Keithley 2002 repair help

[nikonoid]

Hello guys,

I am new to this forum. I had been reading for a while, but this is my first post.

I have a Keithley 2002 that I am trying to repair. It is quite old, chips are from 1998 the serial starts with 072XXXX. The board looks clean and capacitors seem to be in a fine shape.

The major issue with the unit was the Front/Back switch. For now I hardwired the switch and was able to get the unit to pass self test. Can someone please recommend a source for replacement switch. I believe it is SW-468, but I cannot find a source for it. Would there be any special considerations for replacing it?

Also the unit came to me without a fan. I am not sure what to make of it. It had the opening in the black protective cover and plastic air guide, like in the newer 2007 unit TiN has. For now without a fan I am limiting the use of meter to 10 minutes at a time.  Can someone recommend a replacement fan and what sort of connector I would need to get to install it?

Thank you very much. If I can somehow contribute to the forum, please let me know.

[TheSteve]

Welcome to the forum! The 2002 is a great machine - certainly worth the effort to get it operating 100% again. I think your first call should be to Tektronix/Keithley and ask to speak to the parts department. You should be able to purchase the parts you need direct from the source.

[TiN]

Welcome to forums. K2002 is my second favourite meter  .

Ask Keithley/Tektronix, they might sell you switch for replacement. It's not too expensive considering the cost of the meter.
You can see my articles about 2001/2002 (here and here and here) but from my memory fan is 40x40mm 12V 0.1A. For my meters I've used this one from Digikey. Bit more powerful, as I tend to run meters in warmer ambient.

Also replace all electrolytics on both boards, even if they look good! They are badly thermally stressed in 2001/2002, so don't play gamble here.

If you can take photos of your unit, would be nice to have. What is your firmware version? Would be good to add into collection if it's not the one we already have.

[nikonoid]

Glad to be here.

Thank you for suggestion to call Keithley for spare parts, I did not even think about this option.
The switch was surprisingly less than I expected - $12 and fan surprisingly more - $47. Now I have to wait for both to arrive.

As firmware is concerned, on start is says "Rev. A06   A02 IEEE Adr. = 16". Not quite sure what A02 is referring too.

I will take photos and post in few days. Is it OK to post full resolution photos?

For electrolytics could it be sufficient to test them with LCR miter before replacement? My soldering skills are a bit less than expect, so I would not want to risk replacing something that is not necessary.

I am still going through complete checkout of the meter and just found that resistance reads wrong, but only on a single range (2MOhm) and only in 4 wire mode. Other ranges work fine and in 2 wire mode all ranges read fine. Any suggestions for what to look for? Can I re calibrate this single range?

Thanks.

[TiN]

Go for full resolution. Forum have limitation, so you can upload to my ftp (how-to link in my messages footer), I'll add them nicely to forum for you.
Always a pleasure to see one more 2002 saved from dumpster diving.

Just replace the caps, don't waste time with checking them. If you excercise bit of care, replacement will be just fine. Avoid touching board surface/components with fingers though, handle it by edges only to avoid leakage contamination.

Calibration is bit complex on 2002, so I'd wait till you get switch replacement to see if issue stays. Could be leakage in switch causing problem.

[Samogon]

Replacing caps is mandatory!
And if you hesitate to re solder caps how you are going to replace switch which is much harder to do and requires skills?
I can suggest my help replacing caps and switch, i have proper tools for it (your shipment and components). I have two 2001 on my queue for caps replacement.
K2002 is not device to gain soldering skills. I would start with something less gentle and expensive.

Dont want to be harsh. But i can suggest to buy desoldering gun for it
like https://www.amazon.com/Desoldering-Station-Digital-Temperature-Controlled/dp/B00KQ6PR6K/ref=sr_1_8?ie=UTF8&qid=1494359041&sr=8-8&keywords=desoldering+gun
or more advanced but more expensive
https://www.amazon.com/Hakko-FR300-05-P-Desoldering-Tool/dp/B00KWM69C4/ref=sr_1_4?ie=UTF8&qid=1494359041&sr=8-4&keywords=desoldering+gun
I personally use
https://www.amazon.com/gp/product/B00NS49ZB0/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
but it requires compressor to work. so whole set is bit expansive, but i like it.

[nikonoid]

Thank you, Samogon.

This is a very kind offer and I do appreciate it very much.

I am not a complete novice and have done a fair share of removing large connectors. However I do not solder on a regular basis and I never worked on the instrument this sensitive before, so I may need some guidance.   

I do have a vacuum desoldering station (very old PACE Micro that I bought used and fixed). I am planning to modify one of the tips to work with slightly bigger 40 mills legs on the switch. I also have an ESD mat to work on.

The questions that I would have are:
1) Should I wear gloves? What kind (rubber/cloth)?
2) Is using regular 60/40 solder with flux core good enough, or do I need some kind of low thermal solder for the switch?
3) Is alcohol (or samogon  ) sufficient for cleaning after soldering
4) Are there any special tricks or tips for removing Analog and Digital boards from the case?
5) Should I replace capacitors that are far away from the transformer and voltage regulators?
6) In some videos, including TiN's video of K2002 repairs, I saw new solder being added to the joint before desoldering it. Why is that?

Thanks in advance for answers.

In case someone would find this useful here is a list of every electrolytic capacitor in K2002 with their DigiKey part numbers. They all are Nichicon beside 15000uf that is Illinois Capacitor (Nichicon was not in stock anywhere). Most original capacitors are VX(M) series and these are these UVX series. I am hoping these are essentially the same.
 

Code: [Select]

Index QT Part Number Description                 Cust        Stock Backorder Unit Price USD Ext USD
1 1 UVZ1H102MHD CAP ALUM 1000UF 20% 50V RADIAL C104      1 0 0.94 0.94
2 2 493-1323-ND CAP ALUM 2200UF 20% 35V RADIAL C112,C113 2 0 1.31 2.62
3 1 493-1288-ND CAP ALUM 2200UF 20% 16V RADIAL C114      1 0 0.72 0.72
4 2 493-1358-ND CAP ALUM 470UF 20% 63V RADIAL C116,C117 2 0 0.76 1.52
5 2 493-1322-ND CAP ALUM 1000UF 20% 35V RADIAL C118,C119 2 0 0.7 1.4
6 1 493-1266-ND CAP ALUM 100UF 20% 10V RADIAL C206      1 0 0.28 0.28
7 3 493-15350-ND CAP ALUM 22UF 10% 16V RADIAL C247,C250,C253 3 0 0.38 1.14
8 1 1572-1350-ND CAP ALUM 15000UF 20% 16V AXIAL C611      1 0 5.15 5.15
9 3 493-1287-ND CAP ALUM 1000UF 20% 16V RADIAL C624,C630,C633 3 0 0.52 1.56
10 1 493-14496-ND CAP ALUM 47UF 10% 35V RADIAL C625      1 0 0.45 0.45
11 1 493-1369-ND CAP ALUM 2.2UF 20% 100V RADIAL C912      1 0 0.28 0.28



 

[Samogon]

He he i glad you have got my nickname properly

1 nitrile gloves probably will work keeping body oils off board.
2 60/40 is pretty much standard i use 70/30.
3 i use isopropyl 99% it is much better than 70% (samogon usually 70% but it has surrogates in it and cant be used to clen pcb)
4 most of the removal procedures are stright forward, but take extreme care removing input ferrite filters their plasric eclosures can be easily broken so disconnect cables from input connectors first then gently lift them from the grooves. Also desolder front white wire from analog board.
5 all capacitors have to be replaced, i had error 102.1 and thought it is eeprom, but it was small 47uf cap
6 adding solder really helps to break oxidized surface and adds thermal mass to the tip application point.

