The Datron 1062 Wobbles - Page 1
Forum section: electronics / repair

author: dml
author_rank: Regular Contributor
timestamp: October 02, 2018, 03:13:50 pm
content: Another installment in the Datron DMM weird issues series.Happy with having two of these 1062 units seemingly repaired - revived to the point where they power up without errors and give fair readings in all modes - something was still not right with either of them. The readings wobble at almost-regular intervals, in a roughly recurring pattern. This happens even with the filter on.Neither unit has been calibrated and I expect readings to be off, but wobbling/jumping of multiple lower digits isn't on my list of expected behaviour for a precision DMM. This is my first experience using these things so I could not make direct comparisons except between the two units I had in front of me - and they both do the same thing.I confirmed the problem using a precision 10k resistor in ohms mode and a precision DC source configured to 50.000mV. Both cases wobble by about the same relative magnitude. This suggested an issue somewhere on the analog board or analog-digital interface, which is known to contain ...interesting design choices.So I set about testing the opto-isolators used in the digital-analog pathway as these have been reported as a recurring issue. Some trickery has been used to perform logic using opto-isolators with known CTR differences to produce something like gate delays for correct combining of signals. Other threads (and pages) cover this in more detail. The optos are known to degrade, changing their timings and messing up signals.I tried to characterize the ones most involved in this process (M1 .. M4,M5,M6) and came to a few conclusions. Those marked white are 'fast', or high CTR. Those marked red are slow, or low CTR. In at least one case resistor values are changed to set timings outwith the CTR alone. Fun stuff. Basically if you plug in a new device without measuring it first, you might just make things worse.All the new devices I had ordered were just slightly better than 'slow' - ok for slots marked red, but not suited for use in the slots marked white. Fortunately the originals were still good enough and all produced similar measurements.While debugging, I tried to use the original optos, arranged as they were when the unit arrived. Two of the original optos were faulty/dead so I used the replacements for those slots - the rest remained original.After mucking about endlessly with different patterns of optos, I concluded that:its easy to mess things up by using the parts with the wrong spec. typically the unit will report bad readings or just show an error.they were not involved in the problem I was debugging, having no effect on the jumping valuesthe optos were good enough to produce the correct signals for the null-detection events described in the service manual and nothing I did with the optos improved on that situation. it either continued to do the exactly the same, or didn't work properly at all.So I decided to give up on that exercise and leave the optos alone. I'll measure them more properly another day but for the unit under test at least, they're ok.I then hooked up my scope to monitor the A2D integration circuit......to view the waveforms described in the service manaul:The scope showed the expected signal when a voltage is available at the input:However the waveform was not stable! The spacing between events in the integration cycle was varying erratically.I indicated this variation in red on the diagram above.Here are two images superimposed, capturing the integrator exposed to a constant, precision input voltage:Anyway the erratic movement of the integration period and the jumping of the lower digits on the display readout seemed to be related, both apparently cyclic and similar period range. It's also pretty certain that a change in integration period is going to change the measurement.Something funny going on with the timings driving all this stuff.I checked voltages around the board looking for ripple and other effects. Nothing apparent.I did the usual hand-wavey routine of replacing caps near sensitive areas and playing with decoupling capacitor sizes etc. Tried different caps at the 40v DC convertor etc. etc.. None of this really made a dent in the problem. So having got bored with that I went back to debugging it properly.Using the service manual & schematic, I followed various signals back from the integration circuit to the Ferranti chip that handles the A2D logic and found the wobble present on most of those signals. This was interesting. I followed this as far back as the digital board.At this point I started looking at the 6800 CPU clock and found that to be erratic as well...The manual has a very small paragraph on 'line locking' which describes a the rejection of mains 50/60hz from measurements by phase locking with the mains transformer. That got my attention.The schematic for the line locking circuit shows that it is primarily a digital circuit, with a small analog portion which tweaks the master clock. Sticking probes on various parts of this circuit presents a confusing picture of clocks masking other clocks, counters, dividers and so on. Not very straightforward.Without bothering to figure out the details of how that mess compares the line and reference clocks, it is already clear the analog part takes output from that comparison and uses it to drive a kind of variable capacitor, implemented with varicap diodes and some other sensitive stuff, which in turn sets the frequency offset for the clock, which in turn drives the 6800, the Ferranti thing, the A2D period and everything downstream from there.To confirm I was in the right area, I stuck a capacitor across the 10M resistor across the varicap diode. This effectively removes the variable cap and stops the line locking.The master clock stops wobbling. The integrator stops wobbling.At this point I notice the adjustable/trimmer coil on the board, and in the line locking schematic. I usually never touch those without a really good reason, and this seemed like a really good reason. I turned this perhaps 120' clockwise and watch the 6800 clock change from jumpy through a slow ramp, to a dead stop. A bit more adjustment finds the ramp will reverse direction. So I find the midpoint between these two extremes and put the lid + shield back on the DMM.A few measurements later, the jumpy digits have gone. Readings are rock solid in all modes, all ranges.I won't be surprised to find this is all documented properly in the appropriate part of the calibration guide. Which I haven't even started reading properly. But I guess solving it that way would have been less fun...

