the manual these stop when Glug is dead, indication a problem with the HD63B21PIAThe Glug drives and Glug signals all look OK.Integrator on TP201 looks exactly like the example in the manual -the manual doesn't state the PK-PK voltage, mine is 3v. As the Glugs are working OK I'm assuming this signal is OK.So I'm still looking around the area of IC315 and IC314. I might build up these ICs on a bread board so I can see exactly what should be going on.....I'm a bit stuck at the moment so cup of tea and some thinking before wielding the hot air light sabre destroyer.Dek

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2020, 07:16:50 pm
content: 3 V pp sounds reasonable for the integrator and doubt there is something simple to change the amplitude except a wrong reference voltage.I one suspects the 68D21 IO chip, it could also be the socket - maybe move the chip a little out and back in.The counter 6821 part is relatively close to the battery - so maybe some hidden corrosion ?

author: grizewald
author_rank: Frequent Contributor
timestamp: March 14, 2020, 10:24:14 am
content: Quote from: Dek on March 13, 2020, 06:15:49 pmThe other difference I spotted was that your meter appears to be one of the very early ones which has PCBs 3 and 5 instead of 13 and 15.For your glug death problem, are you seeing regular activity on pin 1 of IC315? Those pulses should arrive on a regular basis and prevent the output at pin 10 from changing state and signalling "glug death" to the CPU.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 14, 2020, 10:40:46 am
content: Looking closer at the logic around IC315, the GLUGDEAD signal is active high, so it should always stay low.The flip-flops in IC315 are arranged input to output, with the first input on pin 3 being tied to +5V, the first input is always high. Then the clock which should be arriving on pin 9 clocks that 1 through each flip-flop in turn. So, the 1 will arrive at the output on pin 10 four clock periods later, unless the flip-flops are reset by an incoming CLR (active low) on pin 1.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 14, 2020, 10:44:01 am
content: If you want me to get the 'scope on my 7061 to capture you some comparative traces, just shout!

author: Dek
author_rank: Regular Contributor
timestamp: March 14, 2020, 02:42:35 pm
content: Hi Grizewald,Thanks for the help, I'm going to need itYes, I think they are the earlier PCBs.Pin 1 of IC315 is seeing a 1.5khz pulse train - going low for about 50ns so 315 should be getting reset.Also I only see Q1 (pin 2 and 4) go high and pin10 never goes high.Pin 15 (Glug dead signal of the HH63B21) is low.So that possibly leaves a problem with the Glug Dead input bit (pin 15) on the PIA being faulty?AHHHHHH !Just before sending this post, I was checking that all the buttons on the front panel worked....which they do.Did reset and it says self test passed, 7061 Initialised with no Glug Death displayedTurning the meter off and back on again shows Glug Death but clears with a front keyboard reset.Quick check and :-DC V are working at least up to 20vOhms - a 10k resistor on the 10000 range reads 9.903 KOhmthe same 10k resistor on the 1000 range reads 0.003,6 KOhm so something looks a bit screwy there.Possibly someone has mucked about with the calibration???So at least 2 steps forward.edit: RTFM !Need 4 wires connected for ohms measurement.Which results in some sensible readings :-)Looking much better.Now if I could work out what's causing the Glug Death on startup?

author: grizewald
author_rank: Frequent Contributor
timestamp: March 14, 2020, 05:59:01 pm
content: Quote from: Dek on March 14, 2020, 02:42:35 pmExcellent. That shows that your glugs are being generated and that the watchdog isn't causing the problem.Quote from: Dek on March 14, 2020, 02:42:35 pmThat is encouraging, but very strange!It almost sounds like a problem with how things are enabled during the start up sequence at power on. You might like to look at the GLUGDEAD signal at power up with respect to pin 23 on the PIA to see what state it is in when the PIA gets enabled.Quote from: Dek on March 14, 2020, 02:42:35 pmI don't know if the calibration constants have a checksum so that the CPU can detect any corruption, so it's possible, but I'd say unlikely. If you go into calibration mode, you should be able to check the calibration date to see what it was last calibrated.What kind of readings do you get on the various voltage and resistance ranges with the inputs shorted?Have you checked all the power rails on the two boards for correct voltage levels and if there is any unacceptable ripple on any of the rails? It's a very old meter and the smoothing capacitors could be tired. If there's loads of ripple, the meter will do all kinds of unpredictable things.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 14, 2020, 06:31:18 pm
content: When you check the rails, pay attention to the correct test points in the service manual. As the analogue board is floating, you don't want to try using the digital ground when looking at anything on the analogue board and vice versa!

author: Dek
author_rank: Regular Contributor
timestamp: March 14, 2020, 06:42:45 pm
content: Hi Grizewald,I edited my previous post.I didn't realise for ohms measurement I needed 4 wires even when making a 2 wire measurement.So with the meter connected correctly I get good readings on a 110k and 39k resistors.With the meter ip shorted, all zeros on all DCV and Ohm rangesVolts read OK on all ranges ( max ip 20v DC)I switched off the meter, powered back up. Glug Death flashes up but the meter functions OK without doing the reset.There is a section in the manual about adopted settings on power up and reset - I wonder if something is set incorrectly...Off to RTFM again :-)I'll also check the GLUGDEAD signal at power up with respect to pin 23 on the PIA as you suggested.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 14, 2020, 06:57:15 pm
content: I have the original cables that Solartron supplied with the meter. The cable that just has positive and negative connections for voltage measurements has all four wires in use. The Hi and Lo sense wires are connected to the red and black leads just before they terminate in the banana plugs.If your glug problem now seems to be fixing itself, maybe something has a temperature fault. It could be as simple as a poor connection to the PIA or one of the other socketed chips.Do you know much about the meter's past? If it's been stored in a cold garage for some time, you might just have some moisture in there which will cook out if you leave the meter powered on for a couple of days. However, I wouldn't trust the meter to leave it unattended with power on right away. The Schaffner mains filter in the power inlet has a nasty habit of shorting out and boiling the pitch that the filter capacitors are potted in. When they fail, there's lots of smoke and a terrible smell that is very hard to get rid of!

author: Dek
author_rank: Regular Contributor
timestamp: March 14, 2020, 07:33:16 pm
content: Quote from: grizewald on March 14, 2020, 06:57:15 pmYou might be right about a temperature fault.The 20v is now reading 18v.I switched off the meter for 10 mins to cool - (the lid was getting quite warm above the transformer and oscillator section), now it's cooled a little, it's reading 20v again.I'll check the PSU rails tomorrow as well at the glug / PIA reset.The display still shows Glug Death on startup, but I'll leave that one for now.No idea about the meters past - the seller said it had been in a cupboard and was working before, but now shows glug death...yeah right, but at least I knew I was taking on a project :-)Roger on the Schaffner mains filter.Dek.

author: Dek
author_rank: Regular Contributor
timestamp: March 15, 2020, 04:43:35 pm
content: Bit more work todayChecked Glug dead sinal in respect to PIA CS and indeed the glug dead is high for about 0.8 seconds during start up.So this explains the Glug Death message on start up, but not why watchdog IC315 doesn't get the CLR pulses... OK perhaps, put that to the back of the que for a moment.Checking the supply rails, not all is good.TP Voltage ActualTP 901 +36v 32.5 with 1.4v rippleTP 902 +15 +15. 100mv noiseTP 903 0vTP 904 -15 -15 100mv noiseTP 905 -36v -35.5 150mv p-p noiseTP 907 0v=========================TP510 9V unreg 9.3v + 1v rippleTP509 5v 5.1v ripple + noiseTP501 5v 4.95v 50mv noiseTP502 5v 5.03 10mv noiseTP503 5v 4.63 (downstream of D501 so OK)TP504 5v 5.1 v 20mv noise=========================TP505 +40v +40vTP506 +32 unreg +28.8 +500mv rippleTP507 -32 unreg -30.7 no rippleTP508 0vSo I think look at changing some caps and see if that fixes the bad +36v and +32V supplies and also 5v supply associated with the processor (IC501 & C517).

author: grizewald
author_rank: Frequent Contributor
timestamp: March 16, 2020, 07:03:42 am
content: It certainly looks like a recapping might help your glug death problem. Otherwise, you'll need to dig deeper into the clock generation circuits and see what happens at power up. There must be some kind of clock coming from PCB 8, or otherwise the GLUGDEATH signal would not get asserted to start with, so it's kind of confusing that it clears itself after 800ms.These meters do run fairly toasty. The case is designed to act as a heat sink for the transformer, so it's quite normal that it gets hot in that area.

author: Dek
author_rank: Regular Contributor
timestamp: March 17, 2020, 10:41:21 pm
content: Bit more work..Changed one of the 5v regulators IC501 and associated cap - improved the noise and ripple significantly on TP509.No change though to the instrument. :-(Also changed C508 and C509 on the +- unreg 32v supply - no change in the instrument behavior.Now looking for the source of the 1.4v ripple (and low volts 34.1v) on the floating +36v supply on PCB 5. Trying to decide what to attack first ;-)I have a hunch about the bridge rectifier Q901 but the PSU is a weird configuration (to me) and I'm not sure of the function of the 39ohm resistor in the raw dc supply to the LM340 regulator.Also in the process of making a connector - now just need to find some pins of correct diameter and will have a half decent input method.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 18, 2020, 10:24:43 am
content: That's a very nice bit of turning work there! How I wish I had a lathe sometimes.Sorry to hear that your fixes so far have had little effect on the meter's problems, but it sounds like you're moving in the right direction at least.The resistor? I think it's there to ensure that the two regulators start up properly, given that IC901 provides the ground input for IC904 and its 8.2V zener which pushes the ground up to the level needed to produce an output voltage higher than the allowed input voltage.Your low voltage on the 36V rail could be due to the zener being damaged. It might be a good idea to remove it and test it to check if it still has the correct breakdown voltage. If you search for Application Note 103 by Ti/National, it shows a great variety of power supply configurations based on the LM340 series.

author: tggzzz
author_rank: Super Contributor
timestamp: March 18, 2020, 11:13:24 am
content: Quote from: Dek on March 17, 2020, 10:41:21 pmIIRC you need a diameter of 1.3mm. I used the wrong end of some pogo pins, but IIRC many are 1.2mm. I suggest the good old suck it and see method

author: grizewald
author_rank: Frequent Contributor
timestamp: March 18, 2020, 11:20:21 am
content: Quote from: tggzzz on March 18, 2020, 11:13:24 amIt just occurred to me that I have some genuine leads with the Fischer connectors here, so I took one apart and my micrometer measures them at 1.28mm.Your memory is still working well!

author: Dek
author_rank: Regular Contributor
timestamp: March 18, 2020, 01:17:45 pm
content: Quote from: grizewald on March 18, 2020, 11:20:21 amTGGZZZ,Good tip re: the pogo pinsGrize, thanks for the confirmation on the dia......you have let slip you don't have a 7 1/2 digit micrometer :-)Thanks,Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 18, 2020, 02:41:19 pm
content: Quote from: Dek on March 18, 2020, 01:17:45 pmThat's probably a good thing!

author: Dek
author_rank: Regular Contributor
timestamp: March 19, 2020, 07:09:41 pm
content: Bit more of a poke about today - measuring the various rails:39v raw DC to R901 -3.7v drop across R901 = 35.3v into the regulator (-drop out), no wonder why the 36v rail is low.The 36v rail varies from 32v to about 34vR901 is running very hot - too hot to touch for more than 1-2 seconds (could be something on the 36v rail failing and drawing too much current?)So what should the raw DC from the bridge be at the ip of R901?Volt drop across the zener is 8.35vJust in case, replacement bridges, diodes should be here tomorrow. (they will come in handy even if I never use them)Bit more thinking to do before firing up the hot air light sabre destroyer again.Dek

author: Kleinstein
author_rank: Super Contributor
timestamp: March 19, 2020, 07:55:36 pm
content: The large drop on R901 indicates too much current there. It is really 34 V at the input to IC301 ? If yes, this could indicate a problem (near short) with C902 (the one before the 36 V regulator).The voltage at R901 relative to ground should be more like 40-50 V (2x 20 V from the +-20 V supply) + some 8-10 V from the lower ~10 V for the 5 V regulator minus some drop for the capacitor.Diode can usually be checked in circuit. They usually fail with a short or rarely open. Both cases should be visible with the diode test in circuit.

author: Dek
author_rank: Regular Contributor
timestamp: March 19, 2020, 08:44:41 pm
content: Hi Kleinstein,I was looking at the voltage ip at the 15v and -15v regulators and was wondering why they weren't balanced.Yes the ip to the +15v reg is about 34vA quick check of the resistance between the ip of the 15v and ip of the 36v regulators and they are dead short......getting somewhere :-)I'll have PCB5 out and check C902.Dek

author: Dek
author_rank: Regular Contributor
timestamp: March 19, 2020, 10:13:02 pm
content: Quote from: Kleinstein on March 19, 2020, 07:55:36 pmSpot on Kleinstein.:clapI swapped out the cap and now the 36v rail is a rock solid 35.5vQuick check of the old cap and on my £15 ebay tester reports the cap as a 0.31resistor.Using my 2aa battery box (fluke 187 reads as 3.2243v, the Solartron measures 3.2241v (floating between 3.22404 and 3.22410) not sure if that is indicative of a problem of just my un-shielded hacked up test lead picking up noiseChecking Ohms ranges with the Dekabox, all looking quite good.The display still flashed up Glug Death, but I'll take that as a win for tonight :-)Dek

author: grizewald
author_rank: Frequent Contributor
timestamp: March 19, 2020, 10:51:12 pm
content: Nice result and great diagnosis from Klienstein.Considering how R901 has been fried, you probably want to pull that and check its resistance. To save yourself future grief, a wholesale replacement of all the electrolytic capacitors would probably be a good idea. If one has failed short like that, then there's a very good chance that the others won't be far behind it.

author: Dek
author_rank: Regular Contributor
timestamp: March 20, 2020, 08:33:13 pm
content: I'm an idiotJust testing the 32v unrequlated supply, slipped and touched R510 and R511 causing a short between the + and - railsDisplay now blank, no keypad leds and no key press beep.The led D509 near T501 is off.All psu rails appear to be OK including 32v unreg and the 40v for the VFD.Looks bad.... was getting so close.I'm off for a beer and start again in the morning....

author: grizewald
author_rank: Frequent Contributor
timestamp: March 21, 2020, 09:34:04 am
content: Really sorry to hear that!Shorting those resistors would do nothing on one end, but short +32V with -32V at the other end, quite possibly taking out the bridge rectifier. I'm having trouble understanding why this would have taken out the 24KHz filament drive (as indicated by the LED not coming on), but that would certainly explain the blank display.I'm always paranoid about probing live circuits and prefer to turn the machine off, make the connections with test clips and then power on again. At the very least, I'll use probe covers on my multimeter probes so that slipping off a pin or probe point can't cause a short. It's a lesson that is very painful to learn the hard way.

author: Dek
author_rank: Regular Contributor
timestamp: March 21, 2020, 11:32:09 am
content: Hi Grize,Painful lesson indeed, blood and guts paid for by the instrumentI was doing one last check on all the rails before investigating further the Glug death on start up.I'm kicking myself because the 32v unreg supplies (I think) are only needed for the expansion ports (I have the memory removed at the moment) so it was an unnecessary test....Not getting the CLR on IC315.No CS on the PIA - looks like the processor / digital side is toast. Can't see any activity on the data or address bus.No output from the intergrator.No response from the keyboardGlugs and Glug drive appear to be OK which is ironic because the reason for tracing Glug Death in the first place!I fear that multiple components have been damaged and the meter is scrap, BUT there might be some hope - processor reset is floating about 2v...now off to try to see if it is the power fail circuit...... or am IDek

author: grizewald
author_rank: Frequent Contributor
timestamp: March 21, 2020, 11:55:46 am
content: I'll keep my fingers crossed that it's just something simple. Are you sure you only shorted the 32V unregulated supplies? I really can't see how that could knock out the CPU and filament drive.I guess you just need to go methodically through the various power rails and see if any are missing or being overloaded.