Hope it helps

[nikonoid]

Thanks a lot. This will be very useful when parts arrive.

Today I checked date codes on capacitors and they are all originals from 97 and 98. So I will be changing all of them.

[nikonoid]

While I am waiting for parts here are some images.

Digital side:


Appears to be revision 06K:



Firmware was possibly upgraded to A06 by Keithley. See the glue residue from previous sticker. The transformer looks very cruddy.







Analog side. See the plastic air guide, like in the later TiN's unit. The hole in the black shield was cut too.



Revision E of the analog board:


Capacitors to be replaced and again very questionably looking transformer. Should I bee worried?



ADC Board:


Precision resistor networks:



These regulators get really how and their heat sinks look a bit rusty. Is that normal?



More to follow after I receive parts and attempt to fix it.

TiN: Thank you or hosting photos.

[Samogon]

Tranformer is ok. They all look like this, melted, but it is not.
Accorfing to photos, you got nice unit. No acid damage.

[nikonoid]

Thank you for reassurance on the transformer, Samogon.

Today I took the analog board out. It took quite a bit of effort. I think the manual is somewhat incorrect. They are recommending desoldering the white wire when disconnecting the rest of input connectors. In practice it seemed that the best course of action is disconnect the rest of front and back inputs, keep the white wire attached until the Analog Board is out of the case. Only then you have direct access to the back side of the board and can easily desolder it.

The good news is the the faulty inputs switch is off. Once I customized the vacuum suction tip to have wider opening: 0.052", removing the switch was a breeze.

Will be changing capacitors in few days.

Meanwhile I have another question, the board I have seem to have a bit of dust on it, mostly by the fan (including spots under the shield).
Should I blow this dust away? Is there a preferred method?

Manual suggests compressed nitrogen, but I do not have it. Would air from a compressor suffice or should I use IPA instead? Thank you very much.

[macboy]

Meanwhile I have another question, the board I have seem to have a bit of dust on it, mostly by the fan (including spots under the shield).
Should I blow this dust away? Is there a preferred method?

Manual suggests compressed nitrogen, but I do not have it. Would air from a compressor suffice or should I use IPA instead? Thank you very much.

Don't use air from a compressor unless you have very good filtration. By that I mean 0.02 micron or better coalescing filter to remove oils in addition to particulates. Otherwise you will contaminate the board with compressor oils. You should be able to get ultra low residue canned duster from you usual electronic supply shops. Don't use the normal consumer grade stuff, in most countries it has a bitterant that will leave residue everywhere.

[nikonoid]

Thanks for the suggestion. Would "MG Chemicals Super Duster 134" or "MG Chemicals Super Duster 152" be a good enough for the task?

[macboy]

Thanks for the suggestion. Would "MG Chemicals Super Duster 134" or "MG Chemicals Super Duster 152" be a good enough for the task?

Either looks fine, both are "pure" (single ingredient) and residue free. The 134 is non-flammable and approved for use on energized circuits. The 152 is flammable under some conditions, but I think it is less expensive as well. Use with good ventilation.

[nikonoid]

Just ordered one. Thank you.

Changing the front/back switch was a breeze. I am surprised that changing capacitors is more difficult than I expected. Holes in the board are very very tight, so vacuum desoldering iron does not help much.  I have to move each leg a little bit at a time. Any additional tips there?

[Le_Bassiste]

well... not directly related to desoldering the caps, but to general handling of the p.c.b.
i'd suggest that you do not try to clean the entire pcb by any means, in order to absolutely avoid any contamination. just clean the areas that you have been working on, and you should be fine. this is IIRC also stated somewhere in the repair manual.
a pcb being contaminated by residues of foreign matter (that is otherwise just sitting on the components, not doing any harm at all) dissolved by cleaning agents, and then evenly distributed across the whole board will probably send your fine meter to the point of write-off.
of course, ymmv, and i'm being probably a tad too paranoid about this. it's just that my kitchen-sink equipment would simply wreak havoc on the pcb if i would try to do this, even with gallons of cleanest IPA, N2 and whatnot at my hand...
so, the way i tackled that on my 2002 was just to get out a pair of nitriles, stop breathing, remove all caps, clean the areas with KONTAKT LR and cotton swabs, mount the new caps, then again clean the areas and close the box, and breathe.

[nikonoid]

I am almost done recapping. Digital and analog boards are done. More high res photos and more questions to follow soon.

Meanwhile I am facing a new dilemma now. According to manual display board contains one electrolytic capacitor: 2.2uf 100v. However when I opened that board I saw the his:


I have never seen electrolytic like that. Should I replace it? What do I replace it with? I could not find the same Philips available for sale.

Thanks in advance.


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[coromonadalix]

they are series 139  Aluminium electrolytic capacitors  long life  models, i would substitue only with an original part ...  your dallas DS1243 "black chip" 64K NV SRAM with Phantom Clock is dated form year 96, if its not dead  change it.

WARNING :  i dont know if it retain any calibrations, i dont think so

[nikonoid]

Thank you, coromonadalix,

This 2.2mf 100v capacitor is a hard to find item: discontinued and non stocked.
I managed to locate and purchase 25 pieces, but I only need one.

If anyone here is recapping their Keithley and need one, send me a message and I will hook you up.

[Samogon]

You went too far with recaping

[Smith]

The new units have the same cap, but manufactured by BC. I replaced the cap on mine after replacing the VFD. It went bad within 8 hours and overloaded the 60v of the VFD. I placed the old Philips cap of the old VFD on the new VFD, and it's still working fine.

[nikonoid]

Smith, thanks for the photo. This is right on. And here is my display board:


As soon as I opened it, I could see signs of extreme overheating. See by the contact switches. The board became more transparent and showed rainbow colored fringe, sort of like you get with a drop of gasoline in the water. The damage almost looked worse on the switch side than on component side. The heat pattern seemed to be centered at Q901 component. It is near capacitor, but not directly next to it. The transformer also looked burned. That said when I powered the meter up, the display worked without any problems. Please let me know what you think.





Here is the rubber keyboard. I cleaned it and switch contacts with IPA. The discoloration on rubber looks strange:


When I removed analog board and lower shield, I also discovered another spot that looked like overheating. It is right at the spot where lower shield has a heat transfer sponge. It is in the lower right corner of the photo below and also magnified in the photo after, where I was able to photograph rainbow discoloration on the board.



What could it be? Should I be concerned?

Here is a quick grab of the back of a digital board and also the ESD safe sponges I used with IPA for cleaning:



And finally the badge of honor 




Please let me know if you have any ideas about two overheated spots (display and analog boards). Thank you.