author: capt bullshot
author_rank: Super Contributor
timestamp: October 02, 2018, 04:20:36 pm
content: Interesting and well done. These things are full of surprises.

author: Gyro
author_rank: Super Contributor
timestamp: October 02, 2018, 04:24:33 pm
content: Line locking was first introduced on the 1051. It looks as if the 1062 circuit is more stable - the 1051 pll takesagesto adjust and and lock.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 02, 2018, 04:28:07 pm
content: BTW, to all the Datron fanboys:I've got a 1061-S86, does anyone know what option S86 is? I can't find anything, neither in the manuals nor in the almighty internet.

author: dml
author_rank: Regular Contributor
timestamp: October 03, 2018, 08:20:51 am
content: Quote from: capt bullshot on October 02, 2018, 04:20:36 pmYup they sure are. On my travels with the probes I found a few interesting signals I'll probably need to visit again. I also need to figure out a way to test some of the other options.I recently scraped up a broken 1071 as well. I believe you have one of those? I'll share a backup image of the ROMs from that soon with the date codes.The 1071 is mostly the same inside, for the main boards. However the line-locking circuit is completely absent from the digital board, with a big crystal can in its place - nothing to adjust. On closer inspection there are some differences in components used and test point locations but the schematics are very close. There are more options fitted including the ratio and current options.The 1071 board seems to be much older than the other two - marked '84 versus 86/87 and this might be why the components are different in places.

author: dml
author_rank: Regular Contributor
timestamp: October 03, 2018, 08:22:16 am
content: Quote from: Gyro on October 02, 2018, 04:24:33 pmIt did seem fairly easy to correct so they probably redesigned the circuit. I'm wondering if mine were both off because they were original set for a 60Hz line.

author: dml
author_rank: Regular Contributor
timestamp: October 03, 2018, 08:24:18 am
content: Quote from: capt bullshot on October 02, 2018, 04:28:07 pmI wasn't able to find anything either. It's not on the list of options from any of the units I have.It'll probably turn out to be a socket/interfacing difference or maybe a specific extra in the firmware. Strange that it's not documented.Sharing photos of the front/back boards and the back board may help figure out what it is.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2018, 09:27:00 am
content: Quote from: dml on October 03, 2018, 08:22:16 amThere is a chance that 50/60 Hz could make a difference. The clock circuit looks like a rather odd semi-digital PLL and no obvious 50/60 Hz switching, though there could be somewhere in the software / programmable logic part. That PLL part still looks strange, as the adjustment steps seem to be rather coarse. From today's perspective the whole ADC circuit looks quite complicated.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 03, 2018, 05:56:52 pm
content: Quote from: dml on October 03, 2018, 08:24:18 amSo that's the photos. Pretty obvious the Model number 1061-S86, and a bit more subtle the "ACF" right below the AC and DC keys. The pull tab (Operation) is in French. Otherwise identical to the 1061A, nothing special on the rear side in comparison with the 1061A. These are the only Datrons I have, no 1071.What's different: pressing both the AC and DC keys doesn't switch to AC+DC RMS measurement, the instrument stays AC coupled. I couldn't make out a significant difference between AC and ACF mode at various voltage / frequency combinations. So the "ACF" mode might be the S86 option, but I'd like to know the intention of this option.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 03, 2018, 06:15:37 pm
content: More pictures of the 1061-S86AC converter and currentGPIB / IEEE-488

author: capt bullshot
author_rank: Super Contributor
timestamp: October 03, 2018, 06:16:25 pm
content: More pictures of the 1061-S86Analog boardDigital board