author: Dek
author_rank: Regular Contributor
timestamp: March 21, 2020, 02:34:23 pm
content: Quote from: grizewald on March 21, 2020, 11:55:46 amYes definitely R502 R503.Looking at the photo, I wonder if I inadvertently shorted the trace between the two resistors to R502? (not sure what this trace is).Also spotted a bent leg, but as this is A5 address line, I don't think this is related to the original problem.I'll carefully check all the rails again, then go through the power fail and watchdog circuit to see if that is holding the processor reset.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2020, 03:38:24 pm
content: The bend pin could still cause some problems - possibly only after bending the board a lot.AFAIK the computer chips (6809 and 6821) are not so rare and should still be available in some form.Bend back the A5 pin and checking the supply and reset part would be the next points. If the short was from the + or - 32 V to one of the computer inputs this could definitely cause some damage. So it may be worth to follow the trace in question.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 21, 2020, 07:11:21 pm
content: If I had to hazard a guess, I'd say that the track which runs between those two resistors goes to the GPIB chip.What bothers me is that you say the LED on the board doesn't light up. That implies that the +5VD rail at TP509 isn't coming up for some reason. That rail also powers the clock circuit on PCB 8, and if you don't have a clock and you don't have the filament drive which is derived from +5VD and the 24KHz signal from the clock divider circuit, then that would fit well with +5VD not coming up.What do you see at TP509?

author: Dek
author_rank: Regular Contributor
timestamp: March 21, 2020, 07:56:31 pm
content: Hi Grize,TP509 = 5.06v49mhz and 24mhz signals are alive.I see the 24mhz on the filiment drive fets Tr501 and 502.I pulled the GPIB interface adaptor to see if I could see the trace - the socket base is almost solid, so checked the continuity of the trace between the resistors to the IC socket - it goes to pin 18 which is the clock signal.This is generated directly from the processor (pin 34 E) and is also to nand gates in IC118 which generated /OE and /WEThis line appears to be 3to ground, so it looks like I've most likely fried everything connected to this signal line.I think I will take out any IC that has direct connection to the E clock line which I think is the processor, the GPIB and the 74LS00 IC118 and see if that clears the short.Then it's a case of a new processor and 7400 - I'll leave the GPIB out for now and see if it comes back to life.So there might be still some hope left.How does that sound as a plan?What a muppetDek.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2020, 10:26:06 pm
content: The analysis sound logical. Removing chips from a socket should be relatively easy, and LS00 are still available. For a first test one could just cut the leg in question and add it back if the 74LS00 was not at fault.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 21, 2020, 10:45:01 pm
content: That really is exceptionally bad luck to short 32V to the E line.You have almost certainly fried both the GPIB processor and the main processor. For the soldered stuff, Klienstein's suggestion is the right way to go. Just snip the pin on the NAND gate mid way and you can just solder it back together with a blob of solder if the chip is OK.A little hint on replacing soldered logic ICs from the days when I used to do it for a living: just cut all the ICs pins close to the package body to remove the IC. Then with a pair of fine tweezers and your soldering iron, melt the solder on the pin and swiftly pull the remains of the pin out. Then resolder each hole to get fresh solder in the hole before you then clear each hole with a vacuum desoldering gun. This way, you limit the amount of time that the hole and via is heated and eliminate any mechanical stress during the process.Other methods, like trying to desolder the IC complete risks damaging the pads, through hole plating or tracks. You really don't want to make things any worse than they already are.

author: Dek
author_rank: Regular Contributor
timestamp: March 22, 2020, 07:53:12 pm
content: Thanks Grize and Klein,Your help (and encouragement) is much appreciatedI've found suppliers of all the ICs and not too expensive, should have these in about 5 days.I also noticed that IC105 (138 decoder) is also connected to E so I have orders spares for that too.Hopefully the damage is limited to those ICs and not propagated further.As you have suggested, I'll snip the IC leg connected to E to check which IC is taking this line to ground.Dek

author: grizewald
author_rank: Frequent Contributor
timestamp: March 23, 2020, 04:04:19 pm
content: Well spotted that E also goes to IC105, I completely missed that one when I was looking for other parts connected to that node.Still, the big plus with the Solartron meters is that all of the various ICs are still available. If you'd fried the ASIC in an HP meter, finding replacement parts could be a major headache. Hopefully the damage is limited to just those components connected to the E node. I'll keep my fingers crossed for you!

author: Dek
author_rank: Regular Contributor
timestamp: March 25, 2020, 05:59:51 pm
content: Now, just waiting for the 138 decoder to arrive and then time to see if I can resurrect this puppy.One thing that is bugging me is the filament drive led is still off. As far as I can tell, with the 5v and 24khz /24khz lines live, the LED should be on....It was the processor that was holding the E line to ground, wonder if it's possible to test if the 7400 and the 74138 survived without taking them out and without socketing the new processor?Dek

author: grizewald
author_rank: Frequent Contributor
timestamp: March 25, 2020, 10:17:55 pm
content: Quote from: Dek on March 25, 2020, 05:59:51 pm??Your processor looks like it is already socketed.As you don't have low impedance to ground with the dead processor removed, then the other two chips are at least not shorted and shouldn't damage the new processor. To test them in circuit other than looking at the impedance on the pins which are connected to the E node sounds kind of tricky.I still don't understand why the LED in the filament drive went down. Maybe it's just a coincidence and the LED itself is actually dead? If you have the 24KHz signal coming into the transformer T501, do you have the expected waveforms appearing on the output?

author: Dek
author_rank: Regular Contributor
timestamp: March 26, 2020, 08:51:45 am
content: Quote from: grizewald on March 25, 2020, 10:17:55 pmI should have worded that better... is it possible check the other ics in circuit to prevent damage before installing the new processor in its socket.I guess other than a simple "check the inputs are not shorted" there is no easy way to tell.I'll check the F1 and F2 output waveforms....F1 appears to be accessible, but F2 only looks line I can access it from the underside.Guessing F1 and F2 should be approx 5v square-ish with a slight DC offset bias and 180 degrees out of phase?Dek

author: Kleinstein
author_rank: Super Contributor
timestamp: March 26, 2020, 09:14:51 am
content: A first test would be checking the supply. However the xx138 may already be needed for such a test. These of often used to generate the chip select signals - so a broken / missing xx138 could cause the RAM and ROM to work against each other.Checking for signals that are shorted could be possible - so bring the data-bus to a defined state, e.g. where the CPU can write, or at least no other chip gets control over the data bus.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 26, 2020, 11:41:55 am
content: Quote from: Dek on March 26, 2020, 08:51:45 amSomething like that. I'll see if I can get the 'scope on mine later and capture the waveforms for you.

author: Dek
author_rank: Regular Contributor
timestamp: March 26, 2020, 07:04:17 pm
content: I took the LED out of circuit and tested on the bench PSU, it works OK !TP509 4.95vHooked up to F1 and F2 look OK.I'm guessing that there needs to be a filament current for the LED to be on? as at the moment the display is all off.Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 26, 2020, 09:38:37 pm
content: That looks good, but I'll still try to probe mine in a bit.Have you replaced the 74F138 yet? I notice that there's a signal "-DSPDAT" generated by that chip which goes to the display. I also noticed (rather ominously) that the E node also goes to IC208, the 63B21 PIA. Did you replace that chip as well as the processor?

author: Dek
author_rank: Regular Contributor
timestamp: March 26, 2020, 09:56:23 pm
content: Quote from: grizewald on March 26, 2020, 09:38:37 pmThe 74F138 should be with me tomorrow or Saturday, I have a new processor, PIA and the 74LS00 ready to install.Nothing changed yet, the fact the filament LED was off was bugging me, so needed to check that first before moving on.I was thinking of just trying the new processor and PIA before pulling out the two logic ICs but worried that if one or both of the logic chips are fried it could damage the new proc or PIA.So the plan now is to replace the 2 logic ICs as well as the proc and PIA all together and see if the meter comes back to life.Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 26, 2020, 10:18:24 pm
content: Here's what I'm seeing on the F1 line:About 3V DC offset and 10V P-P. I couldn't see anywhere I could safely put the channel 2 probe to measure F2, but I'd say yours is probably OK. The voltage levels could well be different if the display is actually on, so you're probably right about the LED not coming on unless the display is actually on.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 26, 2020, 10:20:01 pm
content: Quote from: Dek on March 26, 2020, 09:56:23 pmI think that's a sensible plan. There's no need to take additional risks.

author: Dek
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:27:23 am
content: Hi Grize,Thanks for checking the filament drive on your meter. Glad mine also appears to be OK.So, next step put in the new ICs and hope for the bestDek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 27, 2020, 10:21:52 am
content: Good luck!

author: Dek
author_rank: Regular Contributor
timestamp: March 27, 2020, 06:53:57 pm
content: Extraction of the two logic IC went OK, snipped the legs as suggested.I don't possess a desolder gun so heated the individual pad with soldering iron, then with a tiny nozzle on the hot air gun, carefully blown out the solder. Quick, minimum heat - works well.Still no 74F138, hopefully will drop through the letterbox tomorrow.Wondering if best to put in some turned pin DIL sockets or solder ICs direct to the pcb as per original? What do you think.Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 27, 2020, 08:04:35 pm
content: That's a nice clean job! It really is so much kinder to the PCB to do it this way.There's absolutely no harm in using quality turned pin sockets. If they're good enough for the microprocessor, they're also good enough for the glue logic around it.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 27, 2020, 08:13:34 pm
content: On the subject of solder removal, I have one of these:While I'd love a Pace desoldering station (I used to use one at my job fixing minicomputers several decades ago), I just can't justify the price. The tool pictured is called an S-993A and it works very well. The tips are quite expensive and not the best quality in the world, but the smallest tip has a 1mm hole and clears holes on sensitive PCBs very nicely. It's always a better option than a spring loaded solder sucker. The recoil on those is guaranteed to rip the pad and track clean off the board if it hits the PCB when you press the button.

author: Dek
author_rank: Regular Contributor
timestamp: March 28, 2020, 01:50:25 pm
content: I'll have to treat myself to a desolder gun :-)Postman came, postman went - no 138sI have 74ls138, but with the gate delay being 3x the f type, I don't recon it will "cut the mustard"I'll go and find something else to break and wait for the next post on MondayDek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 28, 2020, 01:53:05 pm
content: Quote from: Dek on March 28, 2020, 01:50:25 pmCareful with that axe, Eugene

author: Dek
author_rank: Regular Contributor
timestamp: March 30, 2020, 04:17:49 pm
content: The 74F138 arrived and the dog didn't shred the packet....good omen I think.Popped the 138 into its socket having installed the other ICs over the weekend.Prayed to the magic smoke gods (several times).Turned on the meter andYESpowered up with the usual Glug Death, which disappeared after a few seconds.Now does the meter actually 'meter' anything -2aa batteries - check- agrees with my Fluke 187 looking goodQuick check on the Dekerbox and looks good.So it appears that I'm back to where I was about a week agoOne thing I was going to look at before the 36v disaster, was when the meter is set to DCv and nothing connected, the reading floats up to between 2v and 4v. The display immediately reads zero when the ip lead is shorted. Is this normal behaviour?When connected to my 2aa batter pack, reading varies between (approx) 3.18736 and 3.18750. Is this to be expected.Considering what happened I've had a lucky escape.Dek.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 30, 2020, 04:44:48 pm
content: With the input set to hight Z (not 10 M) it is normal that the open input drifts at some speed. This can be used to estimate the input bias. The rate is given by input bias divided by input capacitance. If needed one could add more capacitance if one can not get the actual capacitance from a schematics or similar. Where the input settles depends and is not very significant and eve of it drifts all the way to overload this can be OK. Ideally one would check the drift rate with a few starting points (e.g. +-10 V, +- 3 V, 0 ).Some 3.2 V for 2 new alkaline cells is about right. The comparison with the Fuke meter is more important that just the number , it can change a little between brands and with temperature and age.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 30, 2020, 05:05:51 pm
content: Quote from: Dek on March 30, 2020, 04:17:49 pmCongratulations!Quote from: Dek on March 30, 2020, 04:17:49 pmPerfectly normal.Quote from: Dek on March 30, 2020, 04:17:49 pmIt sounds like a lot of variation for a battery pack and may be connected to your Glug Death problem. I'd do the same test on mine, but there's an HP 34401A sitting on top of it running a 24 hour log of a reference at the moment and I don't want to turn the 7061 on as the heat will mess up the readings from the HP.I'm a bit short on bench space!These meters are rather noisy, but 140μV seems kind of excessive. I'll see what mine does when my logging is finished.

author: Dek
author_rank: Regular Contributor
timestamp: March 30, 2020, 05:59:07 pm
content: Hi Klein and Grize,Thanks, I am very relieved that the meter has come back from the dead !My cobbled connection using just grabber leads temporally stuffed in the front socket might be adding to the noise.I have some pogo pins on order to make up a 'proper' connector which I will build into a shielded lead and modified binder plug shown below.I'll also graph the history readings so it will be more meaningful to review.Thanks again for your help so farDek.

author: Dek
author_rank: Regular Contributor
timestamp: March 30, 2020, 06:58:20 pm
content: Quick graph of the meter after its been warmed up for about 3 hrs.I'll have to have a go at running a program to check min/max SD etc. (RTFM first though!)Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 30, 2020, 09:22:11 pm
content: That looks a lot more reasonable!If you do your plotting with gnuplot, it will work out min, max, SD, variance, median, mean and all that stuff for you. Not to mention that so will your meter if you select the statistics program, but it's much more fun to graph things and much better at showing hidden features of the collected data.This is from the log that's running on the HP meter at the moment.

author: Dek
author_rank: Regular Contributor
timestamp: March 30, 2020, 09:36:56 pm
content: I manually bashed the history figures into Excel - got fed up at 200Ran the stats prog for about 5 mins.N 1497Mean 3.186598SD 6.649784E-5Var 4.421963E-9I'll have to order a replacement GPIB controller IC and try gnuplot.

author: grizewald
author_rank: Frequent Contributor
timestamp: March 30, 2020, 09:58:53 pm
content: There's a great little project here on EEVBlog that will turn an Arduino board into a USB-GPIB interface if you do get that replacement GPIB chip. See:https://www.eevblog.com/forum/projects/ar488-arduino-based-gpib-adapter/It works just as well on my 7061 as it does on the 34401A.

author: Dek
author_rank: Regular Contributor
timestamp: April 01, 2020, 05:16:57 pm
content: Quote from: grizewald on March 30, 2020, 09:58:53 pmLooks like a useful project so I've ordered a GPIB connector and an Uno.Now back to tracing the Glug Death fault.I'll check that there isn't a power supply rail or one of the clock signals that's being lazy on start up. Or possibly the 1.5khz clear signal to IC315 isn't being 'slow' .etc (trying not to blow it up again)It appears to be only a problem for on power on- If I reset the meter, it will self test and initialise OK.Dek

author: Dek
author_rank: Regular Contributor
timestamp: April 02, 2020, 08:48:47 pm
content: Been looking at the clock output form PCB 8, not happy with what I'm seeing.Looking at TP101, the clock signal is an approximate sinewave and I'm assuming this should be a 5v square feeding the hex inverter IC 430.Whats more strange, the output of the inverted is not a square wave, nearer a sine, in fact so are several of the other inverter outputs. (wonder if I'm missing something here - scope ground was to the IC 0VThe clock is very unstable, ranging approximately 49.1084 to 49.155.I've checked this with a frequency counter and it agreesBit of a think needed.<edit> looking at this !https://www.eevblog.com/forum/metrology/solartron-7061-and-thermal-noise/25/