[Le_Bassiste]

yes, these pesky VFD transformers are real cookers! don't know how to handle that, though.
as for the discoloration of the area under the heat sponge, that is not caused by overheating. it is caused by the sponge itself, sweating out some of the silicone over the years. if you look closely, you will find traces of that on top of the pcb as well, where it crept through the vias. the extra conformal coating on the pcb in the area, where the sponge sits, was discussed before:
https://www.eevblog.com/forum/repair/repair-94'-keithley-2002-8-5-digit-dmm-good-ol'-cap-leaks-destuction-derby/msg1016968/#msg1016968
i _assume_, that the seemingly useless and contradicting application of the coating over guard traces was intended to protect that area against moisture getting trapped underneath.
so, i would go with TiN's notion here: leave as is.

edit: see also:
https://www.eevblog.com/forum/repair/repair-94'-keithley-2002-8-5-digit-dmm-good-ol'-cap-leaks-destuction-derby/msg1017683/#msg1017683

[Le_Bassiste]

hmm, after another look at your photos, i now see that it is not the transformer that caused the brown spot on the pcb,    it's one of the two push-pull transistors on the primary side.
given the age of the instrument and the fact that the supply of the display is still giving the right voltages for filament and anode, you probably shouldn't be too worried about it.
the only thing that you could do is to exchange the transistors with some better ones. don't know from top of my head whether these are BJTs or MOSFETs, have a look at the KEI2001 schematics (VFD supply should be same as in KEI2002, but is located on digital board) on TiN's site to find out.

[nikonoid]

Thank you, Le_Bassiste.

After reading your post I decided to put the instrument back together and test. I did not bother to put push rods back in, since I will be disassembling it again.

The good news is that it runs ok. On 10 volt range it is about 15ppm away from my 34401a. I measured a 10 Ohm standard and got 10.000065 - not too bad either.

The bad news is that after being assembled it repeatedly failed the self test with 306.2 error: 1.92?A ohms source. This corresponds to 2 MOhm range. This range was not working properly even before recapping.

I used 34401a to check the current on the 2 MOhm range and it was fluctuating wildly between 2.8?A and 4.0?A. Then all of the sudden it snapped to 1.90?A and became stable. I checked the resistance measurement in 2 MOhm range and it worked fine. Ran self tests and they all passed.

I power cycled the meter and again everything passed.

This situation with an intermittent current source problem has me completely at the loss. I would appreciate any troubleshooting ideas. Thanks.

[nikonoid]

Today I checked the base level noise and got 4uV. This did not improve after an hour of a warm up.
I used a copper strand with Q-tip to short the inputs. This seems too high.

My 34401a starts at 4uV but fairly quickly settles to 0.3uV.


Le_Bassiste, I think you reported something similar with your 2002. How did you get it resolved?
Is there something I can do to troubleshoot this? Thanks.

[Muxr]

Welcome to forums. K2002 is my second favourite meter  .

What would be your most favourite meter?

[Le_Bassiste]

Today I checked the base level noise and got 4uV. This did not improve after an hour of a warm up.
I used a copper strand with Q-tip to short the inputs. This seems too high.

not sure as to whether you are talking about "noise" or "offset" here, so i assume that you mean "offset"
after cold start, (instrument was off for 24 hrs) my specimen needs about 2.5 hrs to fully settle from approx. 6µV to some constant offset value (set to DC, 200mV, 10NPLCs, no FILT, input shorted).
i'm not concerned about the constant offset, as that will be fixed, once the instrumet has got a calibrate/adjust run in a cal lab. also, that constant offset value is "rock stable", i.e., even after a continuous run for a month, that offset didn't change at all (well, it did change with ambient temperature, but always came back to same value for same temperature).
but i am wondering whether the amount of initial drift of several µV has to be considered too high, and might point to some hidden fault. otoh, the instrument passes all self tests. 
so, would be great if you could post some of your offset values wrt time here 

as for "noise", have look here, where i posted some values that may be helpful for further performance verification of your 2002. note that i've used quite a small sample window of just 50 samples, so take them with a grain of salt. should be fine for a q&d performance check, though.
https://www.eevblog.com/forum/repair/keithley-2002-repair/msg1188312/#msg1188312


 

[nikonoid]

Thanks.

I checked the noise and so far it seems around +/- .5uV and the offset is around -4.2mV even after warmup.


I am still trying to troubleshoot 1.98uA source and currently suspecting problems with either Q244, Q242 or CR217. Do you know what parts they are? Here is a photo of one of them.



Also here is a thermal shot of the meter after warmup. Surprisingly ADC is very hot as well as area around 3 electrolytics by display board (they themselves are cold - cold horizontal line in the lower right corner)




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[Kleinstein]

Have you checked input leakage / bias in voltage mode ?
A jumpy test current could also be due to leakage at the input. The ohms current source is reasonably well protected and thus not that likely to show a defect. Input protection and switching is much more exposed to things like ESD.

[Le_Bassiste]

I checked the noise and so far it seems around +/- .5uV and the offset is around -4.2mV even after warmup.

looking good. mine is around 3.4µV offset at 23°C, hovering about 40 nV/°C

I am still trying to troubleshoot 1.98uA source and currently suspecting problems with either Q244, Q242 or CR217. Do you know what parts they are? Here is a photo of one of them.

 it's a siliconix J2611 JFET.
note colour dots on top. these are factory-selected and marked, so it might be better to not try and change/remove them, until you are absolutely sure that they've gone south.

Also here is a thermal shot of the meter after warmup. Surprisingly ADC is very hot as well as area around 3 electrolytics by display board (they themselves are cold - cold horizontal line in the lower right corner)

on the ADC, the CPLD can get quite warm, no reason to worry. same holds true for the three electrolytics next to the smal silver box. the temperature of that box is actually very high, but its nickel plating is fooling your TIC into false temp readings (because emissivity of metallic platings is much lower than that of the other things around the box).

did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?
are the current jumps still noticeable when measuring a resistor in non-auto-ranging mode?
(i must admit, a little bit poking in the dark here 

EDIT: did you read this?
https://www.eevblog.com/forum/repair/keithley-2002-repair-help/25/#msg1227932

[nikonoid]

Have you checked input leakage / bias in voltage mode ?
A jumpy test current could also be due to leakage at the input. The ohms current source is reasonably well protected and thus not that likely to show a defect. Input protection and switching is much more exposed to things like ESD.

How do you check for input leakage?

Also, I am dealing with two separate issues, one is 4mV offset with shorted input and this might be normal.

Another is non working 2MOhms range (selected manually) and 306.2 error. The current source sometimes is at 1.98uA and is stable as it should be and sometimes is as high as 7.9uA and floating up and down by a lot.




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[nikonoid]

did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?
are the current jumps still noticeable when measuring a resistor in non-auto-ranging mode?

I manually set 2MOhms range. While in this range sometimes current jumps and someones is it stable. I have seen it change both ways without any mechanical interaction on my part. I will watch it a bit more. It almost seems thermal rather than mechanical. However I will try the knocking test tonight.

 it's a siliconix J2611 JFET.
note colour dots on top. these are factory-selected and marked, so it might be better to not try and change/remove them, until you are absolutely sure that they've gone south.

Thank you, yes I do see the yellow mark. Actually on Q242 and Q244. Now that I know they are JFETs they are less of a suspect.

