Fluke 732A with popcorn noise SZA? - Page 1
Forum section: electronics / general

author: TiN
author_rank: Super Contributor
timestamp: September 18, 2018, 10:33:03 am
content: I know, we already have many 732 threads. Yet, I don't remember anyone talking about popcorn noise issue in them as of today.Well, there is place for everything, so couldn't resist recent lures, and bought one of those 699$ sick 732A's.Box arrived yesterday, and quick check revealed poor darlington on AC mains board cooking to +130 °C.Quick swap with TIP122 (original is lower voltage TIP120) fixed that issue, so I hooked the output to some random meters:After a day of runtime, I observe "popcorn spikes" on the 10V output , and buried in the noise 1.018V output.Yes, I am sure it is not the 3458A (meter used for 10V 732A log.Jumps are random, at -0.4 ppm level.Guess I am looking at replacing SZA263 reference chip with a replacement? Anybody else here have this failure mode in their 732 unit, to share the experience?Reminds myjumpy LTZ1000A chip, which I still have somewhere.I have 8842A for donor parts, so probably least resistance path is to steal SZA chip from it and put into this 732A. Of course I'll have to test SZA in 8842A first, to make sure that one is quiet.Also will replace all electrolytic capacitors in 732A, but I doubt those can cause the noise like this.

author: Andreas
author_rank: Super Contributor
timestamp: September 18, 2018, 04:36:54 pm
content: did you check the zener itself?might be a mis-handled output stage or capacitor.with best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2018, 08:06:12 pm
content: The noise does not look 100% like the popcorn noise seen with the jumpy LTZ. This one is more like random drops - the more usual type of popcorn noise is with same size steps. It is still possible to get peak like behavior if the lower state is only rarely populated and short lived.

author: martinr33
author_rank: Frequent Contributor
timestamp: September 19, 2018, 04:17:07 am
content: 732as can pick up noise from transformers through induction, switch mode power supplies - and UPS systems. I'd get it away from your meters, and maybe even try to run it off of a battery, or a clean 30V supply.I often wonder if Fluke used the SZA in so many meters because they needed a few really good ones for the 732a, so I wouldn't replace it just yet (they may also have gained some economies of scale from doing that, so it is just a hypothesis).

author: TiN
author_rank: Super Contributor
timestamp: September 19, 2018, 04:24:06 am
content: Sure, more testing is required before doing any ref lobotomy. I also have spare 5700A refdac , but rather not take that one apart just yet.Graph chart have averaging math applied, so what matters are raw sample (circles in blue).

author: Pipelie
author_rank: Regular Contributor
timestamp: September 19, 2018, 05:20:03 am
content: the curve doesn't look likes popcorn at all.it probably has a fault in power supply.and due to the averaging math applied on Keithley 2002, I can't find those pulse on curve of 1.018V.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 19, 2018, 08:36:38 am
content: Dont sacrifice the 5700 RefDAC, it hasnt done anything wrong.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2018, 02:50:23 pm
content: Checking the supply (e.g. measure in parallel to output) could be a good idea. The faulty TIP120 might not have been the only problem there.

author: floobydust
author_rank: Super Contributor
timestamp: September 19, 2018, 06:59:50 pm
content: Quote from: Kleinstein on September 19, 2018, 02:50:23 pm+1I could not find a TIP120 Darlington on the 732a schematic, andnever met a transistor that runs hot because it is defective. It's usually an overload causing it to heat up. A shorted cell roasts many gel-cell chargers.My second guess is RF interference from cell phones etc. as the 732a does not incorporate any ferrite beads or common-mode chokes that I can see. It's from an era when the airwaves were quieter. I would try some clamp-on ferrites to the outside test leads and see if the (~4uV) jumps clear up.edit: it's Q1 on the A3 pre-regulator board.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: September 19, 2018, 07:24:25 pm
content: The TIP120 is listed as Q1 on the A3 pcb in the newer 732A/AN manual.When he gets the parts to change on the power supplies and new batteries, the problem just might clear itself. The 18.6v supply to the reference would also be a good place to monitor without going too deep.