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2020, 07:45:17 am
content: The frequency is quite high. So it is normal not to have a nice square wave any more. Loading by the scope probe and the way the ground is connected can also effect the picture.The signal after the inverters may look a little better, but even these are not that much faster. There is also a good point to have a more sine like clock: it is causing less RF emissions.The larger problem is likely the not so stable frequency. This could be the PLL not locked. So how does the voltage at the VCO control input look like. This can be tricky to probe in some cases if it is high impedance and can thus pic up noise.

author: coromonadalix
author_rank: Super Contributor
timestamp: April 03, 2020, 02:52:14 pm
content: what probes do you have ? are they x10 or x100 ?? it may affect your signal quality ? as kleinstein wrote

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 04:56:07 pm
content: Quote from: Dek on April 02, 2020, 08:48:47 pmNow my very busy working week is over, I got some time to come here and catch up.As others have said, it may simply be your 'scope making it look like that, but I'll have a probe of mine after dinner and post some screen captures. The raw clock output and the control inputs are kind of sensitive to being probed.When I was looking at the clock on mine, I found that the very simple setup instructions in the manual of 'adjust L401 on PCB8 until TP402 reads 3.0V +/- 0.2V' to be woefully inadequate. For a start, you can't probe TP402, even with a X10 probe and expect the PLL to lock as the probe steals enough current to stop the PLL from locking. Instead, I removed PCB8, powered it from one of my bench supplies and then used a counter, with its timebase derived from a GPSDO to adjust the free running frequency of the oscillator to as close to 49.152MHz as I could get. That ensures that the frequency should be in the range that the PLL should be able to achieve a lock with.I discovered how sensitive the clock control at TP402 was when I was trying to monitor TP403 and TP404 (the 50Hz from the mains and the 50Hz created by dividing down the 49.152MHz clock) while also monitoring TP402. The PLL never even got close to locking. When I removed the probe from TP402, it got much closer to a lock, but never actually achieved a stable lock. I'm guessing that either the varicap on PCB 8 or one of the other components is a bit 'tired'.If you probe just TP403 and TP404, you can see if the PLL ever does achieve a lock. On mine, the leading edges of the signals cross slowly back and forth, but never actually align and stay there. It's almost as if the time constant for the PLL is wrong.Oddly, when you look at the circuit diagram of the optional crystal oscillator, it doesn't even use the control signal from the PLL, which makes me wonder how important the lock to the mains frequency actually is. I tried feeding in a reasonably accurate 49.152MHz from a signal generator with PCB8 removed, but it did nothing to improve the approximately +/- 12μV of noise that I see on my 7061 when measuring on the 10V DC range with the inputs shorted.Earlier this week, I had an order from Mouser arrive and part of that order was some 49.152MHz crystals, so I'll be making up an oscillator board that I can try as a replacement for PCB8. It won't be locked to the mains, but it should at least be fairly stable.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2020, 05:16:36 pm
content: How important the frequency lock to mains is depends on the signal measured. If everything inside the meter is fine an not much internal hum, there can still be hum from the signal to measure. With a short there should be essentially no hum and thus low importance of the PLL lock. The same is true for good quality reference with cables that don't pick up much hum. Especially something like an AC measurement may pick up quite some hum and thus be more sensitive to the PLL lock.For a test one could check with an intentional 50 Hz signal to see how good this is suppressed to get an idea.With just a fixed clock and no PLL there may be a slow beat frequency visible.Especially a passive PLL filter can be very sensitive, so the measurement could effect the PLL to much. This can be DC loading and AC coupling of some mains hum.Adjusting the PLL to a give frequency is probably OK, provided the control voltage is at a defined state (e.g. mid range).The PLL lock does not have to be in phase, a constant phase shift can be acceptable (depends on the detector and filter).For the PLL it would be nice to have some lock indication.Edit: A fixed frequency should not be such a big problem. With my ADC I use a fixed frequency and not even well matched length. The noise is still very low (e.g. < 1 µV).

author: Dek
author_rank: Regular Contributor
timestamp: April 03, 2020, 06:46:29 pm
content: Hello Klein, Coro and Grize,Yes, looks like you are correct, my setup (100mhz scope cheapo x10 probes) appears to be distorting the clock signal due to its limitations, so at least nothing appears to be seriously wrong there.I've rechecked this with a 200mhz scope x10 probes (supposedly 350mhz) and with slightly better results, but the clock frequency is varying significantly, drifting from max to min in about 10 sec (by eye) (screen grab TP101)Section 4.2.5 where the manual 'explains' clock adjustment, I see on my scope a 50hz pulse that slowly varies between 1.62ms and 1.17ms(TP402 WRT TP508)I was expecting to see a DC voltage here, so somewhat confused.Dek.Checking TP403 and 404, I see that there are not precisely locked. Persistence set to infinite, shows the drift of TP404 wrt triggered TP403.

author: branadic
author_rank: Super Contributor
timestamp: April 03, 2020, 08:14:55 pm
content: QuoteYou can't expect to see a 50MHz square wave signal with a 100MHz scope. It needs >250MHz at least to get something that barely looks like a square wave.-branadic-

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 08:15:20 pm
content: They look more locked than mine! I'll grab a shot of how (not) well locked my clock is next.First up is TP101 (yellow trace) vs IC430 pin 6.You could almost be forgiven for thinking that the inverter isn't actually inverting. Then again, the clock period is just over 20ns and the propagation delay of the inverter is around 4ns, so actually, the output of the inverter is going low just as the clock signal rises past the logic 1 threshold.On the clock control signal from the PLL chip at TP402, what you will see is the signal go positive when the clock's leading edge comes after the 50Hz leading edge, when the clock leading edge comes before the 50Hz, the control signal goes negative and when both edges are simultaneous, the control output goes tristate.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 08:47:44 pm
content: Here's the PLL on mine having a hard time.Ch1 is the 50Hz mains referenceCh2 is the comparison 50Hz divided down from the clockCh3 is the clock control output from the PLLThe rising edge of the comparison clock is after the rising edge of the 50Hz mains. Therefore the clock control output goes high in an attempt to pull the clock back to match the 50Hz rising edge. The only problem is, with a 10X probe on the control output, it can't pull the edge back far enough.So I disconnect my probe from TP402 and a second or so later, the phases lock.So maybe I was not remembering my last testing properly as it appears that I do now have a locked PLL as long as I don't try robbing the varicap of electrons to observe the control signal.If this doesn't happen for you, it may be a good idea to try my way of setting the frequency of the oscillator outside of the meter and then see what happens when you put it back in. It's incredibly frustrating trying to set it to exactly 49.152MHz though as the adjustment slug is very sensitive. Obviously, you can only adjust the slug with a plastic tool, but even then, you'll think you've got it set perfectly and then when you take the plastic screwdriver out of the slot in the slug, the frequency will hop up 5KHz.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 09:21:44 pm
content: Actually, my memory does still work properly. Zoom in a bit and watch for a while and the instability of the lock reveals itself.280μs after.-170μs before.So in my case, I have a +280μs / -170μs variation around the locked point and it can swing from one extreme to the other and back again in just a few seconds. Maybe I'm expecting too much to think that it should be able to lock and hold the lock considering how small the variation is in mains frequency.

author: tggzzz
author_rank: Super Contributor
timestamp: April 03, 2020, 09:43:56 pm
content: Obvious questions.Have you checked the noise on power lines in that area?Is there noise on the varicap control voltage?Have you checked whether C101 C102 have dried out?

author: Dek
author_rank: Regular Contributor
timestamp: April 03, 2020, 10:04:16 pm
content: Quote from: grizewald on April 03, 2020, 09:21:44 pmGrize, I'll do the same experiment tomorrow, but the quick check I did today seems to replicate what you are seeing almost exactly.tggzzz C101 and C110 are a tants and C102 is an aluminum so all worth a check as this board runs like an oven!(Still trying to track down the cause of the transient Glug Death when powering on).

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 10:07:10 pm
content: Quote from: tggzzz on April 03, 2020, 09:43:56 pmWell, as you can see from post #74, the purple trace is the control voltage and it looks like there's about 300mV p-p of noise there. However...Here is where you find TP402 on this board:Sitting right next to that bridge rectifier and the mains transformer, I can't exactly imagine it being particularly quiet either!C101 is a dipped bead tantalum and C102 is a metal cased wet tantalum. They both look OK, but when the Peak ESR70 tester I have on the way to me arrives, I'll test both to see if they are still within reasonable limits.I've checked all the power rails on both analogue and digital boards and there is no significant ripple on any of them.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 10:08:22 pm
content: One other thing I have noticed about this meter which is worth bearing in mind is that the very convenient fasteners which hold the top cover down are actually a hidden danger.The fasteners grip the steel pin which runs through the middle of the hole in each receiver:The danger is that every time you open and close the case, the friction between the two metal parts shaves off tiny flakes of metal. For three out of the four points, there are PCB traces and sensitive circuit paths right below the receiver and when I inspected the PCB at these points under a strong light, I could see tiny glittering flakes of metal on the PCB!The solution is to just put some adhesive tape at the bottom of each receiver to stop the metal dust before it falls on the PCB.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 10:12:15 pm
content: Quote from: Dek on April 03, 2020, 10:04:16 pmYou're not kidding about how hot the clock dividers sections gets! Even with the top off now while I've been collecting these traces, the metal box gets so hot it's uncomfortable to touch it. Your screening box doesn't even have the holes in it that mine has so I hate to think how hot it gets in there.It may be worth doing what they did on my later model meter and drilling some holes in the lid. It may extend the life of the parts that live inside the box.

author: tggzzz
author_rank: Super Contributor
timestamp: April 03, 2020, 10:16:49 pm
content: Quote from: grizewald on April 03, 2020, 10:07:10 pmProbably helps mode locking onto the ~50Hz

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 10:17:12 pm
content: Quote from: Dek on April 03, 2020, 10:04:16 pmIf you're seeing a similar variation, then I'd have to say that this is perfectly normal and is neither the cause of your initial glug death, or the (to my mind) excessive noise on my 10V range. It's a red herring.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 03, 2020, 10:19:30 pm
content: Quote from: tggzzz on April 03, 2020, 10:16:49 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2020, 08:22:25 am
content: For comparison one could check the jitter on the 50 Hz mains signal send to the PLL: look at the slope some 1 second after the trigger point with some persistence. A reasonable DSO should be able to do this.The position of the PLL and testpoint 402 is really not good - quite close to the transformer, so one might get magnetic coupling and thus 50 Hz FM modulation on top. If right at the phase comparator output this point is a sensitive point and should be well shielded.I don't think the filter caps in the loop filter should be so critical. Tantalum caps usually fail short, but not with slowly rising ESR. LOW ESR is likely also not really needed. It would be more the capacitor at the supply that could be a problem.

author: Dek
author_rank: Regular Contributor
timestamp: April 04, 2020, 10:05:27 am
content: Quote from: grizewald on April 03, 2020, 09:21:44 pmMine is very similar, so possibly red herringI haven't yet took out PCB8 to set the 49mhz accurately against freq counter / GPSDO and the lock does drift backwards and forwards over about 5 secondsMeter has only been on for about an hour, lid is off the clock enclosure.Variation after a few mins is -221us / +482usWhat I did notice it takes "forever" for it to lock on power on. Quite hilarious to watch the second trace whizz back and forth for over 20 seconds until it stabilises.Could be due to not having the clock set close nominal free running frequency and the PLL having to work hard resulting in the Glug Death until it locks? Glug death only flashes up for about 4 secs and previous test the glug death signal is only high for about 0.8 secs on start up anyway, so not sure if this is related.I see about 0.2v ripple on the 5v supply at the pins on the clock board.Good spot on the swarf emitted by the cover screws[ Specified attachment is not available ]

author: tggzzz
author_rank: Super Contributor
timestamp: April 04, 2020, 10:33:53 am
content: Quote from: Kleinstein on April 04, 2020, 08:22:25 amI don't disagree, but would a tant on the way to becoming a short exhibit increased leakage? If so, could that affect the loop, either because of a R101/2/3-to-leak "potential divider" shifting the lock range, or by introducing droop?

author: grizewald
author_rank: Frequent Contributor
timestamp: April 04, 2020, 10:49:15 am
content: I just timed how long it takes for mine to lock from power on and it took around 13 seconds. (The meter was stone cold as well, this being the first power up of the day.) So I'd say it was unlikely that seven more seconds to lock would explain your glug death problem.My trusty Brymen 867 measures the supply to the oscillator board at 5.06V with 0.005V ripple.The control trace really isn't that well positioned. The PLL and phase comparator is the second chip to the right of the linear regulator with the heatsink just to the right of the transformer in this picture:The control trace from the 4046 comes out on the bottom of the PCB and then pops up to the top through a via just at the right hand end of the shielding box. Then it runs along the side of the box to the test point and back to the bottom of the PCB again where it comes up through the PCB to the pin which connects it to the oscillator board.Taking the oscillator board out shows this:The control signal is the pin at the far left of the shielding box and it's interesting to see that on my 7061 at least, Solartron have placed a little ferrite on the control signal and 5V supply pins. They look very much like an attempt to mitigate some of the noise coming in on those two pins.You probably don't have those on yours Dek, what with it being an older revision. I wonder what that ferrite would do for the ripple on the 5V supply to your clock board...

author: Dek
author_rank: Regular Contributor
timestamp: April 04, 2020, 11:07:29 am
content: Humm,Checked the 5V DC supply on the boards again, this time with the Fluke, 4.995V and 1.236 mv ripple.Also checked under the clock board, it has the same ferites.I checked TP101 again and the clock frequency jitters terribly. Its almost impossible to determine an accurate reading.Using the stats on the scope Min 48.3 and max 49.75mhz.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 04, 2020, 11:32:09 am
content: Quote from: Dek on April 04, 2020, 10:05:27 amIf your oscillator board is way off frequency, it might explain the much larger variation that you have once the PLL actually locks and possibly explain the slightly longer time to lock from power on. I doubt it has anything to do with your glug death though as all the glug timing is derived from the main clock. Logically, if it's a bit fast or slow when it starts, the rest of the timing will also be affected the same way and it shouldn't make any difference.I'm thinking that your glug death problem lies somewhere with IC313. I notice it has two split pads on the outputs - SP303 and SP302. On my machine, both are soldered. The -CLR signal which is supposed to reset the watchdog so that it doesn't assert GLUGDEAD comes from IC313 via IC314 and has a test point at TP304. IC424 plays a key role in switching the glug timing between the full speed forcing wave frequency of 24KHz for 4 digit measurements or the much slower 1.5KHz speed used for more digits. If there's anything that's not 100% with IC424, it could mess up all the timing as the whole circuit starts up.I'm grasping at straws here as the whole timing circuit is very complicated, but there does seem to be some condition which isn't established properly as your meter starts up, so these areas would be good places to look at.Just saw your new post as I went to post this. That ripple really doesn't sound nice, nor does the jitter. I'd certainly be having a look at the smoothing capacitor for that 5V rail. You do have a nice ceramic package there for your VCO chip, while mine is the humble plastic package. If you have a signal generator that you can generate a 49.152MHz sine wave with, it would be an interesting test to pull the clock board out and inject 49.152MHz from the signal generator at the clock board's output pin and see if it makes the glug death problem go away. It would, at least, determine if the clock board is the problem or not.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2020, 11:55:26 am
content: It is quite normal that the PLL lock take quite some time, as the filter is relatively slow with caps of 1 and 22 µF and 100 K resistors.I would not expect the PLL lock to effect the GLUG dead signal. This would more like the input amplifier coming out of saturation slow on start up and maybe the supplies coming up in a slightly different sequence / speed.The placement of the 4046 for the PLL is really odd. With resistors in the 100 K range I would consider the VCO control signal sensitive, even if there is a 1 µF capacitor. I think this deserves a.Leakage of the capacitors at the filter would at first one cause a phase shift and this should not effect the ADC.The nasty part would be some signal from the comparators / forcing signal to couple into the PLL - this could cause extra INL errors.The ceramic vs. plastic case for the VCO chip should not make much difference - this part is not about precision or lang time stability.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 04, 2020, 12:15:27 pm
content: Quote from: Kleinstein on April 04, 2020, 11:55:26 amI agree and I won't waste any more time on it. As I now have a 49.152MHz crystal, it's time to dig into my parts drawer for some capacitors and replace the oscillator board with it and completely ignore the PLL, just like Solartron's optional crystal oscillator does.Quote from: Kleinstein on April 04, 2020, 11:55:26 amThe only thing the ceramic package normally gives is a larger working temperature range. As Dek doesn't have any ventilation holes in his screening box, the ceramic package was probably needed to cope with the temperatures inside.