Here is my test setup: I put the K2002  into 2Wire Ohms on 2MOhm range. I use Fluke 187-II in micro current mode (100 Ohm impedance) to measure current produced by K2002 between Input High and Input Low. K2002 shows 0.000100MOhm when working well or higher number when not. While monitoring the current, I use 34401a to measure voltages relative to Input Low of K2002.

One noticeable difference is on CR217. When working well one leg is at 304.3mV and another at 0.64mV and fairly stable. When not working properly one leg is still at around 300mV but fluctuates a bit, while another is at 1.5mV and fluctuates a lot.

Also, when working well very stable 12.7017 is seen on legs on Q211 and Q212 and in contrast when misbehaving I can see around 9v on these legs and it fluctuates a lot.

It would be much easier if I had schematics. Any help is appreciated. Thank you.

[Kleinstein]

The changing voltages around Q211, Q212 might be a clue to the issue. However without a schematics, it is not that easy. There is a reverse engineered schematics for the ADC part around. Chances are the input stage is somewhat similar to the K2001.

Measuring leakage / input current is relatively easy. There are two options:
1) use a 10 M or similar resistor across the inputs in the volts (e.g. 2 V) range.
2) have a low leakage capacitor (e.g. 1 nF) across the inputs an watch is charging / discharging.

[nikonoid]

Measuring leakage / input current is relatively easy. There are two options:
1) use a 10 M or similar resistor across the inputs in the volts (e.g. 2 V) range.
2) have a low leakage capacitor (e.g. 1 nF) across the inputs an watch is charging / discharging.

1) 10M standard resistor on 2V scale is giving me around 60uV if I stay 3 meters away and stay very still.

2) I did not have a 1nF capacitor handy, so in a pinch I used 200nF GenRad 1409 capacitance standard. It showed about 1.4V and was discharging approximately at .1mV per second.

Is this good?

did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?

Mechanical knocking on the board does not change a thing.

I am planning to do some offset voltage plotting after I get GPIB connection working.

The search continues...

Is there a way to test Q211, Q212 and Q213 in circuit? I do not want to desolder them unless I have to.

[Kleinstein]

60 µV at 10 M is only 6 pA. 0.1 mV/s for a 200 nF capacitor would be 20 pA - this includes capacitor leakage and maybe dielectric absorbtion.
So leakage seems to be very low - usual specs are something like below 50 pA.

Checking JFETs in circuit could be difficult - the error still seems to be more intermittent or just marginal. So one would not a expect a fully broken part, but more like a parameter shift, excess leakage or unwanted oscillation. This might be even hard to detect on a separate part.
It might be worth checking the drive signals, that control the JFETs - there have been faults in the LM339 chips and even the shift registers used for control. The fluctuating state might also be due to oscillation - so maybe time to use a scope.

It might also help to get at least a crude schematics of the suspected area. From TiN's tear-down the circuit seems to be similar to the K2001 meter for which schematics are available.

[nikonoid]

Thank you fro validating leakage spec.

I really like the suggestion to use scope. This will be my next thing to do.

So far I validated R322, CR217, K201, R334 and R342 (parallel to L206). I used every resistor in the chain as if it would be a current sense resistor, to make sure it is about the same wrong current going through all of them, when device is misbehaving.

I doubt it is the return circuit responsible for the issue (Q246, Q245, Q244, and Q242).

So I am left with everything prior to R322 to look for problem.

The repair manual is referring to /OHMD line, OHMREFHI and REFBUF. What are they? Thanks.

[Kleinstein]

There is quite some description in the repair manual:
The /OHMD signal is a digital control signal to set the current source to 2 µA. Due to the / in the name, likely active low. OHMREFHI and REFBUF are a 14 V signal and a buffered 7 V reference.

The 9.6 µA and 2 µA current source seem to use many of the same parts. So one might exclude those used for the 9.6 µA current source. The change from 9.6 µA to 2 µA seems to be changing the reference voltages - thus not in the super higher impedance part. My suspect would be more like U232. It would be really worth checking those ref voltages used for the current source. It may help to make a drawing of the current source - there is quite some descriptions to help with this. However it seems to be a little different from the K2001.

[nikonoid]

The 9.6 µA and 2 µA current source seem to use many of the same parts. So one might exclude those used for the 9.6 µA current source. The change from 9.6 µA to 2 µA seems to be changing the reference voltages - thus not in the super higher impedance part. My suspect would be more like U232.

You are right on the money! When I checked U232 I discovered that when meter is properly producing 1.98uA in 2MOhm range, on U232 I get:
Pin 9: Low
Pin 10: 12.7175
Pin 11: 12.7154
Basically, Pin 10 connected to Pin 11, as they should.

When current is misbehaving, I get:
Pin 9: Low
Pin 10: 12.7175
Pin 11: FLOATING!!!!

To me it is a clear proof of U232 fault. Thank you, thank you, thank you!


Additionally, I checked U221 register that according to manual supped to be: 11101001, but in my case is 11101011 regardless of meter working properly or not (Bit 2 is different).  It is possible that my firmware is too old and manual was based on new firmware.

Also in checking the repair manual in details, I found surprising number of inaccuracies, like U232 is listed as "CENTRIGRADE TEMP SENSOR LN35DM", instead of "IC SWITCH QUAD SPST 16SOIC".

[nikonoid]

Now that I am on the way of fixing the 2MOhm range and waiting for parts, I am looking for the reason for overheating front panel.

hmm, after another look at your photos, i now see that it is not the transformer that caused the brown spot on the pcb,    it's one of the two push-pull transistors on the primary side.
given the age of the instrument and the fact that the supply of the display is still giving the right voltages for filament and anode, you probably shouldn't be too worried about it.
the only thing that you could do is to exchange the transistors with some better ones. don't know from top of my head whether these are BJTs or MOSFETs, have a look at the KEI2001 schematics (VFD supply should be same as in KEI2002, but is located on digital board) on TiN's site to find out.

I took an infrared image of the front panel after the meter was on for a while and it shows very noticeable overheating in the same area where the board is discolored. See attached IR photo.



I do not have a high resolution thermal imager, so the photos are somewhat blurry. Nonetheless I can see that the VFD transformer gets hot, but only to 60C; At the same time 5 transistors in the vicinity and 4 somewhat large 1/4W resistors get to about 80C. This is with the panel open i.e. some ventilation. The temperature is probably much worse when panel is closed. In overall thermal shot of the front panel I can see the temperature bleeding into area of banana terminals and can attribute to EMF voltages.

I checked resistors and they have proper values and 4.7k resistors had a voltage drop of 31V DC, corresponding to dissipating 200mW for a resistor specked at 250mW. Maybe what I have is completely normal.

Can someone please take a thermal photo of the front panel of K2002 after it had been running for a whole? No opening of the unit required. Thanks.
 

[Samogon]

Tomorrow will make thermal images, going to calibrate some agilent 34401a DMMs.

[Le_Bassiste]

Now that I am on the way of fixing the 2MOhm range and waiting for parts, I am looking for the reason for overheating front panel.


I took an infrared image of the front panel after the meter was on for a while and it shows very noticeable overheating in the same area where the board is discolored. See attached IR photo.
I do not have a high resolution thermal imager, so the photos are somewhat blurry. Nonetheless I can see that the VFD transformer gets hot, but only to 60C; At the same time 5 transistors in the vicinity and 4 somewhat large 1/4W resistors get to about 80C. This is with the panel open i.e. some ventilation. The temperature is probably much worse when panel is closed. In overall thermal shot of the front panel I can see the temperature bleeding into area of banana terminals and can attribute to EMF voltages.