author: dml
author_rank: Regular Contributor
timestamp: October 04, 2018, 09:15:50 am
content: Thanks, I'll compare with my own units to see if anything stands out - looks identical so far though. I assume the output board etc. looks the same? It's not visible in the pics.Nope I don't see any reference to 'ACF' or 'S68' in the docs. No idea what that is. Unless it's meant to be an optional AC line filter? Your board does have the line locking circuit.  on second thoughts, that doesn't make sense as it would interfere with ac/dc mode choice.While looking at this, my 1071 was sitting turned on and I heard a relay click, then 'Error OL'.And then smoke and smells. Tantalum fires!I can see which one roasted - hopefully its not more extensive than that because it was sitting in DC mode and the failure is on the AC board. Probably took one of the rails down. That's what I get for playing with it before checking the caps properly.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 04, 2018, 09:26:27 am
content: Quote from: dml on October 04, 2018, 09:15:50 amFrom my experience, that stuff is robust enough to just work again if you replace the burned tantalum only.

author: dml
author_rank: Regular Contributor
timestamp: October 04, 2018, 10:37:48 am
content: Quote from: capt bullshot on October 04, 2018, 09:26:27 amYep - removing the AC board allowed the unit to start ok. Replaced the cap and the AC board is back. Looking at replacing the others and will measure each one in the process.

author: dml
author_rank: Regular Contributor
timestamp: October 08, 2018, 09:01:05 am
content: Quote from: capt bullshot on October 03, 2018, 05:56:52 pmI checked the photos a bit more closely - there are no differences in the board layouts or component assignments. Just different components in a few places. Nothing interesting.Your unit is using normal-looking 4076 registers where others are using some sort of custom/hybrid version in the same slots. While these look a bit strange they aren't going to affect operation. Other differences just look like component choices changing over the years.You have a rear input socket but no ratio socket. My 1062s have neither (option LS101). My 1071 has both rear+ratio (option 40).Looking at photos of other 1061 units I see option 40 is common. You appear to have option 41 and it is listed on the back, but separately from S61. That might be a nonstandard config.I don't see any other differences beyond the 'ACF' thing on the front - and didn't find any reference to this in the docs I've seen. So good luck solving that one!It might be a good idea to upload a copy of your ROMs in case the option is hiding in there. May be a chance to preserve it and for someone else to figure out what it is. Also the '88 date suggests its a later revision than the ones already saved on KO4BB. Regarding the ROMs I should add that all 4 need to match, including the one on the IEEE board. If the unit is booted with different versions on the two boards, it freezes with 0.00V reading and 'rem' lit on the front panel.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 08, 2018, 10:03:19 am
content: Thanks for your investigation.I have a broken ALL03 programmer, so either I'll have to repair it or get a new one (TL866 or similar) to read out the firmware.

author: dml
author_rank: Regular Contributor
timestamp: October 18, 2018, 09:42:18 pm
content: Brief update on this line locking thing.I got around to checking the procedure in the service manual. It is covered, but different from what I tried to do. It seems I got close though. The coil needs trimmed until 1.05v measured on one of the test points and that's all. I think I had set 1.1v or so. I didn't find tweaking it made any difference to readings but decided to follow the wisdom as written. Still I'm happy to have found out what that circuit was doing and what effect it had on results.I also mentioned previously the 1071 has no line locking circuit at all, so nothing to adjust when using it on a 50hz line!Well not quite. According to the service manual I need to replace a bunch of components including the crystal. o__O and its not an easy frequency to find. Fortunately it can be generated a with a divider. This is something I'll have to come back to when I find more time.I should add that the two 1062 units now agree closely on measurements, after the line locking adjustments and some component replacements. The 1071 is off though. I'm hoping the 50/60hz thing is responsible for much of that.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2018, 08:33:48 am
content: The adjustment for the line locking PLL is likely effecting the locking range. So once good enough to get a reliable good frequency lock, the exact setting should not have an effect. The 1.05 V might give slightly more headroom for possible drift or grid frequency changes in the future.Having the wrong crystal / or more like different line frequency should not result in a different reading, but mainly in much less power line frequency suppression. So 50 / 100 Hz residuals will cause a little more scattering in the readings. The effect would be most prominent at 1 PLC and least (essentially not) visible at something like 6 PLC or 60 PLC - so the meters set for 50 Hz line frequency have a slight advantage: the common 10 PLC mode provides simultaneous 60 Hz line suppression.There is a possible small (a few ppm) change in the calibration when changing the crystal, but this is more like a side effect and not related to the line frequency. So it would also apply if the meter would than be used with 60 Hz again.