author: Dek
author_rank: Regular Contributor
timestamp: April 04, 2020, 12:53:07 pm
content: Quote from: grizewald on April 04, 2020, 11:32:09 amUnfortunately I don't have a sig gen that will run to 50mhz, only 25mhz so I think I'll leave the 49.152 clock signal for now. Appears that this is not the cause of the Glug Death (thanks Grize and Klein).Might repeat what you did and set 3v on the clock control and adjust the frequency on the bench. I'll be able to check the noise and jitter to see if that is a incoming supply problem or just the smoothing on the boards itself.The glug death is very consistent so should be possible to trace this with a methodical approachIt will be interesting to see the results of your crystal mod.Dek

author: Dek
author_rank: Regular Contributor
timestamp: April 04, 2020, 08:44:06 pm
content: Looking at the source of the 800ms delay on the clear signal line which is allowing the clock on IC315 to output the Glug Death on its Q4 output.Comparing when the 3 / 1.5khz signal pulses start on the clock input of IC315 (shown green) and when the clear signal arrives on pin1 of the same IC, indicates a delay in the signal path back to IC310 -(shown in Red).24.575mhz clock (derived from the 49mhz clock) on IC310 and IC313 appears simultaneously with the 3 / 1.5khz pulses on IC315 - so no problem there.However, the QD output of IC310 is stuck low as is unused QC, but QB and QA show pulses. (shown orange) - could this be the source of the clear delay?Haven't quite worked out what IC424 a quad multiplexer brings to the party yet, haven't found where the strobe and select lines are on the schematic! There is a note under this IC A -> Y= 4 NINES and B-> 5,6,7 NINES (shown in grey)With IC310 output stuck 4 NINES are not being made?Dek

author: grizewald
author_rank: Frequent Contributor
timestamp: April 05, 2020, 12:22:16 am
content: I have to say, I'm starting to dislike whoever drew the circuit diagram of the 7061's digital board.The choice between the 3A or 3B inputs of IC424 is made by pin 1 of the chip. That's actually located on the next page where the -4NINES signal is generated by the processor writing to the I/O port implemented by IC401 which is mapped into the memory space at address 0x100. Bit seven at that address lets the processor select 4 nines mode or not 4 nines mode.Meanwhile, back on the previous page, IC301b is sampling the -GLUG signal at the full 49.152MHz clock rate. The output of IC301b is fed to the GLUG counter circuit and to the clock input of IC310 and to 3B on IC424. IC310 is configured as a simple counter and each rising edge on its clock pin causes it to increment the count by one. As the high bit of the nibble is connected to input 3A of IC424, then when -4NINES is asserted (low) the 3Y output of IC424 will receive the number of clock pulses fed to IC310 / 8. Otherwise, if -4NINES is not asserted (high) - (which should be the condition when the meter starts up in the default 5 digit mode) then input 3B of IC424 is selected and output 3Y then matches the output of IC301b (the -GLUG pulse sampler) and is not divided down.IC310 is clocked all the time, so pin 11 should go high once for every eight rising edges at the CLK input on pin 2, regardless of whether the microprocessor has asserted -4NINES via the I/O port or not.From what you are saying, IC310's two most significant bits are either not counting, or the outputs are blown.The problem I see with this explanation is that IC310 only matters if the meter is starting up in 4 nines mode. I'm pretty sure that the default - when the meter displays "7061 INITIALISED" at power up - is 5 nines mode, so IC310 shouldn't matter. On the other hand, if the meter says "7061 RESUMED" at power on and it was in 4 nines mode when it was last turned off, then IC310 does matter.What happens if you change the number of digits to 4 once the meter has recovered from its initial glug death? I'd expect it to go back to the glug death state again.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 05, 2020, 01:16:19 am
content: Here is pin 2 (ch1) and pin 11 (ch2) of IC310, from power up:and zoomed in to the trace once both signals are running:Sorry for the photos, but for some bizarre reason, the utility I have always used to grab screen shots from the Rigol suddenly bombs out trying to connect to the scope. The noise and over/undershoot on the traces is purely from my crap, but safe, probing setup.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 05, 2020, 06:23:05 am
content: It is well possible that during start up the 4 nines modes is used for a quick self test.The scope trace from the start-up looks a little like there is something on that kind going on un the center of the screen.A brocken IC310 is a possible explanation. Maybe also check to solder joints, not just QC,QD, but also the C and D inputs.

author: Dek
author_rank: Regular Contributor
timestamp: April 05, 2020, 10:21:34 am
content: Quote from: grizewald on April 05, 2020, 01:16:19 amSwitched the meter on this morning and without any scope connections, checked 4 digits - Overload message - switching back to 6 digits OK.Shows overload on all ranges when in 4 digits.I think you mean IC310Checked the lines ABCD LD ENP ENT that are tied to 5v, all OKPin 11 and 12 low - no sign of lifePin 13 and 14 nice 340ns pulsesI see the wide pulse on start up followed pulse train as per your screen shot, but nothing on Pin11On one start up while the clips were connected to IC310, the meter turned on with no Glug Death and a proper self test and initialise. - I also seen the expected pulse train on pin 11.Checked 4 digits again and same "OVERLOAD" message.Rechecked pin 2 and pin 11 of IC310 and pin 11 still dead. ( tried triggering on pin 11, nothing apart from a slight ring on the falling edge of pin 2 squarewave)This area of the board shows some signs of corrosion due to the proximity to the battery - I might resolder the pads of this IC and see if that makes a difference.<edit> After posting this, checked 4 digits again and switches OK without OVERLOAD ERROR , still with Glug Death- pin 2 shows change from 1.5khz to 24khz[ Specified attachment is not available ]

author: grizewald
author_rank: Frequent Contributor
timestamp: April 05, 2020, 10:32:37 am
content: Yes, I do mean IC310. Sorry for the confusion.I was wondering if this could be caused by leakage from the battery. In your trace where there is actually some life on pin 11, the spacing of the pulses is inconsistent, as is the level of them. I think we've found your problem!

author: grizewald
author_rank: Frequent Contributor
timestamp: April 05, 2020, 10:44:42 am
content: Quote from: Kleinstein on April 05, 2020, 06:23:05 amSelf test is a good explanation. The service manual does not discuss self test much, except to say that when it has a ROM set which makes the self test a manually initiated routine that it tests 1V and 10V ranges and lists the result as 4 digit values.

author: Dek
author_rank: Regular Contributor
timestamp: April 05, 2020, 02:53:53 pm
content: Well, that was effective as trying to solder titanium to a lump of coalNo change in the meter behaviour.Meter still shows OVERLOAD when selecting 4 digits and G.Death on startup.IC310 is badly corroded as is IC312.I think there is only one thing for it and that is to exchange IC310. (Might do IC312 while I'm at it).Checked againpin 11 lowpin 12 lowpin 13 1.5khz pulsespin 14 1.5khz pulses (wasn't expecting QA and QB to be identical)For a moment I thought that I could connect say QB to the cut leg of pin 11, but the timing is wrong.Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 05, 2020, 10:32:44 pm
content: The alkali that leaks from the battery when it gets too old is really nasty stuff. It is normally potassium hydroxide or sodium hydroxide. It has obviously leaked out further than the first pictures you posted would suggest.If you don't remove every trace of the leak and neutralise the alkali, then it will continue to damage both the components and the PCB, eating away at the vias, particularly if they are under an IC and the alkali has seeped in between the bottom of the IC and the PCB.The best way to stop this getting any worse is to thoroughly wash down the PCB over the complete area affected by the leak several times with a dilute solution of spirit vinegar and a brush. Then with multiple washes of distilled water (if you live in a soft water area, tap water is OK) followed by a final couple of washes with pure isopropanol. If you can see places where the alkali may have got in under the ICs, make sure that you can run the cleaning solutions underneath them. Use an air jet to make sure that the cleaning agent gets blown out from under any ICs after each wash. If there are vias under the ICs, you will need to remove the ICs to properly deal with the vias.From your picture, I can see that the solder in many of the vias has turned grey and dull. This is from the battery leak and it would be wise to test each via to make sure it still works, desolder any grey vias, neutralise any remaining alkali and then fill them with fresh solder. If you don't, you may find them going open circuit as the alkali can get down into them and continue eating away at the through hole plating from the inside. You will also discover that it is really difficult to remove the grey solder as it doesn't want to melt properly any more. I had to resort to a very thin stainless steel needle to completely clear some of them and then install a thin wire through the via and scrape back a little solder resist on each side to solder the wire to the tracks that the via connected.If the PCB has internal layers, it is often a case of "beyond economic repair" when they get hit by a battery leak as repairing vias that connect to internal layers is very difficult and time consuming.Leaking batteries are a real menace on test gear of this era. I'm still working on fixing a Solartron 7151 I bought which had been subject to a catastrophic battery leak. I saved some pictures of the horror and what I've had to do so far to repair the damage.At first, it didn't look too bad once I'd removed the leaking battery:But the more I looked, the worse it got. Just about every via here is open circuit and the 75150 chip in the first picture is dead.Then I discovered that the alkali can actually wick under the layer of solder resist and spread even further than you think it has. As soon as I poked at the solder resist, it would simply flake off, so I had to start stripping that back until I got to the point where it was no longer flaky, scrape off a little more to be sure and then clean everything again starting with the vinegar solution.The pictures are all clickable for full resolution.If you can see that IC310 and 312 are badly corroded, then the best thing to do is remove them, clean again, repair any open vias and install replacements. It's always better to err on the side of caution and limit any further damage as much as you can. I'll keep my fingers crossed that the damage doesn't extend too far!Hopefully, all this hard work will give you a perfectly working reward.

author: Dek
author_rank: Regular Contributor
timestamp: April 06, 2020, 06:27:24 pm
content: Hi Grize,Wow that's some repair project.I've extracted IC310, the tracks and pads look OK some surface corrosion.GND and VCC pads are connected to internal ground planes which makes it difficult to remove the cut legs using minimum heat.I doubt 'thermals' weren't invented in the 80s !Now reading your post above I will certainly rework the vias under IC310 before fitting.For now I will re-solder IC310, (delivery expected tomorrow) hopefully this will resolve the G.Death issue, then look at repairing the remaining corrosion after.Popped a few holes in the clock circuitry enclosure lid. (I thought this was stainless steel but actually it's folded from copper sheet).... and now looks like a bought oneDek.<edit>Managed to work a few of the vias - the soldering iron and hot air nozzle combination works well.and the full horror of the battery leakage on the nearby components.Extent of corrosion

author: grizewald
author_rank: Frequent Contributor
timestamp: April 07, 2020, 06:59:47 am
content: Nice job!I'll have to give your trick with blowing the solder out with hot air a try on my next repair. It certainly avoids cooling the solder down just when you need it to stay liquid.

author: Dek
author_rank: Regular Contributor
timestamp: April 07, 2020, 09:21:59 am
content: Strictly not my trick.I was watching some guy on You tube extract ICs whole, would heat the IC legs in the normal way, pull out the IC and while the board was still hot, blow out the holes. I'm cutting the IC legs and extracting individually, then once the legs are removed go round with the iron and hot air to clear out the holes.The only issue is that the tip of the nozzle can scratch off some of the solder resist. The solder resist on my 7061 PCB seems particularly fragile in that respect.I've since polished the end of the nozzle smooth to see if this helps and use thin strips of capton tape in an attempt to protect the solder resist.The other thing to watch is keeping the soldering iron with minimum solder, the hot air 'jet' can blow blobs of solder about so I also put capton tape to stop small solder spheres from disappearing under nearby ICs !The replacement 74HC161s have arrived ...will it beor"Tune in later to find out"Dek.

author: Dek
author_rank: Regular Contributor
timestamp: April 07, 2020, 06:45:58 pm
content: SuccessSoldered the new lucky IC , prayed to the magic smoke gods and turned on.SELF TEST PASS7061 INITIALISED.Super-bloody-duper4 digits mode now doesn't flash up OVERLOAD but measures correctly.A quick look Ohms and dcV working.Obviously lots more to do in checking all the ranges etc, but it appears that the meter is now working.I have the parts to build a Arduino GPIB interface which will help with further checks and tests once the new GPIB controller arrives.Thank you Grizwald and Kleinstein for your help, very very much appreciated

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: April 07, 2020, 06:54:07 pm
content: Nice work Dek, glad you got it going.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 07, 2020, 06:55:17 pm
content: That's great news Dek! Congratulations!

author: coromonadalix
author_rank: Super Contributor
timestamp: April 08, 2020, 10:27:44 am
content: another "oldie" saved

author: Dek
author_rank: Regular Contributor
timestamp: April 09, 2020, 05:26:36 pm
content: Now on with a bit of maintenance.I've ordered some UV curable solder resist to tidy up the area around the battery. I'll take out the battery and other components to give this area a good clean and re-coat of resist.While at it I'll clean out the holes of the spare IC and refill with solder etc.Something strange going on with the mains filter, if its a filter at all! No sign of earlier eruption / fire / explosion though.It's been got at in it's previous life, botch job on the soldering and installed upside down.New filter on the way and I'll be a lot more comfortable once it's fitted with the new filter.Arduino GPIB interface under construction - it will be interesting to see the meter noise and compare this to Grizewalds 7061I'm a bit suspicious as with the inputs shorted it does look noisy - maybe more fault finding fun to come !

author: grizewald
author_rank: Frequent Contributor
timestamp: April 10, 2020, 12:40:37 pm
content: Good idea on using some UV solder mask to replace what's missing. I did that also on my 7151, although I found it impossible to buy locally as it's classed as 'dangerous' here in Sweden and can only be sold to companies. Thank goodness for eBay!Your mains input selector/filter/fuse holder is very much missing the filter part! (It's the silver box at the bottom of the unit in my photo here.)Whoever bodged yours deserves a good slapping for leaving those mains carrying leads with no heat shrink just waiting to be accidentally touched while the case is open and you are working on the meter.Good to hear you have already located a replacement. Hopefully it's not a "New Old Stock" as the filter is primed to burst into flames by age, not use. Mouser have brand new units in stock otherwise (Mouser part number 631-FN372-2-21)Once you have all your remedial work finished and if the meter is still as noisy as mine, it would tend to confirm that the noise I'm seeing is typical for this particular design. Certainly, Klienstein thought that this may well be the case for these meters and I know from the work that Mickle T. detailed when trying to solve noise problems on the 7081 (which has a very similar ADC design to the 7061, that it's hard to achieve large improvements. So far, I can certainly agree that using an LTC1052 instead of the original ICL7650 chopper amp gives around a 30% improvement. My gut feeling is that every further 10% improvement from there is likely to be exponentially harder to achieve.Still, there are quite a few things I have not tried yet, partly because I don't want to risk ruining the calibration and adjustment that I have had done to my meter. A noisy but accurate meter is better than a quiet meter which is inaccurate to an unknown degree!