Can someone please take a thermal photo of the front panel of K2002 after it had been running for a whole? No opening of the unit required. Thanks.

here you go: my 2002 after approx. 120 hours uptime, sitting between a cooking 2001 and a reference box, ambient at 30°C

[nikonoid]

Le_Bassiste,

Thank you for posting. Your meter shows 22C degree rise over ambient (great point to bring ambient temperature into this).
I was getting 41C max with ambient at 22C to amount to 19C rise over ambient. Give or take the same result as yours.

My heat center is also exactly at the same spot as yours. It looks like it is not the transformer but resistors and transistors that heat up the most.

This is bound to create temperature differential in connectors. How could this be on such a precision instrument?
Do I now have to use low EMF cables?

Samagon,
Can you please upload your thermal photo?

[Kleinstein]

I agree that for a precision instrument so much heat is not a good idea. But on the other side low noise amplifiers need a certain current and thus generate heat. The heat is one reason I do not like the rather power hungry VFD or even Nixi displays. So a simple B&W LCD also has advantages.
Still I don't see a need for the power supply here likely the driver for the small transformer needs to get so hot. At least today there should be more efficient DCDC converters - even if it has to be low noise / low EMI. It looks like the transistor is the main heat source - except in a linear supply this should not happen so much.  The temperature really seems to be on the high side - the discoloration of the board is not a good sign, even if this might have happened in a hot environment. So there might be a fault causing too high a consumption.

However the main heat is not so much near the terminals, but more to the other side. Just plain copper wires are already not that bad with thermal EMF. This is also just a regular DMM, not a nV or special low level meter.

[Samogon]

Here it is, after approx 1 hour of warm up.
Ambient temp 24C

[nikonoid]

Here it is, after approx 1 hour of warm up.
Ambient temp 24C

Also a 20C rise in temperature and the same spot! Thank you very much.

I guess there is nothing wrong with temperature rise on my K2002...  Great news after all.

[nikonoid]

Parts arrived. I am looking at replacing U232 and desoldering it looks a bit daunting with guard traces and precision resistors in a close proximity:



Using fan is out of the question because of thermal concern for resistors. I am considering an enamel wire to bend legs up one by one, as I am desoldering them. And also use of no-clean RMA flux, because I will not be able to clean under microchip after soldering replacement.

Any suggestions for a proper procedure?

How about not desoldering the microchip at all, but just adding a new one on the top of non working chip?  It is just an analog switch and the only fault is that one channel sometimes does not turn on completely....  Would that be too crazy?






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[Samogon]

Just cut legs off. No heat, no contamination.

[nikonoid]

Just cut legs off. No heat, no contamination.

Sounds brutal, but effective

Almost like quote from a mafia movie: "oh, just cut the legs off"

All joking aside I was afraid of leg moving enough from just being cut and damaging the pad. I practiced on two throw away SOIC 16 chips and the procedure worked like a charm.

Thank you, Samogon.


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[nikonoid]

I finally assembled the instrument back together, gave it 24 hours of warm up and now started to verify performance before sending it to be calibrated.

I would like to thank TiN, Le_Bassiste, Kleinstein, Samogon and others who helped me though the process .

Performance evaluation in NORMAL speed mode (NPLC1, I think):

Copper short on 200mV range after long warm up fluctuated a bit between -1.3 and -1.6uV.  StDev=93nV over 1000 readings.

5 volt reference from DMM check plus ("5.0000" as of 2 years ago when I bough it) gave me 5.0000295 or 5.9ppm out.  StDev=1.3mV (0.26ppm) over 1000 readings. So pretty good for the reference and a meter.

Bias with 10MOhm resistor on 200mV range measured  120uV (for some reason it was a bit less when meter was not fully assembled and not completely warmed up). So 12pA of leakage

Resistance worked fairly well comparing to my other meters and standards, however noise on resistance was somehow higher than what I would expect, especially at 1MOhm:

Code: [Select]

Nominal(Ohm)    Measured      ppm      StDev     StDev(ppm)   Readings
10,000,000.00   9,999,711    -28.9     15            1.50     1000
 1,000,000.00   999,942.6    -57.4     1.5           1.50     1000
   100,000.00   100,000.89     8.9     0.074         0.74     1000
    10,000.00   10,000.131    13.1     0.0038        0.38     200
     1,000.00   999.98898    -11.02    0.000131      0.13     200
        10.00   10.00081      81       0.000010      1.00     200

Please let me know if these look ok before sending for calibration an if there are any additional parameters/test I can ran before shipping it out. I am considering using 4GTE company. They have Fluke 5700a they can calibrate it on. If you have other recommendations for good and reasonably priced calibration lab in US, it would be great to know.

On the strange side, I found two things: meter is reporting internal temperature 27F above calibration temperature. How could it be? The calibration date is from 1999 (about a year after the chips date).

Another peculiarity is that at some point meter gave me very jittery Ohms reading. When I went to General menu to check Line Sync is was displaying 400Hz despite my power being 60Hz. Single power cycle of the meter fixed it and now it properly shows 60Hz. Have you seen something like this?

[Samogon]

What 4GTE quotes are?
Simple cal without cert quoted 600 from Tektronix

[nikonoid]

I thought 4GTE was $240. I will have to call them tomorrow to confirm if it is with data or not. They previously did a good job for me.

Any other suggestions?

[MadTux]

Someone lucky scored another crusty 2002 for a spectacular price.
Of course I missed i t ;-(
http://www.ebay.com/itm/KEITHLEY-2002-Digital-Multimeter-8-Digits-Factory-Calibrated-/282569341996?epid=1701817052&hash=item41ca71fc2c:g:AkkAAOSwWdZZZsb0

[nikonoid]

That was actually me. I could not believe it when I saw it by pure chance and pulled the trigger right away.

It looks bad, but it will be fun to restore it. Arriving today. I will post some pictures.


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[lowimpedance]

That was actually me. I could not believe it when I saw it by pure chance and pulled the trigger right away.

It looks bad, but it will be fun to restore it. Arriving today. I will post some pictures.


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Lucky indeed 
Will be interesting to see the before and after pictures 

[nikonoid]

Ohhhh HORROR!!!!!

The package arrived today. I was very surprised that it was not a box, but bubble wrap blob. Oh horror!

I started opening and realized that few layers of thin bubble wrap were put right on the meter. No cardboard or peanuts anywhere. No other cushioning at all. Oh horror!

When I took the bubble wrap off the meter inside did not have the metal case! Oh horror!

More over it did not have a analog board shield at all. And apparently it was stored somewhere without the shield as the analog board is full of dust. Oh horror!

Looking the the board closely revealed few.... mice droppings on analog board! Oh horror!

Upon quick inspection I found at least two places where the analog board was apparently burned through. It appears that internal traces overheated and charred the PCB above them to charcoal. Oh horror!



I uploaded high resolution pictures to TiN, to I can post them.

Seller was able to power up the DMM up, but I am not as brave. I am going to follow Dave's typical advise "don't turn it on, take it apart".