author: dl1640
author_rank: Regular Contributor
timestamp: September 24, 2018, 11:56:04 pm
content: A local second-hand offered me about 1300USD for one 732A without batt. I am struggling.通过我的 PRA-AL00 上的 Tapatalk发言

author: TiN
author_rank: Super Contributor
timestamp: January 01, 2019, 03:21:39 pm
content: I decided that January 1, 2019 is a good day to get back on this project. After bit of struggle I got into oven box and took the reference board out to the day light.Nasty corrosion on the tracks and even component legs was immediately detected.Maybe it cause excessive leakage and unwanted resistance paths? Maybe not... But this is not the way precision circuit should look like, so off we go, to remove all parts, inspect them, clean them and refub the PCBA.Mark all the connections and remove the wiring harness from the board. Done..Now we can go ahead with removing each resistor, carefully marking where it was originally located. One could argue that soldering stress cause resistor to drift/shift...You right, but because I got this unit with zero history, and it must be stabilized afterwards for weeks/months anyway, I don't see much harm already.We can however see lot of area to clean up, and it would be nearly impossible with all those tall Fluke's wirewound sticking around.To be continued....

author: chuckb
author_rank: Frequent Contributor
timestamp: January 01, 2019, 04:13:08 pm
content: TiNYour bravery really has no bounds!Have a great new year.

author: branadic
author_rank: Super Contributor
timestamp: January 01, 2019, 06:09:35 pm
content: Good luck on that road and happy new year.-branadic-

author: alm
author_rank: Super Contributor
timestamp: January 01, 2019, 08:38:20 pm
content: Let's hope that the soldering heat does not affect the annealing of (manganin?) wire on those mica card resistors, and affect the temperature compensation. Although I would expect the resistors that are critical for stability to be the hermetically sealed ones.

author: chuckb
author_rank: Frequent Contributor
timestamp: January 02, 2019, 10:31:54 pm
content: FYII will be trying some low temperature solder to help minimize thermal shock on sensitive components. Mouser has -https://www.mouser.com/ProductDetail/910-SMDIN52SN48Which melts at 118 deg C verses 182 deg C (?) for normal 63/37 solder. This low melting temperature along with a normal heat sink clip should minimized thermal shock to resistors and zener diodes.I don't know the thermal EMF generated by this solder but it can't be worse than the 34uV / deg C for a normal Kovar / copper connection.I will also use a dedicated soldering iron tip for this solder to minimize contamination with normal solder.

author: lowimpedance
author_rank: Super Contributor
timestamp: January 03, 2019, 02:08:18 am
content: Happy New Year indeed !.Just a thought, since you have infinite patience for restoring gear that most would have junked after one look at it, how about re creating the PCB (I assume its bog standard FR4 by the look) and transfer the now removed precious parts to the new PCB !?! .As I said just a thought.

author: TiN
author_rank: Super Contributor
timestamp: January 03, 2019, 04:15:33 am
content: Quote from: lowimpedance on January 03, 2019, 02:08:18 amThat idea crossed my mind, but decided not to do it as LTFLU/SZAs are not trivial to source and trim procedure for them is tedious. Making board for just one unit repair rather pointless, as there is long list of more interesting projects already. So for this 732A I'm going for least resistance path to get it going and be done with it.Otherwise we will end up with putting LTZ1000 chipsy inside , you know how that goes...I have better reference box to work with than this old 732A, and need attention there before the exciting calibration this spring..

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2019, 08:10:43 am
content: It looks like some of the flat wire would resistors are not actually used, but just there to be eventually used if jumpers for trimming are set different.The corrosion does not look that bad to do real damage to the traces. So a good cleaning should be enough to restore it.Any idea what caused that type of damage ?Kind of looks a little like a leaking cap, but I don't see a candidate for this.The corrosion looks bad, but it is still possible the popcorn noise is still from the reference chip. So one may want to do a check before full assembly back to the oven.