author: tggzzz
author_rank: Super Contributor
timestamp: April 10, 2020, 03:42:55 pm
content: Quote from: grizewald on April 10, 2020, 12:40:37 pmIf you can, find a saturated Weston standard cell (or perhaps other types of cell) and use that as a "noiseless" input signal. Variations in the output are then due to the DVM, or temperature shift, or bad measurement practice.This is my 7081 measuring such a cell over a day; the vertical axis is 10µV/10ppm and 1µV/1ppm. The temperature variation was ~1C, and you can see a ~40µV change in the voltage. That dwarfs the noise, which is <1µV.I don't know what the three impulses are, but they aren't popcorn noise. I've seen popcorn noise 15V/1.5ppm, on an old zener reference.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2020, 04:12:54 pm
content: With Weston cells there is always some danger that they may not be absolute stable because they where moved the last few days. The saturated cells also have quite some TC, so they need a really stable temperature. Still it should not be 40 ppm/K.With a high impedance source, input current can be an issue too. There can also be variations in gain, especially if using the 1 V range. The Solartron meters use a well trimmed zener current and with the 7081 they use a relatively odd construction with opto-coupler and a DAC to set the current. These opto-couplers can fail, running the zener with a wrong current and thus much higher than intended TC. So chances are there is something wrong with the 7081 or maybe the weston cell.For just the scale factor one could try measuring the internal zener reference - this would reduce the possible error sources.Anyway testing the pure ADC noise (near zero) and scale factor variations are 2 different things both have there justification.Usually the noise near zero is the first step as it is easier.If changing from the ICL7650 to an LTC1052 allready did improve the noise quite a bit, this indicates that the OP was a significant source and the meter may not be far away from the intended performance. These old Solartrons are relatively noisy by today's standard even if they claim high resolution. By design the resolution is limited, as they only use a single reference level. So even if one can reduce the additional noise there will likely be a limit from quantization noise, at least for the not so very slow speeds.

author: tggzzz
author_rank: Super Contributor
timestamp: April 10, 2020, 05:34:23 pm
content: Quote from: Kleinstein on April 10, 2020, 04:12:54 pmFor saturated cells the tempco is about 40µV/K, so roughly 40ppm. From a Muirhead Journal article on Weston cells, second column line 6...

author: Dek
author_rank: Regular Contributor
timestamp: April 10, 2020, 10:18:55 pm
content: I ran a few programs today - each over 1 hour, no filter, range 10Meter warmed for 3 to 4 hrsTypically looks bad asMax 6.341934-5Min -5.531311-5Mean -3.559899-6SD 1.638492-5Var 2.684659-10Rms 1.676719-5Just watching the display over approx 2 minutesRange 1 range i.e 0.000,000.0 last two digits vary 0.000,012.3 to approx 0.000,002.4Range 10 i.e 0.000,000 last two digits fluctuate -0.000,044 to -0.000,013Range 100 0.000.00 last two digits fluctuate 0.001,19 to 0.000,54Range 1000 0.000.0 last two digits fluctuate 0.003,1 to -0.000,1Now trying to understand whats happeningMaybe I need to take a closer look at the floating PSU now the Glug Death is fixed.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2020, 08:10:43 am
content: Some 16 µV RMS noise is not that far off what grizewald measured for his meter. So this level may be kind of normal.Changes to the mains grid could contribute to less accurate operation of the PLL. The operation with a fixed crystal clock would give a hint.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 11, 2020, 11:00:19 am
content: I think I was seeing more like 12μV p-p, but let me repeat exactly the same test as you have made Dek.I just turned the meter on, so I'll let it warm up properly and then run the STATSprogram for 10V range, shorted inputs, no filter for 30 minutes or so and then we'llhave two similar apples to compare.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 11, 2020, 04:34:06 pm
content: Time for some results!My results are from 1836 samples in 7 digit mode, so just a shade over 1 hour.Code:DekgrizewaldMax 6.341934E-05 4.768371E-06Min -5.531311E-05 -1.049041E-05P-P 1.525878E-05Mean -3.559899E-06 -2.975560E-06SD 1.638492E-05 2.836091E-06Var 2.684659E-10 8.043415E-12RMS1.676719E-05 4.110641E-06My 7061 has the original ICL7650 installed. Just for fun, I'll quickly power down the meter and swap it for the LTC1052and run another hour once the meter has re-stabilised (which shouldn't take that long as it's nice and hot now.)It certainly looks as if Dek's meter has some additional noise compared to mine.Did you sort out all the various noisy power rails that you reported in your initial rail check Dek?For reference, here's what I found when I checked all my power rails:7061 Rail CheckDigital BoardTestPoint 0VRef Spec Actual / Ripple501 508 +5V +/- 0.25V 5.09V / < 0.01V502 508 +5V +/- 0.25V 5.07V / < 0.01V503 508 +4.5V +/- 0.5V 3.96V (off) 4.50V (on)504 508 +5V +/- 0.25V 5.07V / < 0.01V505 508 +41V +/- 2V 41.7V / < 0.01V506 508 +32V +/- 5V 32.5V / 0.12V507 508 -32V +/- 5V -32.7V / 0.02V509 508 +5V +/- 0.25V 5.08V / < 0.01VAnalogue BoardTestPoint 0VRef Spec Actual / Ripple901 903 +36V +/- 2V 36.1V / < 0.01V902 903 +15V +/- 1V 15.4V / < 0.01V904 903 -15V +/- 1V -15.2V / < 0.01V905 903 +36V +/- 2V -35.8V / < 0.01V906 903 +5V +/- 0.25V 4.98V / < 0.01VReference Voltage302 303 -10V -10.024V / < 0.01V301 303 +10V 10.024V / < 0.01V

author: Dek
author_rank: Regular Contributor
timestamp: April 11, 2020, 05:34:23 pm
content: Hi Grizewald,Thanks for that.I'm running another prog for an hour to get another set of results, but looking at your last run, my meter is much noisier.I haven't looked at the power supply since my first check, so it likely the rails still have significant ripple.I'll check as soon as the current prog runs for over the hour.To ensure comparable results, did you check with DMM or oscilloscope?Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 11, 2020, 06:16:16 pm
content: I checked with my Brymen 867S in DC+AC mode. The AC part always starts off at an alarming value, but if you wait a while, it slowly comes down to the actual value.Getting a scope to give an accurate reading of ripple is next to impossible unless you can keep the ground lead to just a few millimetres and that's pretty much impossible in this case.I'll post the results from the LTC1052 in a bit.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 11, 2020, 06:44:47 pm
content: Updated with results from the LTC1052 based on 1841 readings for comparison.Code:Dekgrizewald griz/LTC1052Max 6.341934E-05 4.768371E-06 1.001358E-05Min -5.531311E-05 -1.049041E-05 -4.291534E-06P-P 1.525878E-05 1.430511E-05Mean -3.559899E-06 -2.975560E-06 3.347188E-06SD 1.638492E-05 2.836091E-06 2.600971E-06Var 2.684659E-10 8.043415E-12 6.765054E-12RMS1.676719E-05 4.110641E-06 4.238953E-06It's a slight improvement. The only reason I don't leave it in the meter is that when I put the LTC1052 in the hot meter and the LTC1052 comes up to operating temperature, I'm left with a 355μV offset from 0V. When the first auto-zero comes around after 15 minutes, the meter zeros out the offset, but I'm not confident that this isn't going to influence the meter's calibration, particularly from the point of view of linearity over the range. So, until I get get a larger set of voltage reference artefacts ready to compare them between the OP amp it was calibrated with and the LTC1052, I'm holding back from making it a permanent change.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2020, 07:26:46 pm
content: The step from 2.83 µV to 2.6 µV noise does not look large, but when doing the difference of squares it is quite a bit. I would not have expected that much noise from the ICL7650.The SD reading may sill include some drift part, if not absolutely stable temperature, or history before the data.set was started.The observed offset of 355 µV is really large. Both the ICL7650 and LTZ1052 are normally low offset (e.g. a few µV) and also the bias should not be so large. There is a gain of 5 for the OPs offset, but it should still not be that high.A factor effecting this could be the odd lack of decoupling at the AZ OP - normally a no-go.If I remember right from an old thread about the 7081, there was quite some noise from the comparator reading: so of the type that one reading is to high and the following reading to low. If one has a long enough data set, one could look at the Allan deviation plot to see if the noise is more white, with 1/f type or with anti-correlation between adjacent points.

author: Dek
author_rank: Regular Contributor
timestamp: April 11, 2020, 07:29:10 pm
content: Code:Dek grizewald DekMax 6.341934E-05 4.768371E-06 9.775101E-5Min -5.531311E-05 -1.049041E-05 -8.249282E-5P-P 1.525878E-05Mean -3.559899E-06 -2.975560E-06 -4.682388E-6SD 1.638492E-05 2.836091E-06 2.665109E-5Var 2.684659E-10 8.043415E-12 7.102809E-10RMS 1.676719E-05 4.110641E-06 2.705930E-57061 Rail CheckDigital BoardTestPoint 0VRef Spec Actual / Ripple Dek501 508 +5V +/- 0.25V 5.09V / < 0.01V 4.90V/ <0.01V502 508 +5V +/- 0.25V 5.07V / < 0.01V 4.99V/ <0.01V503 508 +4.5V +/- 0.5V 3.96V (off) 4.50V (on)2.02V and falling(off) 4.90V (on)504 508 +5V +/- 0.25V 5.07V / < 0.01V 5.05V/ 2mV505 508 +41V +/- 2V 41.7V / < 0.01V 40.82/ 8mV506 508 +32V +/- 5V 32.5V / 0.12V29.0/ 0.15v507 508 -32V +/- 5V -32.7V / 0.02V -29.2V/ 9mV509 508 +5V +/- 0.25V 5.08V / < 0.01V 5.06V/ 5mVAnalogue BoardTestPoint 0VRef Spec Actual / Ripple901 903 +36V +/- 2V 36.1V / < 0.01V 35.6V/ 1mV902 903 +15V +/- 1V 15.4V / < 0.01V 14.98V/ 0.5mV904 903 -15V +/- 1V -15.2V / < 0.01V -14.96V/ 0.5mV905 903 +36V +/- 2V -35.8V / < 0.01V -35.4V/ 0.8mV906 903 +5V +/- 0.25V 4.98V / < 0.01V 5.10V/ 0.7mVReference Voltage302 303 -10V -10.024V / < 0.01V -10.03V / 4mV301 303 +10V 10.024V / < 0.01V 10.02V/ 2mVSo the +32V rail looks like it needs some attention.As a simple comparison looking at the + and - 32V rails on the scope, +32V has 50mV ripple and the -32V rail 5mVNot sure whats going on the the RAM back up battery- D502 could be duff, so this also needs looking at.Along with the new filter I've on order from Mouser, also a LTC1052 .<edit> D502 was toast, the poor battery was back feeding the 5v rail..... temporarily soldered in a 1N4148 and things look like they should be.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 11, 2020, 08:27:48 pm
content: Quote from: Kleinstein on April 11, 2020, 07:26:46 pmThe drift of the meter has always troubled me. If you remember on my 7061 noise thread, I posted this plot:Here, I've turned off the regular 15 minute drift correction. The trend, over an eight hour period, is always downward and seems too large to be only caused by the very small drop in room temperature over the eight hours.I understand why the meter would need to perform a drift correct periodically, but I don't see why that drift should always be in a negative direction.Neither do I understand why the drift corrections would be as large as they are in this 24 hour plot of a Weston cell with drift correction on.The contrast to the plot which tggzzz posted earlier is striking.I'm wondering if there's some kind of leakage at play here. I have not had the analogue board out of the meter, so I'm thinking that maybe I should give the whole analogue board a good clean and several washes with iso just to rule out any surface contamination causing these effects. Even if this is part of the problem, I don't understand why there would be a seemingly constant negative drift over many hours; surely it has to level out at some point?Quote from: Kleinstein on April 11, 2020, 07:26:46 pmWell, the meter was calibrated with the original ICL7650, which includes setting the zero point, so that may imply that the original ICL7650 has a large negative offset due to a problem with the OP amp itself and that switching to the LTC1052 shows how much of a negative offset the calibration is actually masking.The only range which actually failed calibration was the current range. The zero reading for current (even after calibration) shows 0.0261mA. I noticed that this offset appears on thesecondreading shown after enabling the current range. This would imply some kind of leakage which occurs after the shunt resistor is switched into the measurement path for the first time. A faulty relay perhaps or some kind of leakage around that part of the PCB?Quote from: Kleinstein on April 11, 2020, 07:26:46 pmI can easily collect measurement data of any reasonable length. Could you suggest how much data would be needed and how I'd go about processing it to create such a plot?In the mean time, I should probably collect a 24 hour log with inputs shorted on the 10V range and drift correction off with the new LTC1052 installed to see if the overall drift tendency is any different with the new OP amp.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 11, 2020, 08:32:18 pm
content: Quote from: Dek on April 11, 2020, 07:29:10 pmI'm not surprised that diode was toast after its alkali bath. Well spotted!I think we both saw that the +/- 32V rails were only used by the expansion sockets? That's why I haven't bothered swapping the capacitors on that rail yet, but I will and it will certainly be an issue for your memory expansion board if I'm remembering correctly.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2020, 09:16:24 pm
content: For a reasonable Allan variation plot it would need some 1000 points - so about 1 hours worth of data with a short and preferably little drift and no jumps from drift correction.I used a program AlaVar5 , that needs ascii (CSV) data with time and voltage. AFAIR space separated and as an odd point that took me some time to find out the values in the first row needs to include a decimal point.The plot from the drifty part with a short suggest a peak to peak noise of some 10 µV and thus some 1.7-2 µV RMS. So the 2.8 µV number likely includes some drift / jumps.This number looks better, but still higher than the about 0.7 µV of noise I can account for from the theoretical side.There are a few possible sources for drift: one is the amplification to produce the +- reference. The other parts are the precision resistors. these have quite some effect: a 1 ppm drift in the resistor ratios (reference part) can result in 20 µV of drift for the ADC. For this reason the resistors used should be really good quality, but could still show some slow drift (e.g. warming an releasing humidity).The OP used in the inverter for the reference (e.g. make -10 v from the +10 V) can also contribute to drift. It's drift is amplified about 2 times.Leakage (getting less when warming) could also be a factor.The rather jumpy curve with the drift correction suggest that there is something not working very well with the drift correction - like too much noise. AFAIR there was a problem with the 8081 and too little waiting time. Causing trouble. Because of dielectric absorption the drift correction can be problematic when measuring a voltage different from 0. I don't know if and how they take the effect into account.

author: Dek
author_rank: Regular Contributor
timestamp: April 11, 2020, 09:31:27 pm
content: Quote from: grizewald on April 11, 2020, 08:32:18 pmAh yes, that's a bit or a worry then as the PSU rails look OK.The GPIB interface will be useful to look into this much further, just need the new controller IC to flop through the letterbox!