For decontamination I am considering a large ultrasonic bath with distilled water and IPA. Or possibly using syringe to power wash it. 

Suggestions and condolences are welcome (;



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[TheSteve]

It's beat to hell - looks like a good challenge that needs a lot of TLC. If you're not up for it there are many of us who are. I'd love a go
Paypal ready and willing 

[TiN]

At great price comes great responsibility  .
I don't ever remember 2002 going for that low.

New analog board cover kit is ~150$ from Keithley/Tek.

[alm]

Looks like an interesting repair .

I would remove anything that can trap water (e.g. switches, non-sealed relays) or that you will replace anyhow (electrolytics?). Then rinse with demi water (to get clean water in all the nooks and crannies), then demi water with some mild soap, then demi water again and then IPA to get rid of the water. Then try to figure out what blew the traces (short on the analog board? excessive voltage from power supply?) and go from there.

[nikonoid]

Here are more images. TiN, thanks for hosting.

The digital board looks excellent and has MEM2 option installed while analog is burned and dusty. The date codes are from 1994 and firmware looks like is A05.


Here are some details of bad parts. First image is intentionally overexposed to better show charred parts.


Few more pictures of the bad section. It looks like the electrolyte spill. It also appears that one of capacitors short circuited and that started chain reaction.




I already have few pars ordered from Digikey.

Would anyone know what kind of diodes this section is using? The repair manual lists them as "DIODE, CONTROLLED AVALANCHE, BYD17GSOD-87".

Also, the CR113 rectifier is listed as "DIODE, BRIDGE, VM18". I could not find it and ordered DF01MDI-ND instead. Would that be a good choice for replacement?


Tomorrow I will call Keithley for replacement cover and metal casing. I still cannot believe I got it without the case, but with plastic handle!

At great price comes great responsibility  .

Love the quote.

[saturnin]

Tomorrow I will call Keithley for replacement cover and metal casing. I still cannot believe I got it without the case, but with plastic handle!

If I were you I wouldn't complain. You got exactly what was shown in pictures in the auction. One can always ask a seller about further details, but in such cases (extremely low price), there is a risk someone else would buy it before you get answer It is always lottery, if it is repairable or not. I wish you it was. Looking forward to repair details 

[TiN]

WYSIWYG of T&M world.

I better buy third 2002 pronto, or Keithley freak title might go to nikonoid soon
Even if you decide to give up, just voltage reference parts alone would cost more than what was paid .

[macboy]

Tomorrow I will call Keithley for replacement cover and metal casing. I still cannot believe I got it without the case, but with plastic handle!

If I were you I wouldn't complain. You got exactly what was shown in pictures in the auction. One can always ask a seller about further details, but in such cases (extremely low price), there is a risk someone else would buy it before you get answer It is always lottery, if it is repairable or not. I wish you it was. Looking forward to repair details 

Exactly. I did not for even a second assume that the cover or shield were included. Nowhere to be seen = not included. Dirt and corrosion were assumed to be free though.

[Kleinstein]

The burned trace seems to be in the power supply part. So worst case one could isolate that part and have a new power supply board. Also leakage is not that critical in that area. Chances are that after replacing the caps, a few diodes / rectifiers and with maybe a bodge wire to replace the burned trace it might actually work.  A shorted power supply is more like a good sign.

Really bad would such a burned trace in the Ohms or low current area, where all the guard traces are.

The analog part cover and the case is more like one of the last parts to fix.

[nikonoid]

I am certainly not complaining. This was a great purchase. My reaction was more to a condition of the unit being an insult of a fine instrument.

I like the challenge and am looking forward to restoring it, hopefully with the group's help. The help and advise I got while repairing my first K2002 was invaluable.

For now my plan is this:
1) Remove analog board
2) Identify and replace burned parts and repair traces. I expect replacing bridge rectifier, diodes and caps.
3) Power up the meter and self test with analog board outside of the case
4) Address self test issues
5) Decide on overall cleaning and decontamination procedure
6) Recap, new shield, case

[alm]

I would probably clean the board(s) after removing suspect parts, and before mounting new parts or testing. The leakage may very well mess up the self tests anyway, so I see no reason to test it before hand. It may also make it easier to see defects and nicer to work on.

[TiN]

Yea, item number 5 should be after number 2, not the other way around.
However PSU section for 2001/2002 analog domain is bit tricky, because of capacitor bootstrap doubler. That's exactly what two of the big caps near transformer are for, doubling unregulated +18/-18 to +36/-36 BS rails, which float around input to provide high impedance on main ranges. These rails power many opamps on frontend, and risk is high that one or more of those are taken out.
Schematics for that part almost same as 2001.

[nikonoid]

Thank you for suggestions.

On another note, today I got a call from local calibration lab (New Jersey) telling me that my first Keithley 2002 failed the calibration.

The tech said that meter was in spec everywhere but in DC Volts that were out of spec by a bit. However his attempts to calibrate it were not successful. His exact words were that "unit does not hold cal on DC Volts". He tried several calibrators but could not get new constants to store.

It sounds like EEPROM problem or new constants are too much outside of normal range and firmware is refusing to use them.

I will have it back on Thursday. I am considering doing a dry run calibration myself. Not that it would be a good calibration, but just to see what exactly is not working. I got calibrated K2000, EDC 520A, GenRad 1433-B for resistances and Agilent 6611C for 2 amps range.

I might also consider changing calibration parameters manually though secret menu.

Have you ever had a similar situation with Keithley 2001/2002? Do you have any other suggesting on what to try?


Thank you.

[Zucca]

Have you ever had a similar situation with Keithley 2001/2002? Do you have any other suggesting on what to try?

One fo my 34401A  has same issue... fully story

https://www.eevblog.com/forum/repair/hp-34401a-drift-in-the-2w-r-mode-in-the-100-ohm-range/25/

At the end it was a bad cell in the EPROM that would not get written... and this drive you crazy because you triple check all the analog circuit, but in the end is a tricky digital failure. Fortunately your cal lab pointed you in the right direction, mine was just saying 2WR 100Ohm is out of spec... good luck.

[TiN]

2001/2002 are not friendly for hobby level calibration, meaning that you must run ALL calibration steps without error to allow it to store data into NVM (which is pair of 24C16 EEPROMs in case of 2002). 

Also since you had unit repaired and also it's an old one, you must run low-level manufacturing calibration, otherwise your current, resistance and AC functions will not work/calibrate correctly. For this you need Fluke 5700A/5720A or Wavetek 4808 (due to 200V 30kHz requirement) + HP 3325/3245/similar signal generator for 2V 1Hz AC. I think I have the footage recorded of both 2001 and 2002 cal procedures run, will need to find it and upload to YT, if you interested. 

Then user-calibration or point cal adjustment can be ran to tweak ranges/functions. They still run in groups, so you cannot calibrate single range in traditional manner. GPIB control is required for easy calibration process and advanced error reporting. 

There are few "self-calibration" steps during which meter measure internal signal path sections, comparing to externally measured references, and if any of them failed for whatever reason (gain error or offset error) than meter will throw whole calibration data away and you must start over from the beginnig. It took me a while to sort all this stuff out, and it's definately not an "few hour playjob with access to friend's calibrator" sorta thing. 