author: TiN
author_rank: Super Contributor
timestamp: January 28, 2019, 05:28:16 pm
content: Alright, slowly but surely, we have some progress:Condition before:Board stripped of precision wirewounds and ready for cleaning (SZA and film caps also removed temporarily in process)Condition now:I messed up few parts on PCB, but should be alright still. Also replaced plastic LM358 with hermetic fancy-pancy one.As by product of resistors removal I did actually run temperature coefficient plots on most of them. I can tell so far, that these "gold sealed" Fluke resistors are one of the best wirewounds I ever tested, typical WW's hysteresis is suprisingly very small.Plan to hook up harness and put it all together into oven for a smoke test tomorrow.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 28, 2019, 10:52:49 pm
content: Looks better.I was hoping to see a new xDevs red 732Z pcb.My last 732A purchased at the same time as yours is a high plains drifter. I think it drifted down 60+ ppm before I unplugged it. Maybe mine had the same corrosive damage?Have you considered 1v and 100mV outputs like the 732C?

author: Inverted18650
author_rank: Frequent Contributor
timestamp: January 29, 2019, 05:19:55 am
content: thumbs up brotha, hope it functions better and settles in.

author: TiN
author_rank: Super Contributor
timestamp: January 30, 2019, 01:02:28 am
content: My 732A is not a drifter (afaik), but a jumper.Well, and all that teardown and cleaning was all for nothing.Same ~0.5ppm jumps as before. To prove that it's not fluke in the measuring 3458, second meter connected to 1.018V output shows correlated jumps on it's data too.Data off the chart in first hour or so is just warming up. Oven thermistor measurement provided by third meter (OHM 10K range, dark-red line).Now I swapped original SZA263 reference chip to SZA263Afrom Fluke 8842A donor board. Yes, I know the resistors are not matching this other chip, but I don't care about tempco much at this point, just want to see if jumps are indeed sourced from refamp or not.

author: TiN
author_rank: Super Contributor
timestamp: January 30, 2019, 04:43:01 am
content: Initial data from 8842A SZA263A chip looking promising.Obviously tempco/stability is thru the roof, as resistors on 732A board are not matching the refamp? It's going down at rate of about -2 ppm/hour.But no spikes yet!Setup rig capture from livecam. Excuse the poor IQ.Of course there could be big possibility that SZA263A sorting for 8842A is way less strict than sorting/conditioning process for 732A-grade SZA, so I could end up trimming resistors till the cows come home to no avail.Which make me thinking to ditch all this SZA/LTFLU non-sense and make quick and dirty PCB around good proven LTZ1000A ref design.Just reuse external 732A oven regulation circuit and output stage resistors to form 10V/1.018V/1.0V.I know at least one more person that might be remotely insterested in such a zombie board?What say you? Yay for LTZ-based 732Az or leave the thing rest in peace?

author: Inverted18650
author_rank: Frequent Contributor
timestamp: January 30, 2019, 04:59:23 am
content: Stream it, Ill watch.here now:https://xdevs.com/live/

author: Echo88
author_rank: Frequent Contributor
timestamp: January 30, 2019, 06:01:12 am
content: While a TC-adjusted and stable SZA might have a different drift direction compared to the LTZ and therefore offers value as a reference compared to LTZs which might drift in the same direction, im always for more interesting LTZ-boards.

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 30, 2019, 06:50:11 am
content: A new board makes no sense to me... It's clear, that the LTZ topology does not match the SZA one.So you would only copy the existing SZA 263 schematic into a new layout, maybe a more compact one, but the two core problems will remain.I would at first determine the correct value for R5 to have near zero T.C. for the SZA chip @ 45°C oven temperature.The 2nd problem is the slightly incorrect divider ratio for 10.000 00 V, but I wouldn't replace R27 / R28, because the low T.C. of the output is the most interesting feature. Maybe the coarse trimming by R10 / R11 is sufficiently small, so not to affect the T.C. matching.Probably the timely drift of the SZA from the 8842A is not bad at all.. you'll have to see before you scrap the whole thing, like you determine stinkers in your LTZ group.PS: Maybe that initial drift of -2ppm/hr is also caused by hysteresis effects due to soldering heat, like on LTZ1000 chips.Frank

author: TiN
author_rank: Super Contributor
timestamp: January 30, 2019, 07:32:23 am
content: How is it clear that LTZ no match to SZA chip? I must have missed that part as in my books both are in same performance level when *equally* biased.Another available option is to scrap my second bad 5700A REFDAC A11 board to donor dual SZA reference hybrid off it and make board around it. Since we have already external oven, maybe inner hybrid oven can be turned off (omitting lot of parts to drive it) or at least ran at wee higher setpoint, like 55C (if my math right, this 732A have oven about 49-50c). But this path will need to redo output network if we'd want to use 13V output to get lower noise.Does not look like it's slowing down...