author: grizewald
author_rank: Frequent Contributor
timestamp: April 12, 2020, 09:01:54 am
content: Quote from: Kleinstein on April 11, 2020, 09:16:24 pmDo you have a reliable link for that program? The site it originates from (http://www.alamath.com) doesn't seem to exist any more and I wouldn't touch any of the download sites it appears on with a bargepole.What bothers me about all of this is that some of our thoughts about fixing the problems are not based on the correct assumptions in my opinion. I don't believe for a second that these meters showed any of these kind of problems when they were new. We must assume that the Solartron engineers who designed the meter knew what they were doing. Therefore, the root causes of the noise and drift problems we are seeing must be rooted in degradation of components or increased PCB leakage over time and not in any fundamental design error on Solartron's part - like "forgetting" to decouple the integrator OP amp's rails.The noise levels on my meter make the last 1.5 digits a random number generator, even with filtering enabled. There's no way Solartron would have been able to sell the meters if this was the case when they were new.

author: Dek
author_rank: Regular Contributor
timestamp: April 12, 2020, 09:26:41 am
content: Quote from: grizewald on April 12, 2020, 09:01:54 amI tend to agree.Switching to DCI (rear terminals shorted) and DCI see that last two digits jump about the same as the DCV (although the meter has only been warming for 1.5hrs).

author: grizewald
author_rank: Frequent Contributor
timestamp: April 12, 2020, 09:30:07 am
content: To have a proper look at the stability of the voltage reference section, I could make a high quality test lead that I can connect to test points 301, 302 and 303 and bring the voltages out to my scope without coupling a load of noise to them in the process.I have some three core shielded PTFE cable. The three cores are PTFE insulated multi-stranded silver plated copper wire. All three wires are twisted together. The cable is thin enough that I could slightly enlarge an existing hole in the rear panel and be able to connect it and close the case.What I'm a little unsure about is how to actually connect the shield. The whole analogue board is floating, so I really don't want to connect the 0V test point to mains ground via my Rigol scope. I don't really want to connect the shield to the 0V point either as I'm going to end up coupling whatever the shield picks up into the analogue board's floating ground.I also have a battery powered Micsig scope which would get rid of the mains ground issue. I suppose I could connect the shield to the Micsig at the Micsig end and leave it open at the other end, but then if I include the 0V point from the meter inside the cable to give the Micsig the ground reference to measure the voltages against, that ends up being the same as connecting the shied (and whatever it is picking up) to the floating ground anyway.

author: grizewald
author_rank: Frequent Contributor
timestamp: April 12, 2020, 09:33:53 am
content: Quote from: Dek on April 12, 2020, 09:26:41 amNo nasty offset on your DCI reading though?I'm starting to think that my best plan of attack is to get the analogue board out of the machine and clean it properly and see if there's a mechanical reason for my DCI offset. It's at least a more concrete starting point than I have now.

author: Dek
author_rank: Regular Contributor
timestamp: April 12, 2020, 10:08:03 am
content: Quote from: grizewald on April 12, 2020, 09:33:53 amDCI offset.........if the meter would just stay still for a few seconds :-)anywhere between 7mA and 40mAIt will run at about 10mA for about 20 sec then increase to say 20mA or more then drop again - looks cyclic.Did I say I could do with the GPIB workingRunning a prog for ~2 mins give me>(No filter 7 digits)N=90Max= 0.05340576Min= 6.347656E-3PP= 0.04705810

author: Kleinstein
author_rank: Super Contributor
timestamp: April 12, 2020, 11:14:51 am
content: The Very noisy DCI part looks like a different problem. Even with the noise is it at least reading a current only approximately right.An offset of some 5 mA sounds like too much for just leakage on the board. The current ranges should use some 200 mV range, so one could check that lower voltage range too.For the reference section, sensing the internal reference and checking the reference part for noise with the scope are two parts and could be done separately. When using the scope there would be a connection to mains ground. This makes some sense as the ADC can cause spikes from the switching that could effect the reference part.Connecting the internal reference to the input is separate and not really testing the stability of the reference. It is more checking the rest of the instrument and taking the reference out of the equation for a large part. So reference drift would essentially not effect those measurements. One can still check (include or not) the amplification from 7 V to +-10 V. These signals are relatively low impedance, so there would be no need for special shielded cables - just connect the input, not ground !For the noise, there are 2 points that have changed since the meter were build / designed: the mains grid got more noisy from an increasing number of PV and some wind turbines. SMPS can also contribute to mains noise. So the PLL for the frequency can have a harder time than it was some 30 years ago. The other point is that there is more RF around, especially mobile phones. So there can be possible new EMI susceptibility issues.For the high resolution end, it is quite normal that the RMS noise is at about step of the last digit. So it is kind of normal that the last digit does move, up and down. The current 2.8 or 2 µV RMS noise is something like 3 or 2 times higher than this convention. So the noise is already better than 6 digit level - though not really 7.5 digit level. The drift level without the zero step every 10 minutes is relatively high. Still this is from quite some time ago - the expectations may not have been that high back than. given the drift level the zero step every 10 minutes makes about sense. If it was much better the time interval could have been longer. I don't know if there are specs for this and maybe the noise.For the Allan variance, I could use the program if you provide some data (e.g. CVS data in some form, e.g. zip compressed).I found the software on TiN's xdevs server - if you dare.https://doc.xdevs.com/docs/_Freeware/

author: grizewald
author_rank: Frequent Contributor
timestamp: April 12, 2020, 01:46:06 pm
content: Quote from: Kleinstein on April 12, 2020, 11:14:51 amI'll trust a download from TiN's server much more than I would allowing some dodgy website's Javascript, that's for sure! Anyway, I'll be installing it into a Windows virtual machine, so I'll just take a snapshot of the VM before installation, run it, and then revert the snapshot afterwards. Thanks for the detective work!I think Dek's machine still has some more serious problems to be found. Mine might have an unwanted offset on DCI, but it can be nulled out and the meter would have passed calibration on the DCI range without adjustment if it wasn't for the unwanted offset at 0 on DCI.You're right about testing the reference voltages of course - just connect the probe inputs without the grounds!My excuse is I've been really sick for the last week.While the theories about SMPS, mobile phones and increased mains EMI in general are attractive, I just don't buy it. That the mains of 30 years ago was cleaner and more predictable in frequency than it is today doesn't sound that likely. 30 years ago, most of Europe still had a large manufacturing industry with lots of heavy machinery creating interference which was certainly magnitudes larger than what we see today. The Solartron's are also very well shielded boxes with conductive linings on the panels inside with will soak up most RFI sources, just as they are intended to do. Also, if this was the case, then how is it that tggzzz's meter is so quiet? Or, for that matter, my 20 year old 34401A? Nah, I don't buy those theories.I certainly accept that the last digit on any meter, regardless of counts, is generally a random number generator, but the second to last should be stable to +/- 1 count.It's interesting to note that in the 7061 manual, one can find the following statement:DRIFT CORRECTIONThe instrument automatically compensates for internal drift every15 minutes but does not interrupt a GPIB input or a keyboard trigger.Under remote control, drift correction may be turned on and off.For example, in tracking measurements lasting longer than 15 minutes,drift should be turned off for continuity of results.It's interesting that they suggest turning off drift correction on longer measurements. That's all they say about it, so I'm not sure what I should really make of that.I think I may have to accept that the 7061 is no 3458A and keeping hoping that I'll see one pass by me at the right price some day!Anyway, I'll get some appropriate logs and see what looking at the Allen plots might reveal. I'll also connect up my Time current source to the rear terminals and have a quick look at how stable mine is on current readings. Plots to follow!

author: Kleinstein
author_rank: Super Contributor
timestamp: April 12, 2020, 04:25:23 pm
content: The measurement from Tggzzz is in the 2 V range. Here the ADC noise is reduced by the gain to 1/10 , so some 0.2-0.3 µV_rms from the ADC and some noise from the amplifier. The 0-0.5 Hz noise from an ICL7650 should be somewhere then 0.5 µV_pp range, so not very high.For the DC drift, one could check how sensitive the DC offset is to things like loading the 5 V supply on the analog board. There could be some effect via delays in the Glug switching that depend on the supply.

author: Dek
author_rank: Regular Contributor
timestamp: April 19, 2020, 09:36:26 am
content: Repaired the solder resist in the battery area, best I could anyway. It was a very crusty in the area between the battery and the regulator.I attached a small 3.7v lippo via a 330resistor to power the EAROM while the main battery was taken out.All the caps , diodes and resistor were taken out too so that I could give the affected area first a polish with a fibreglass brush, IPA wash then paint with the new solder resist.Looks much better, if not quite factory freshArduino GPIB adaptor built and ready for when the controller IC finally arrives.Dek.

author: Dek
author_rank: Regular Contributor
timestamp: April 24, 2020, 06:56:20 pm
content: The GPIB controller IC arrived today- plopped it in and connected up WaveyDipoles Arduino USB GPIB interfacehttps://www.eevblog.com/forum/projects/ar488-arduino-based-gpib-adapter/Quick check using ++read and heypresto - it sent back a readingTyped the bee(p) command and the 7061 gave a joyous BEEPSo all is hunkydory in Solartron world, for the moment at least.Quick log of the meter with IP terminals shortedDCV Range 1 set 6 digits just to test the logging is working.I manually cut and paste the readings from Putty into excel do a Data to columns and then graph.Now to do some comparison tests with Grisewald and his 7061 - haven't seen him for a while, hope he's OK.Dek

author: Dek
author_rank: Regular Contributor
timestamp: April 28, 2020, 09:18:43 pm
content: Now with the GPIB logging working I have took a series of measurements, generally over approximately 1 hr periods.Graphing the whole hours of data does look like random noise, but 'zooming' in, the noise appears to be not actually random noise but a cyclic oscillation, possibly caused by the PLL not locking (page 4 of this thread).The frequency of the oscillation does appear to be very similar to that of the PLL lock drift.The fact that the PLL doesn't actually lock has always bugged me.I have a 49.125 clock from mouser arrived today, so I will bodge up a replacement clock board Manhattan style and see if that eliminates the variation I am seeing.1st image is from a cold start,2nd image after 4 or 5 hrs

author: Dek
author_rank: Regular Contributor
timestamp: April 30, 2020, 07:29:03 pm
content: Made up a small clock board using a Crystec clock oscillator.Currently logging the meter as it warms up as in the first trace above filter on range 0.1 6 digits (7 displayed).Just looking at the display, doesn't look to be much improvement.Once the data is graphed will be able to see if it's any better and also run a range 10 7 digits log with no filtering for comparison to the second trace above.Dek.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2020, 08:14:38 pm
content: The 0.1 V is usually limited by the input amplifier. So there is not much change from the clock expected. The more critical one would be the 10 V range, as this is often limited by the ADC.AFAIR the solartron meters use a continuous integration with only very rare zero cycles. The noise can be of 2 types: one is noise that actually accumulates over time this is very low frequency noise. The second is noise in getting the comparator levels. This is higher frequency, and causing one reading to read high and the next to read low. So there would be some corelation between adjacent readings.Noise of the 1 st type causes the noise from the average of N adjacent readings to go down with N line square root N or a little slower (if 1 /f noise it present). Noise if the 2 nd type goes down approximately like 1/N, so considerably faster. So looking at the Allan deviation (which is essentially look at the noise with different amounts of averaging) would give a hint if the noise is more the 1 st or 2 nd type.

author: Dek
author_rank: Regular Contributor
timestamp: April 30, 2020, 10:03:23 pm
content: Hi Kleinstein,Well, the new oscillator shows a different result compared to the original clock board.This is a quick trace from the log. ( Range 0.1 6 digits filter on)The large spikes have gone, the oscillation error is more consistent.I think the variation in clock frequency of the original board due to the PLL not locking, randomises the "oscillation" making the error look more like random noise which possibly it is not.I'll run a log set on range 10 7 digits and no filter and see what that looks like.Dek

author: Kleinstein
author_rank: Super Contributor
timestamp: May 01, 2020, 08:28:17 am
content: The 0.1 V range readings look a lot like some beat frequency. I see 3 frequencies that can contribute: mains frequency, the input chopper and the ADC clock. As things have changed the ADC clock should be one of them.The filter function can also be tricky, as it can hide effects of some outliers. Unfiltered data are easier to analyze. One can always apply the filter later in the spreadsheet program if needed.What is the x axis / time scale ? It looks like the oscillation is relatively fast.Is there a very fast mode (more than 250 SPS) with the 7061, so one could see the actual level of 50/100 Hz hum from the input signal ?

author: Dek
author_rank: Regular Contributor
timestamp: May 01, 2020, 04:25:16 pm
content: Quote from: Kleinstein on May 01, 2020, 08:28:17 amHello Kleinstein,Thank you for your reply.X axis is about 1 sample per sec - I'll check exactly later.Looking at the trace today, there are some other repeatable artifacts in the sine wave i.e the small reversal about 2/3 up/down the sine wave.Maybe or maybe not this is significant.I'll run a Range10 7 digits no filter log shortly and also look at the fast burst mode to see if that germinates another clue.Dek

author: Dek
author_rank: Regular Contributor
timestamp: May 01, 2020, 07:15:10 pm
content: Run for about 1 hr.1 sample / 2 secRange 107 digitsNo filter.Will now try the burst / fast mode.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 02, 2020, 03:47:22 pm
content: Nice to see you still working on this Dek. I've been working like a madman since I was last here, hence my silence.I just finished bringing my 7151 back to life - well sort of. In the end, I had to come to the conclusion that the analogue board probably has a dead 68P01 microprocessor. First I found that the transistor that drives the -RESET line to the processor from the watchdog circuit had turned into a 300 ohm resistor, then I found that the EPROM which sits on top of the processor was blank and couldn't be re-programmed either. Even after finally getting a new 2764 to give the processor a program to run, it didn't help. The analog board just sits there turning various relays on and off.Then I noticed that the analogue board is identical in the 7150+ and the 7151, so I took the analogue board from the 7150+, put the 7151 EPROM in there and got a working 7151 for my efforts with the 7150+ relegated to the position of parts mule.The reason why I mention this is the interesting difference when it come to how Solartron implemented the drift correct feature in the 7151. On our 7061 meters, a drift correct is performed once every 15 minutes. On the 7151, it is either performed every 10 seconds, or before every reading when in 6x9 mode. Despite that, the meter's readings will regularly cycle through about +/- 60μV in either direction but at times will remain stable for several minutes.I also received my ESR tester and had to replace a bunch of electrolytic capacitors on the 7151, so I'm about to dismantle to 7061 and do the same thing. I'm hoping to find some which need replacing and that doing that might bring a little more stability to my 7061.Your last graph seems to show the same behaviour as I see on mine, plenty of random noise plus significant changes on each drift correction. We obviously bought the same part at Mouser as I also have a Crystek 49.152MHz oscillator ready to replace the PLL controlled clock. It looks like it made a difference for your meter so I'll get mine built and see what effect it has on mine.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2020, 04:35:07 pm
content: The noise is really quite high, even for the Solatron design that is not really low noise. It looks like both "high frequency" noise going up and down fast, like an error for the comparators and also some low frequency noise, a little like popcorn noise.The quite regular up and down from the 0.1 V rangs is no longer there, at least not much. So this is more like a thing of the input amplifier, and not of the ADC itself. Even with not so perfect frequency match it is not likely to have so much hum to cause the regular signal for the 0.1 V range. The very good 50 Hz suppression due to the PLL is more a thing if the signal to measure contains quite a lot of hum.Not having decoupling at the ICL7650 is suspicious. Like with other slow OPs there could be ripple from the -15 V side coming through and cause some trouble (not just noise, but also INL errors).

author: Dek
author_rank: Regular Contributor
timestamp: May 02, 2020, 06:48:11 pm
content: Hello Grizwald,Glad to see you back, beginning to worry the plague had bitten you :-)I was looking for a 7150+ before I bought the 7061, nice little meters for not much money. Still looking for a clean one at a reasonable price.The 49.152 oscillator works quite well. The outout was about 7v Pk-Pk so I've added a divider to lower this a bit - The frequency drops to about 49.15189mhz with the main cover on, the PCB is too high to get the clock enclosure lid on at the moment.I'm attempting the test Kleinstein suggested which is the burst measurement. I have set up the meter to do this on pressing the trigger button and the readings can be see in memory- Hist.Burst measurement although about 1000 sps it is only in 4 digit mode.You can also set up the sample mode and select say 5 digit mode, this will take 100 readings in about 2 seconds which might provide a more usable resolution.I'm now trying to configure the GIPI adaptor to upload the readings from memory automatically so they can be fed into excellThe DUmp command only returns 1 result, where it should dump all for the readings in memory.I'm having a fiddle about to see if I can get the memory to automatically upload to PuttY, then set up a macro in the GPIB controller to do the same.I've also a LTC1052 to swap over to see if that helps with the variation we're seeing.Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 02, 2020, 07:43:10 pm
content: Quote from: Dek on April 19, 2020, 09:36:26 amTotally not needed. The EAROMs are early versions of what we now call flash memory. They keep their contents without power.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 02, 2020, 07:53:32 pm
content: Quote from: Kleinstein on May 02, 2020, 04:35:07 pmTrue, Dek's noise is an order of magnitude more than mine, so there's still something basic wrong....Quote from: Kleinstein on May 02, 2020, 04:35:07 pmThe thing is, the original design does not have any added decoupling for the ICL7650. If that actually is a problem, then the meter would have had these noise issues when new. Logical deduction says that can't be the case.The PCB layout is such that the ICL7650 is right next to the smoothing capacitors from the +/- 15V rails. So I'd say the designers realised that there was no need for extra decoupling.(The ICL7650 is the LTC1052 in my case.)