[Le_Bassiste]

For decontamination I am considering a large ultrasonic bath with distilled water and IPA. Or possibly using syringe to power wash it. 

Suggestions and condolences are welcome (;

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no condolence but congrats, nice find!
as for the suggestions: clarify first with component manufacturers as to whether the ultrasonic can harm any components. if in slightest doubt, don't use ultrasonic. just, don't.
also, the shroud over the analog board (or, to be more precise  , the absence of it) will really bite you when trying to debug the analog section (fingers crossed that you don't have to do that) , as it is ultra sensitive to any temperature fluctuations across the board.


...my 0.02.

[Dr. Frank]

..
For decontamination I am considering a large ultrasonic bath with distilled water and IPA. Or possibly using syringe to power wash it. 

Suggestions and condolences are welcome (;

Suggestion: Never use ultrasonic bath on assembled PCB.
This will probably kill many/most of the semiconductors.

Frank

[WastelandTek]

OK, I am subbing to this thread

this is getting really interesting

[Samogon]

Probably it depends on what ultra sonic cleaner to use. Crest for instance is proved to be good for Mac motherboards and cell phone pcb cleaning.

[Zucca]

Suggestion: Never use ultrasonic bath on assembled PCB.
This will probably kill many/most of the semiconductors.

Hi Doctor Frank, do you have any example of that?
I fully agree with you for MEMS devices...

After some googleing:

https://electronics.stackexchange.com/questions/196373/components-to-avoid-using-on-an-ultrasonic-cleaner


Interesting, did not know that...

[MadTux]

I would at least be careful with older chips that have unsupported bond wires (the ones with a lid on it and an open air die).
I don't know wether gold has an endurance limit, but aluminium doesn't have one, thereby aluminium foil is "eaten" by ultrasonic cleaners
https://youtu.be/YR5PgwEDFzs?t=2m20s
https://en.wikipedia.org/wiki/Fatigue_limit

[nikonoid]

That is some fascinating information. I did not expect ultrasonic cleaner to be able to damage the board.

Parts should be arriving today. I cannot wait to dig into this new project.


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[Samogon]

Zucca,
From the link you provided

Ultrasonic cleaning is fairly commonly used for cleaning flux from PCBs during the production process. I think you're assuming the wrong reason for their use in your answer.

But k2002 is not where i will use it.
And its pcb pretty big Crest ultra sonic cleaner of such size would cost ~$2k

[nikonoid]

Picked up Keithley 2002 rejected from the calibration lab. They said that it would not hold DC Volts calibration. They gave me printout with DC Volts measurements they did using Fluke 5720 and on my my 10V standard I got the same reading give or take 0.5ppm.  So I decided that  my standard is not that bad.

I did the spot DC calibration myself. The meter asked me for 2V and 20V inputs and also dead short calibration. The short calibration took a very long time. Maybe 20 minutes or so. I did it with fluke's 88X-Short that looks identical to Keithley 8610. When I plug it in value fluctuate between 20nV and 180nV. I hope that is normal.

After that spot calibration unit is holding 2v, 20v, and 200v ranges very well (even after power down), so clearly EEPROM works. 200mV range is a bit off (by 10ppm or so) but this is only according to Keithley 2000, so I am not sure is that is an issue.


What else could be a reason for meter failing to calibrate at the lab?
Should I try comprehensive calibration myself before getting it back to the lab?
Thanks.

 

[nikonoid]

In a form of a short update, I finally had a bit of time to take the new (burned) K2002 apart.

I removed capacitors and diode bridge around burned spots. One capacitor blew a hole in its side while another leaked from the bottom. Basically a poster project on why you need to change capacitors in this.

As a result, there is one surface trace that was eaten - no big deal, one via that looks burned - no problem. And then a large burn through the board. It is basically ok on the bottom and a large extended black hole on the top.

In attempt to understand the damage I "x-rayed" the board with a flashlight. Then I superimposed top and bottom view in photoshop and traced all the relevant conductors in 4 colors.

Red, Green, Blue and Yellow for all 4 layers of the PCB, with Red being component layer and blue and green two internal layers.



It appears that burned area did not have other top or bottom conductors. What got taken out is 3 internal conductors and 3 vias getting from layer 2 to 3.

Unfortunately as you can see all 3 traces start and end outside of area that I have reverse engineered. They probably are not even a part of power supply.

I will have to do more tracing work just to understand what these traces are.

More photos are coming. Wish me luck.



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[TiN]

I don't miss this . Keithley 2001/2 are great for learning how to decipher cryptic layout. Often I had situation when trace goes under 1206 resistor/cap, and going out from the other side. You think, easy, same trace between the pads, right? Mhm, not quite ,there are two vias hidden under the thing. 

Luck will not help here, but dedication and xxx amount of hours and late nights will do Looking forward for your fixes. 
My take would be to cut whole charred area and air-wire all connections with copper thin wire. You don't want leakage (burnt PCB is conductive carbon) to give you horrors.

[Zucca]

nikonoid I hope you will fix it. It´s so inspiring what you are doing...

[WastelandTek]

I agree that you are going to have to dremel out all the burnt fiberglass...what a task you have set yourself

[nikonoid]

I am actually considering very small drill first and then needle file by hand, as there are many good traces nearby.


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[Kleinstein]

If the burnt part is only on one side of the board, one may not need a full hole in this area. If can be enough to remove the two burned layers for the one side. One could use something like a Dremel tool, or just scraping by hand with a knife.

It looks like there are 3 lines from below in the burned area, and nothing to the right. Likely there are good quality pictures on the xdevs site so one could get an idea where those lines are heading.

[TiN]

Best pic I had of that area is this one. Didn't take more photos of analog section, as there was nothing wrong on my second 2002 in there.

Also suggest not to take shortcuts and desolder all SMD parts in affected by electrolyte leak area (and replace cheap ones with brand new stuff), instead of thinking that little IPA and brush is enough.

[nikonoid]

This is a great photo. Thank you, TiN. I can clearly see here layer 2 and what connects to what. Big help.


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[nikonoid]

Today I found a second contributing factor to condition of the meter. Someone installed a 2 amp mains fuse instead of 1/2 amp. So one short happened due to electrolyte leak, it kept burning the board.

I started cleanup of the PCB burn. Made a decision against using dremel. I decided that any power tool throwing tiny conductive particles all over analog board is not a good thing  . Arming myself with a wooden toothpick and 40x microscope, I removed as much of charred PCB as possible, periodically cleaning fine debris with IPA and Q-tips. After that came 1/16 drill bit in a Pin Vise (driven by hand) where some damage was drilled through and some scraped. Then I finished with an exacto knife. I am still debating with myself if I will remove more material.

I am potentially looking at making 5-7 bodge connections using thin transformer wire. Many of them will be soldered to tiny vias connecting 2nd and 3rd layers, showing on the surface only as a tiny dot. Is this an Ok thing to do or should I opt for much longer budge wire so I can solder it to more reliable spots?

Looking at the board under microscope proved that you guys are right again. I can clearly see electrolyte damage on some chips and traces. I will have to remove at least 7 ICs. Last time when I decided to cut legs of a microchip it worked,  but less than perfectly. I am considering a using a soldering fan, covering nearby areas with either kapton film or aluminum foil for heat protection. Are there reasons not to do this and use some other technique?