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 30, 2019, 11:10:52 am
content: Quote from: TiN on January 30, 2019, 07:32:23 amThe SZA / LTFLU chip and circuitry have a completely different topology than the LTZ1000, due to the fact that in the first case, the anode of the zener is connected to the emitter of the compensating transistor, and in the latter case it's connected to the base.So there is no useful way to connect an SZA chip into a LTZ circuit.Also, the SZA circuit outputs an amplified and buffered voltage (e.g. 10.000V), whereas the LTZ circuit outputs the buffered reference voltage (e.g. 7.2000V) only, and requires an additional amplifier to get e.g. 10.000VQuote from: TiN on January 30, 2019, 07:32:23 amThe 732A has an oven temperature of (48 +/- 2) °C, ( see chapter 3-22), whereas the 732B is set to 45°C (see chapter 4-11).Sorry, that's been my fault to confuse that.Why mixing two different oven assemblies? Can't you use the 5700 reference separately.. it's also not intended to be as stable as a 732A/B, due to its higher oven temperature, i.e. typ. 2x higher drift rate.I really would try to fix that 732A in its original shape.Frank

author: TiN
author_rank: Super Contributor
timestamp: January 30, 2019, 12:08:19 pm
content: QuoteSorry, maybe my posts were misleading. I did not mean to use LTZ in original 732A circuit.Of course whole refamp circuit would be replaced with complete LTZ block to generate 7.xV and amplify that for 10V output, all on fully custom PCBA.Essentially the only part that would be original in such a box is 732A oven box with heaters, external battery/charger management stuff and 10V->1.018V->1V divider circuit.So whole path would be LTZ ref 7.2V -> 10V gain block -> 732A divider -> output terminals.5700A ref is ovenized at about +62°C, but it's not hard to reduce oven temperature for lower setpoint. And stability of 5700A REF (6.7 ppm for 99% spec, annual) is very competitive to 732A (6.0 ppm annual). Of course actual instruments show better performance than spec.QuoteSmaller hybrid oven would allow tighter temperature control, while large original 732A oven would keep all gain circuitry around nice and stable. Essentially like double oven concept in OCXO's.Onboard oven on hybrid already there and cannot be removed from the module, so we might as well use it for benefit.QuoteYea, I'd love that too, but those spikes from original SZA chip bother me too much.

author: TiN
author_rank: Super Contributor
timestamp: January 31, 2019, 04:07:32 am
content: Okay, it lookssettled at about -18ppmfinally.Also running on battery power now, to see how much life does it have.

author: martinr33
author_rank: Frequent Contributor
timestamp: January 31, 2019, 04:58:45 am
content: These things can be microovolt stable over a couple of years. I have two of them facing off over a nanovoltmeter in my uncontrolled garage. Over 10 - 30C and six months, the delta is always less than a microvolt. Checked it just now and it is reading 24.9uV, was 25uV when it was checked against other units in the summer. Local friends with 732as show similar results. I know of about 9 units in the Bay Area (anybody else around here got one)?So I would definitely let it sit a little longer, and see how it starts behaving.Over the same period, it looks like my 3458a walked about 0.7uV. It had been off a while, and it took about a week to get the readings to where they are now.Anyhow, if this unit is stabilizing it is a keeper. Maybe your diligence in cleaning out the gunk was the trick.Also, for lab units - the ones I found always have signs of serious external abuse. I like to pick units by the labels on them. Mine came from:- A Fluke cal lab (from when they were in multiple locations) (labeled as primary standard)- The Australian Air Force (labeled as "calibration not required")- Sandia National Labs (labeled as primary standard)I don't feel so good about units with all the labels cleaned off.

author: TiN
author_rank: Super Contributor
timestamp: January 31, 2019, 10:16:59 am
content: Lucky for me, I know what thesecan be, having access data over a year with bank of 732s. Question is however will this particular 732A specimen will ever get that good.Perhaps I should assemble little breadboard circuit with original SZA chip and run some tests standalone in controlled TEC oven box with teckit.