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2020, 08:56:10 pm
content: Quote from: Dek on May 02, 2020, 06:48:11 pmExchanging the ICL7650 could have an effect: The ICL7650 uses an internal chopping frequency somewhere in the 500 Hz range. With some bad much this may interfere with the forcing frequency in someway, that is in a comparable range. So even if perfectly OK some of the AZ OPs may not like the environment, if they happen to use an odd frequency.With only 4 or 5 digit resolution the fast modes may not be sensitive enough to see hum, at least not very good. One may be able to use a 2nd meter to check for hum - still not so easy, as hum can come via so many paths and also effects other meters / scopes.

author: Dek
author_rank: Regular Contributor
timestamp: May 03, 2020, 11:39:11 am
content: Got the burst measurements uploaded from memory using the GPIB interface.The commands have to be issues in a specific manner when doing this manually.Set instrument range eg 0.1vClear the memory historySet up the instrument to trigger in burst mode - set no. samples required.Set the GPIB to ++auto 2send TRI to start the burst mode.Set to ++auto3Send Du (DUmp memory contents)Send ++Readmemory contents are loaded into the serial monitor screen.Graph below.Sample rate is 1Khz, the sine wave period is 30ms or 33hzI did the same on Range 1 and range 10, but the signal is very "digitised" so not very helpful but is a similar but 'blocky shape'.Would be interesting to see what your instrument shows Grizewald, performing the same testSo that is something to think about.Next I'll swap out the ICL7650 and do the same test.

author: Dek
author_rank: Regular Contributor
timestamp: May 03, 2020, 01:12:49 pm
content: Took out the ICL7650 and fitted the LTC1052Ahh, that's disappointing, no difference (1/2 expecting that ) graph below.While I was in that area, I noticed an additional earth lead that Grizewalds 7061 doesn't have.Looks like an addition ??So removed that hoping that would solve the noise problem.Ahhh, that's disappointingDek.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 03, 2020, 02:53:44 pm
content: A 33 Hz signal is really odd. The amplitude is also quite high - not just a few µV, but mV range. As it gets less visible in the 1 V / 10 V range this would be signal coming in at or in front of the input amplifier.The first point is where is 33 Hz coming from ?Tthe ADC forcing signal should be more like > 1 kHz. The 7650 internal chopping should be some ~500 Hz and mains should be 50 Hz. I would expect the display update in sync with mains, e.g. 50 Hz or 100 Hz, but not 33 Hz. Aliasing from some F Chopper or F_sample +- 33 Hz would be strange.For changing the 7650 to an 1052, in this case the more relevant one may be the one at the input amplifier.The second point is how could there by so much signal at the input, that should not be there. Even the 50 Hz / 100 Hz signal should and is considerably lower amplitude. So it much be something relative strong.2 mV at 33 Hz may already be just visible with a scope - so one could check that the signal is really there. It would be amplified at the ADC, so well visible at some 200 mV there. It may be with the check, just in case the actual signal is at a different frequency and due to aliasing.

author: Dek
author_rank: Regular Contributor
timestamp: May 03, 2020, 03:03:36 pm
content: Quick check on set the range 0.1 using 1 ohm resistor.Seeing exactly the same 33hz noise.(The frequency is assumed that the meter is actually taking the samples at 1khz ).

author: grizewald
author_rank: Frequent Contributor
timestamp: May 03, 2020, 05:02:42 pm
content: Quote from: Dek on May 03, 2020, 11:39:11 amThat might be a bit tricky right now....This is my 7061:That extra earth lead that you have is very perplexing. I've scoured the maintenance manual for DP901 and can find no reference to it apart from the component layout for the analogue board. The pin is connected to the power supply ground and seeing as the whole analogue board is supposed to be floating, I'd say connecting it to earth is probably a very bad idea!

author: grizewald
author_rank: Frequent Contributor
timestamp: May 03, 2020, 05:15:45 pm
content: I've ordered a complete set of replacement electrolytic capacitors from Mouser and will be replacing all of them. Some of them, the smaller values in particular, were borderline when I tested them with my ESR tester, so I may as well bite the bullet and re-cap everything while I'm there.Next task is to get the boards under my microscope for a proper inspection. The analogue board is fairly dirty with dust and metal particles from the case locking screws. Something has to explain the offset I get on the current range and possibly some of the noise I see. Looking at the bottom of the board, I can see some parts on the guard traces particularly where the solder resist is missing. An uninsulated guard and a dirty board is a recipe for picking up noise. So I'll have a really thorough inspection under the microscope before I start cleaning and repairing the solder resist.It looks like my analogue board has been "repaired" before and whoever did it wasn't very particular about cleaning the flux off the board afterwards.My replacement capacitors will be here next week seeing as I ordered from Mouser, so hopefully the meter will be reassembled by the weekend and I'll run the same test as you did. I'm pretty sure I'm not going to see anything like what you're seeing there as I doubt the meter could be calibrated with that much noise.

author: Dek
author_rank: Regular Contributor
timestamp: May 03, 2020, 06:14:20 pm
content: Quote from: grizewald on May 03, 2020, 05:02:42 pmWhat kind of an excuse do you call that !Good luck with the recap.I haven't has the analogue board out yet. When I changed 2 of the PSU caps I managed to do it with the board tipped up.With the noise problem I'm having, it might be a good idea to have the board out, do a thorough inspection and change the remainder of the caps before going much further.Oh, I've also the mains filter to fit. (currently running on a in-line jobbie).Hummm, just spotted the dip switches on your board are set differently to mine. Isn't that something to do with the mains frequency? (now reading the manual)<edit> They are OK

author: grizewald
author_rank: Frequent Contributor
timestamp: May 04, 2020, 07:33:35 am
content: I think that's how my configuration switches were set from the beginning. Apart from the line frequency setting, it seems to ignore the rest of the switches if the scanner board isn't installed. The very top one isn't even connected to anything according to the circuit diagram (which has the switch block upside down compared to the orientation on the board).I spotted a few things when I went over the analogue board last night. I'm even more convinced that it was repaired at some point as I can't imagine it coming from the factory like this:

author: Dek
author_rank: Regular Contributor
timestamp: May 04, 2020, 04:48:12 pm
content: I'll check mine later to see if the solder blob is a standard factory optionFrom Kleinsteins comment about the 33hz noise source, I decided to check the burst sample rate as we areassumingthe rate is 1000 samples per sec.I injected a 50hz 0.1V ac signal with the meter set on DC range 0.1 4 digits.Trace below show the result.One 50hz cycle has the same number of sample points as the noise traces above.The conclusion = the noise is not 30hz but must be 50hz.This is a relief as the source will be easier to trace.With my earlier focus on the Glug death problem, I only briefly looked at the analogue PSU - time for some new capacitors I think.Dek.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2020, 06:04:21 pm
content: Even if the frequency does not match to 100% the 50 Hz part should ideally be suppressed by some 60-80 dB. So those 200 µV of 50 Hz should not cause so much noise. However having quite some 50 Hz background where it should not be indicates a problem.So checking the supplies is probably a good idea. There may be ripple somewhere.

author: Dek
author_rank: Regular Contributor
timestamp: May 05, 2020, 09:36:13 am
content: Recaped the floating PSU - No changeBefore the recap, when checking the 33hz / 50hz noise, I have my oscilloscope measuring the input to the meter to monitor the signal from the sig gen.I removed the sign gen but still had the scope attached to the input and could see 50hz. I'll re do this test to get the values.Not sure if this is real or just the scope leads picking up mains hum.Dek.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 05, 2020, 10:01:40 am
content: The 200 µV of 50 Hz Signal should be hardly visible on the scope. However after amplification at the ADC input the signal should than be very visible.A possible point of failure would be the input protection, e.g. a series resistor to limit the current that could be high resistance. Poor relay contacts may be possible too. Noise could also come in from a common mode voltage, that is input negative side to case. Normally the shielding suppression should better, but it can still help to have an eye on the case shields.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 05, 2020, 12:53:52 pm
content: Quote from: Dek on May 03, 2020, 01:12:49 pmI was doing a little scouting around the Internet for more pictures of the insides of the 7061 and found somethinginteresting regarding your extra wire.I found this picture onwww.pa4tim.nland it shows an early 7061 with PCBs 3 and 5, just like yours:It seems that the extra wire from the analogue board's PSU ground going to a spring copper finger in the middle of the case which connects to the brown conductive shielding was indeed a feature of your revision of the meter but was removed on the later models.I wonder why?

author: Dek
author_rank: Regular Contributor
timestamp: May 05, 2020, 07:41:54 pm
content: Quote from: grizewald on May 05, 2020, 12:53:52 pmThat is interesting, at least it's not some hack by a previous owner.The lid of my 7061 has a piece of PCB that sits above the analogue side, and grounded to the main analogue pcb (not mains earth) by the copper spring.As to why, perhaps they determined it was better to let the the noise out of the meter than keep it inside( well on mine anyway).Checked the +-36v and +-15v rails again, all bang on with ripple less than 1mv.I'm now struggling to think of a way forward.Time to fit the new mains filter, it's doing no good in the mouser bag.Dek<edit>Grizewald, reading your earlier post regarding the recap, I have only changed the larger PSU caps.Possibly a way forward is to replace all the smaller caps too.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 07, 2020, 10:02:41 am
content: I think having a look at what the A/D circuit is actually doing may prove to be more useful. See what the waveforms on your machine look like compared to the reference waveforms in the manual, both when measuring 0V and when measuring a battery. Also, like Klienstein suggests, track the input through the input protection circuitry to the input amplifier and then through that to see if you can see where the 50Hz is entering the circuit.I made a start on removing the smoothing capacitors on my digital board last night, but I was a bit tired and ended up destroying a plated through hole to the ground plane.So I'll leave the rest of that job until Saturday morning when I'm rested and have natural light to work with.

author: Dek
author_rank: Regular Contributor
timestamp: May 09, 2020, 03:21:50 pm
content: Hello Grizewald,I've started tracing the I/P as Kleinstein suggested.Using my Fluke 187.2 AA batteries as the input 'reference'Meter set 10v range 7dig no filter.Meter grounded to the LO side of the spark gap.Measure the DV voltage and the AC mV component.Without any kind of base line as to what level of ac I should be expecting ( I've limited experience on these meters to say the least !) comments are welcome. :-)I'll work through the IP amplifier next.[ Specified attachment is not available ]

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2020, 05:21:01 pm
content: The DC readings should reflect the voltage drop due to the resistance and ~ 10 M input resistance of the fluke 187. So the DC part looks normal.The AC numbers are a little confusing. It looks a little like AC coming in from the probes (e.g. capacitive coupled) and the different reading may reflect the impedance seen at the different points. So the lower impedance points see less AC. In this concept the readings make sense and relays seem to work and the resistors seem to be no open.Measuring the real AC without coupling in extra hum could be tricky. The Level seen before was at some 200 µV, so much less than read here.It the bootstrap /guard signal looking OK ? If wrong this could enter through the JFETs.Another point to test could be input bias current (with a high resistor or drift with a 10 nF range cap).The gate drive with opto-couplers is a nice solution. However those opto's may age, not turning on all the way and thus possibly not turning off the JFETs all the way.A lot of AC signal from the AC unit could be a problem too. So it the output from the RMS part OK ( low voltage, not much AC) ?

author: Dek
author_rank: Regular Contributor
timestamp: May 09, 2020, 05:40:21 pm
content: Quote from: Kleinstein on May 09, 2020, 05:21:01 pmThanks for the reply Kleinstein,Measuring the bootstrap WRT 0v (spark gap low side to R9 R10 R11 Node) 3.19VDC 0.30mV AC 100.00 HzThe guard is interesting! 0v DC 1.89V AC 50 HzI pulled the link plug PL6 to isolate the AC converter, appeared to make no difference.All the readings are fairly repeatable.If there is a better measurement method I'll try with the instruments I have - 200mhz scope, Racal 1998 counter and DMM I have a HP 3457 I can bring in from work.Dek.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2020, 07:03:49 pm
content: 300 µV 100 Hz AC signal at the bootstrap signal is odd, as the real signal measured some 200 µV 50 Hz, though with some harmonics. At least the DC level seems to be OK.From the gate drive there is some load from the -36 V to the bootstrap signal. This could be a way 100 Hz could come in.The guard (terminal) may be just floating and picking up hum.For a test one could use the scope at the ADC input / amplifier output with the meter in the 100 mV range. With a gain of 100 I would expect some 20 mV of hum visible. One could than try a short at the OP1 testpoint (11.3 mV AC noted there) and maybe a few more points to see how much this reduces the hum. Similar one could check in the current range for the hum visible.It is still not 100% clear if the hum is more like capacitivly coupled to the input (would have to be quite a bit) or an error from the amplifier itself (e.g. via the supply to the main amplifier), or the ground point for the divider.There may be quite some common voltage. Is there something like a shield winding in the mains transformer that is not (or wrong) connected ? One could try measuring the common mode voltage / coupling. Ground would be via scope to PE and the interesting point the neg side input, guard and maybe a few case / shield parts if not directly connected.Could it be that one should have a connection to the guard (e.g. to the negative input).

author: Dek
author_rank: Regular Contributor
timestamp: May 10, 2020, 12:37:40 pm
content: Quote from: Kleinstein on May 09, 2020, 07:03:49 pmWith my 2xaa batteries connected to the IP 3.191vMultimeter connected 0V to TR3 (the 50Hz 5.45mV point) shorting OP1 to the input 0V reduces the AC by about 1/2.Multimeter connected to 0v and the integrator IP (handy test link ) 3.191Vdc and 7.75mVac noise. Shorting OP1 makes no change to the noise but drops the voltage to 1.0638vdc(may be a good test to split this link and check circuitry before and after the link to check which section the noise is from.With scope ground connected to the meter PE, probe to the guard terminal - screen shot above.Scope ground 0V to bootstrap 3.2vdc but noisy -screen shot below.PE to the IP 0v measures 4.905PE to + IP 8.096Screen grab below.I have the guard terminal not connected, just 4 wires.The manual states that the standard Solartron cable and connector set does not use the guard pinDek

author: Dek
author_rank: Regular Contributor
timestamp: May 10, 2020, 03:19:31 pm
content: Some progress:-Took out the link TL101With Fluke multimeter from 0v to TR3 and see the previously measured 7 to 10mv AC7061 meter set on the 0.1v range and IP socket shorted (excluding pin 5 guard)The 7061 display fluctuates about ~2 counts of the last 7 digit (approx 0.000,000.5)Temporarily re-connect TL101 and the display fluctuates 2 or 3 counts of the 5th digit. (approx 0.000,035.1)Same test with meter set on range 1TL101 link open, the display fluctuates 2 counts of the 6th digit. (approx 0.000,006.4)Connect TL101, display fluctuates 6 counts of the 5th digit. (approx 0.000,056.1)Same test range 10TL101 link open, the display fluctuates 1 count of the 5th digit. (Approx 0.000,024)Connect TL101, display fluctuates ~3 counts of the 5th digit. (Approx 0.000,047 )Range 100TL101 open 1 count on the 4th digitTL101 closed 3 counts on the 3rd digit.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2020, 03:30:43 pm
content: 7.7 V AC at the integrator input would be a real problem.Even with 14 mV RMS at the input a gain of 100 should only cause 1.4 V. So there seem to be something really bad with the amplifier.The reduction of the DC voltage to about 1/3 with OP1 shorted is the expected value, so the resistors seem to be OK.With the AC part only going down by a factor of 2 (and not 3) is a little odd. So AC wise there seem to be some extra impedance - like some 30 nF to ground, so not just a little, more like part of the filter at the amplifier.Opening the link to the integrator may make some sense, though I would not expect much noise from the integrator (should be rather high impedance anyway).Besides the 50 Hz hum, there also seem to be quite some higher frequency background of a few MHz.The ICL7650 used in the main amplifier can no be used along - it would need some extra support / bootstrapped supplies, that could go wrong. One could also check at lest the normal amplifier function with something like a 150 Hz 100 mV or so square wave signal to the input. Chances are things are somehow not working perfect there.The noise tests with the open close link suggest that there is quite some noise coming from the amplifier. Some in expected for the 100 mV range, but not much for the higher ranges. It is odd to see different noise in the different ranges also with the link open. In this case there should normally no much effect of changing the gain.

author: Dek
author_rank: Regular Contributor
timestamp: May 10, 2020, 03:57:02 pm
content: Quote from: Kleinstein on May 10, 2020, 03:30:43 pmHi Kleinstein,Sorry my typo should be 7.75mV AC (scope shows this as noise and also spikes that align with the rising / falling edge of the glugs)The guard appears to be floating, no continuity to PE or any other part of the board, so is acting like an antenna when looking at the trace on the scope.Dek.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 10, 2020, 06:59:46 pm
content: Quote from: Dek on May 10, 2020, 12:37:40 pmDon't believe the manual!I don't know how they got this wrong when writing the manual, maybe some miscommunication between the engineers and the technical writers, but the guard is most certainly used. In the standard Solartron cable with just + and - inputs, all five wires are used. The +V and Ohms Hi wires are joined just before the wires exit the shield, as are the -V and Ohms Lo wires.The guard is bonded to the -V Ohms Lo pair just before they exit the shield.Furthermore, the guard input is connected to the transformer from the analog board on pin 4 (yellow wire) of the connector at the rear of the analogue board. The yellow wire goes into the transformer windings, suggesting, as Klienstein correctly assumes, that there's a shield winding inside the transformer.Not only that but the guard goes to a track which runs around the entire circumference of the analogue board which is connected to the test point labelled "guard" at the front left of the analogue board. Note though that the guard wire from the inputs is not connected to the rest of the meter unless the meter is powered on and RL6 is energised.I hope you haven't been chasing ghosts caused by the guard input being left floating!Oh, and the guard is connected to the screen of the measurement cable at the far end (furthest away from the meter) The shell of the input connector is earthed at the meter, but the shield is not connected to the shell.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 10, 2020, 07:21:46 pm
content: For some bizarre reason, the leads are all described in detail in the 7081 user manual but not at all in 7061 manual. Here's the relevant description of the two terminal lead from the 7081 manual.

author: Dek
author_rank: Regular Contributor
timestamp: May 10, 2020, 07:31:04 pm
content: Quote from: grizewald on May 10, 2020, 06:59:46 pmHi Grizewald,We'll have to change RTFM to DRTFM :-)I've added another pin to the shorting block so all 5 pins on the instruments front connector are joined.Staring at the display for a couple of minutes, it doesn't appear to have made any difference. I think before drawing a conclusion I'll run 30mins of logging and graph it.Have you got yours back together?Dek.<edit> Thanks for the cable info.

author: Dek
author_rank: Regular Contributor
timestamp: May 10, 2020, 08:04:36 pm
content: Using a small PTFE shorting block with the additiona centre pin added and shorted to all others,it looks the sameWhich is a shame as I like an easy fix no matter how it's attained.Time to give it a rest and have another go tomorrow.Dek

author: grizewald
author_rank: Frequent Contributor
timestamp: May 10, 2020, 08:13:20 pm
content: Meanwhile, I've just got my meter back together after swapping out all the power supply capacitors. Thankfully, what I thought was a plated through hole that I'd damaged, was just a plug of solder which had pulled out from the hole, but I couldn't see that properly until I actually got the capacitor out.I enlisted the help of my daughter to remove the rest of the capacitors so that I could use two soldering irons to melt the solder on both pins while she gave a gentle wobble and pull on the capacitor once I could see that the solder had melted. An extra pair of hands made the job so much easier!So my 7061 now has a full complement of Nichicon electrolytics, except for the three 47uF ones on the digital board. Two of those 47uF capacitors are not actually used for smoothing, instead, they couple the AC input of one bridge rectifier to the AC input of another bridge rectifier. (see below)As these capacitors are not for smoothing, I bought the original Panasonic parts which are specified in the parts list. They have a much higher ESR than the ones which were fitted, but given the unusual use, I decided to trust the Solartron engineers on this one.The rest of the electrolytics I used are all 105°C rated Nichicons. I took a higher temperature rating than the ones which were there knowing how hot it can get inside that case. The smoothing capacitors for the digital board are right next to the very hot transformer, so it seemed sensible to chose capacitors with a higher operating temperature.The analogue board was really dirty when I looked at it close up...This kind of dust and grime was everywhere on the top and bottom of the board - hardly something you want in a circuit where input impedances are in the gigaohms range!So, after replacing the capacitors, both boards were thoroughly cleaned with isopropyl alcohol a brush and cotton buds with particular focus on the analogue board. Both are now sparkling on both sides.The meter is now fully cased again and warming up. I'll let it soak overnight and tomorrow I'll run some noise measurements and have a look at how well it reads very high resistances as well.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 10, 2020, 08:20:40 pm
content: Quote from: Dek on May 10, 2020, 08:04:36 pmThat's a shame.I'm puzzled by your plot.It says "range 10, 7 digits", but the numbers on the Y axis have only 5 digits after the decimal point when there should be 6.So what am I actually seeing here? Does the Y axis go from -50μV to +40μV, implying you have 90μV of noise?

author: Dek
author_rank: Regular Contributor
timestamp: May 10, 2020, 09:14:58 pm
content: Yes,The Y axis scale is +40uV to -50uVMax was +27uV and Min -45uVLog is about 15 minutes.Same graph, but I've formatted the Y axis to show 6 digits.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2020, 09:23:41 pm
content: I had a look at the service manual and it looks like the chopper for the input amplifier is running at 50 Hz. So there is a chance that the source of the 50 Hz signal seen is the chopper amplifier and not mains. With the crystal clock installed there may be a chance to check this: both signals are a nominal 50 Hz, but not locked. So one could trigger the scope from one of the sources (mains or the crystal derived 50 Hz in the PLL part) and watch the 50 Hz hum seen (e.g. at the input of amplifier output). One should be able to see the difference in phase if the source a different.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 11, 2020, 07:50:07 am
content: Quote from: Dek on May 03, 2020, 11:39:11 am50Hz.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 11, 2020, 08:23:47 am
content: And here's one hour of log with shorted inputs on the 10V range.This makes no sense to me. It would appear to indicate that my last digit has about +/- 10 variation at 50 Hz, regardless of scale length.The positive side of recapping and cleaning my meter is that the drift it exhibited previously seems to be gone. I'm also getting much better results reading a 100MΩ resistor.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2020, 09:00:09 am
content: In burst mode the chopper uses 1.5 kHz, so the 50 Hz can not come from the chopper. This may explain why more (some 5 mV versus some 0.2 mV) of the 50 Hz is seen, when in the slower modes.Grizewald's meter seem to pic up less 50 Hz, but there still is some amplitude visible in burst mode.For checking chopper artifacts acting back from the chopper to the input, fast mode with 1.5 kHz chopping may be better choice. So no more difficult separation of 50.x Hz mains and 50.y Hz from the clock. Chances are one would see (with the scope) some of the 1.5 kHz chopper frequency at the input and output of the amplifier. Some amplitude at the output is normal (depends on the exact amplifier design), but there should be very little at the input. The higher 50 Hz amplitude seen at 50 Hz may be a hint. I would consider the 5 mV amplitude rather high for the input side.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 11, 2020, 01:50:08 pm
content: Just to make thinking a bit easier, I fixed the scan of the input amplifier.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 11, 2020, 03:56:59 pm
content: Dek, when you took your burst mode measurement, were you using your crystal oscillator or the original PLL oscillator board?The maintenance manual talks about the input amplifier switching needing to provide an equal number of readings in both positions to effectively remove any drift. The manuals hint at the crystal option having effects on the readings and refer back to chapter 1 of the user manual where they fail to describe these effects (just like they fail to describe the measurement cables).I'm puzzling about what could introduce these 50Hz disturbances into the readings. The opto-isolators which are used extensively in the circuits are known to cause problems on Datron meters particularly when they start to age. Your meter is older than mine and we're both seeing very similar problems, but at different orders of magnitude. Just wondering if that might be a clue.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2020, 05:27:32 pm
content: The ADC and chopper use the same clock source, so there should be the same number of switching events, even if the frequency is far off.The opto-couplers could in deed be a problem. One point that can change with ageing could be the delay times, causing some deviation from 50:50 ratio for the chopper. So a look at TP401, TP402 could help. Pin 1 of IC403 would be a similar signal, just less sensitive to loading. This should give a stable 50 / 1500 Hz signal with close to exact 50% on/off ratio.Another possible problem could be a shift in the offset of TR412. This would cause more AC voltage (50 / 1500 Hz) at the amplifiers output. If needed the JFET offset could be adjusted with R419/R420.I would not expect something really broken in the main amplifier as the chopper amplifier at least to a large part still works.Ripple of the +-36 V supply could also be a factor. The capacitor C412 (33 µF parallel to 5.6 V zener for current sources) could also cause trouble if the ESR goes to high.

author: Dek
author_rank: Regular Contributor
timestamp: May 11, 2020, 05:58:59 pm
content: Hi Grizewald,Yes the burst was with the crystal oscillator fitted.I also refereed back to chapter 1 to read up on the affects, but was met by tumble weed ;-)The 50hz / 1.5khz switching looks OK, the square waves at TP401 402 look Ok too.Checking the voltage at the +ve end of C412 I see a lot of switching noise (might just be my probe setup??)star cove as the ground reference (0Vp)What is strange is the pks of the switching noise appear to be sinusoidal.Can't see any noise that aligns with the 50hz or 1.5kh A B squarewaves.Screen shot below.Using my multimeter I get 5.8V across the cap / zennerI get the same level of noise when looking at the test link point TL101 between the ip amp and integrator.

author: Dek
author_rank: Regular Contributor
timestamp: May 11, 2020, 06:16:04 pm
content: <edit>Probe x was set wrong on my scope.Noise on C412 is shown below, also the chopper signals which look OK.I did pull out one end of C412 to check it albeit with my cheapo transistor cap ESR tester. It looked OKHad a delivery today of a new 33uF tant I can compare and if necessary swap.I'll look at the supply either side of R418Checking the + and - 36v psu rails, both look like this.Not so much ripple but switching noise?Probe ground connected to TP205 0v.About 30mV noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2020, 06:51:36 pm
content: Those noise spikes look odd, but somewhat expected for a circuit with lots of TTL type logic running relatively fast.It is odd the spikes look so random, not really repeatable.The test shown probably does not check R412, but more like the -36 V supply. Chances are both sides of C412 move the same.Normally C400 and R418 should keep the spikes out.Even for just the supply (other than maybe digital 5 V) there should no be such spikes. Also the amplifier output / bootstrap signal should not show this. The not so regular pattern for the spikes could point to the comparators / glug switching as a possible source.

author: Dek
author_rank: Regular Contributor
timestamp: May 11, 2020, 08:26:10 pm
content: Something very strange going on.Touched the earth stud on the back of the meter with the scope probe - same sort of spiky noise.Turned off the meter, same noise on the earth stud.Touched the PC case, same noise.Turned off the PC sameetcI'll take the meter and scope into work tomorrow and see what happens there on a different supply.Maybe flailing about in the dark but I'm now thinking possibly mains borne interference from powerline adaptors

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2020, 05:43:15 am
content: External noise / EMI could be a problem. There are plenty of switched mode supplies around, that could cause trouble. With much EMI around it can be difficult to measure with the scope. A good DMM would ideally have sufficient filtering.

author: Dek
author_rank: Regular Contributor
timestamp: May 12, 2020, 04:35:49 pm
content: Tested the 7061 at work and it exhibits the same drift and reading jumping as it does at home.So that rules out mains interference.Maybe now a good time to take out the analogue board and recap any that I didn't replace previously, which will be everything that is not part of the PSU.Dek

author: Dek
author_rank: Regular Contributor
timestamp: May 13, 2020, 06:39:02 am
content: Changed the 5 remaining Tants on the analogue board, gave it a good clean, appears to be no change :-(Need to do a log and graph, but certainly the second from last digit still jumping."Jumping" is not quite the correct word as the error appears to be cyclic over a period of approx 5 to 10 seconds with a noticeable big jump occasionally.More probing and head scratching required.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 13, 2020, 09:24:17 am
content: That's exactly the same pattern as I see, just an order of magnitude less.

author: Dek
author_rank: Regular Contributor
timestamp: May 16, 2020, 04:21:32 pm
content: Quote from: Dek on May 10, 2020, 03:19:31 pmThinking back to this where removing the test link eliminated the noise and fluctuation, gave me an idea to help with escaping from this rabbit hole :-)To prove that the AD converter is working properly and the glugs are stable, I can inject say 1vdc onto the downstream test link pin.This will feed 1vdc directly into the intergrator. If I get a stable reading then the problem is definitely with the IP ranging or IP amplifier, or opto isolators. The absolute voltage does not matter as I'm looking for just a stable reading.At least it will reduce the search area.Grize' and Klein' what do you think?Dek.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2020, 04:58:45 pm
content: Feeding a signal directly to the ADC (e.g. bypassing the input amplifier) is possible. The obvious choice for a first test would be 0 V.

author: grizewald
author_rank: Frequent Contributor
timestamp: May 17, 2020, 09:47:24 pm
content: Quote from: Dek on May 16, 2020, 04:21:32 pmIt has to be worth a try!