Meanwhile here is more high res photos. This is the initial stage:


And this is after removing leaked capacitors and rectifier and a bit of cleaning:


I am still working on 4 layer PCB diagram on this section. I will post it soon. To create this diagram I took few photos on the PCB with light shining through it to help me see internal traces.

[TheSteve]

I wouldn't be cutting the legs of SOIC packages to remove them - IMHO that is a pretty bad way to do it. I much prefer to put a bunch of solder down the pins on one side, then lift it a little with a sharp knife. Then add some solder to the other side, grab the part with some tweezers and touch both sides back and forth and it should lift right off. With a little experience it is very quick. You can use the soldering iron to wick away most of the left over solder and then finish the cleaning with some wick, then alcohol.
Keithley PCB's aren't top quality but they aren't terrible either.

[nikonoid]

Thank you, Steve.

I will give it a try. I have plenty of boards to practice on. What tip temperature would you suggest?

I much prefer to put a bunch of solder down the pins on one side, then lift it a little with a sharp knife. Then add some solder to the other side, grab the part with some tweezers and touch both sides back and forth and it should lift right off. With a little experience it is very quick. You can use the soldering iron to wick away most of the left over solder and then finish the cleaning with some wick, then alcohol.

[nikonoid]

TheSteve,

I got a chance to try a method you recommended. It is a bit nerve racking to have a bunch of molten solder on the board. I had to get my tip temperature to 550F - this was the lowest temperature that would keep the solder molten when you go back and forth with iron. Hopefully the board is ok.

In the end I modified your process a bit, I would lift one side a little bit, then use desoldering iron with suction to remove solder still connecting lifted left to PCB and only then I would use extra solder on another half with tweeters to remove the chip.

This also gave me another proof that replacing components compromised with electrolyte is a must. Some microchips I was desoldering were corroded to the point that solder would not want to stick to them even with a good amount of flux.

Thanks again, 7 ICs are off the board now. Inspection under microscope showed no lifted of damaged pads.

I have another round of cleaning, then fixing broken lines and replacing parts. Unfortunately some of the parts are not arriving until the next week.

Meanwhile I called Keithley where I learned that top shield for 2002 is a non-orderable part - only internal technicians can order it. I can order top shield for Keithley 2001 and it is $55. Would someone know if top shields for 2001 and 2002 are somewhat compatible? Can I modify one into another?

[TiN]

Okay, you hit two no-no's in my book there.

If you have to increase tip temperature over 240-255C, that is a sign that tip and/or iron you using are not fit for the job. Meaning that PCBA is dissipating more heat than you providing. That means two things : you need larger surface area (bigger solder ball blob or wider more massive tip) or heatflow is not enough (need higher power soldering iron). Raising temperature might look like it's working ok, but you just delaminating PCB. Keithley analog PCB not good to survive high temperature, and those thin pads are easy to peel off. So work on your thermals

Second - pulling SOIC by one side is just making pads even more easy to damage. Keithley PCB have nSMD pads, usually connected just by thin trace without teardrops. I had number of times when pad and trace looked fine even under microscope, but actually were broken apart right at the point where pad connected to the trace. Searching for such faults wasted lot of hours.
After I started using only dual side SOIC heating, never have this issue again. Wide 4mm+ chisel tip with large solderblob flowing back and forth on both sides of IC works great. I'm using 120W handpiece to ensure good heatflow.

Top shield is same on 2001 and 2002, but there are two versions if them. One with cutout and duct for fan, and older one without cutouts, sealed type. If I remember correctly, they even have same partnumber. So I'd just order 2001 shields and be done with it . Make sure you order both top and bottom parts, and don't forget the mounting screw! It's not included with the cover. 

[TheSteve]

A different iron is needed if you need to turn the temp up like TiN posted. With the right tools and some experience it is a very quick process and will never cause any PCB damage.

And Keithley PCB's are pretty crap quality...

[nikonoid]

If you have to increase tip temperature over 240-255C, that is a sign that tip and/or iron you using are not fit for the job. Meaning that PCBA is dissipating more heat than you providing. That means two things : you need larger surface area (bigger solder ball blob or wider more massive tip) or heatflow is not enough (need higher power soldering iron). Raising temperature might look like it's working ok, but you just delaminating PCB. Keithley analog PCB not good to survive high temperature, and those thin pads are easy to peel off. So work on your thermals

TiN, I think I just got lucky on thermals. Seeing your email I went to measure the tip with thermocouple. I loaded the tip with lots of solder for better contact. Two different thermocouples I have gave me 241C and 256C, when iron was showing 550F (288C). I am guessing my iron needs to be calibrated.

I use Hakko 888D with 2mm chisel tip and I like it. Should I consider Hakko 951 or something else?

Second - pulling SOIC by one side is just making pads even more easy to damage. Keithley PCB have nSMD pads, usually connected just by thin trace without teardrops. I had number of times when pad and trace looked fine even under microscope, but actually were broken apart right at the point where pad connected to the trace. Searching for such faults wasted lot of hours.

Just in case I will do a very careful inspection of pads after cleaning and recheck vias in the area.

Top shield is same on 2001 and 2002, but there are two versions if them. One with cutout and duct for fan, and older one without cutouts, sealed type. If I remember correctly, they even have same partnumber. So I'd just order 2001 shields and be done with it . Make sure you order both top and bottom parts, and don't forget the mounting screw! It's not included with the cover. 

Part numbers seems same for the bottom shield but different for top shield. Do I still need to buy shields as a pair, if I already have a bottom shield? Also I do not see the screw on their part list and also suspect I may need the LEXAN SHIELD, CHASSIS LEFT and LEXAN SHIELD, CHASSIS-RIGHT.

Thanks.

[nikonoid]

Another update on repair process. I removed all components affected by the spill. They were easy to identify under microscope. This is a full resolution image of the board after the cleanup. For general areas I used canned air with zero oils in it (MG Chemicals super duster 134). Areas with the spill damage were cleaned multiple times with 99% IPA. IPA was not able to remove all the board discoloration. Some corroded pads were scratched a bit with solder wick and re tinned.

I left some dirt around R319, as I was not comfortable desoldering these components, esspecialy precision resistance network. I might try to clean around them with IPA later.



Remarkably only 3 traces were compromised by the burn. Additionally several traces around C116 were eaten away completely.


I took several pictures of copper and component side of analog board. I used light painting technique described by TiN and lit the board from behind to see internal traces. Two images below had been aligned, and flipped in Photoshop to closely correspond to one another. This way you can see traces, including most internal traces, and switch from copper to component side without loosing what you are tracing, but just hiding/un hiding layers. Maybe these will be useful for someone.




Finally on following two images I colored traces important for my repair. Red is the Component layer, blue and yellow and following internal layers and yellow is the copper layer.



I already started repairs and faced few challenges. My original plan was to solder copper transformer wires to micro vias to fix broken connections. All of my attempts to solder to micro via failed. I even tried to excavate the via a bit by removing a bit of PCB material around it under microscope. So far around capacitor C116 I had to reconnect vias to traces missing .3mm to 1mm of copper. The only way I was able to reconnect them was by forming a "solder bridge" between them. Testing showed them electronically connected and mechanically sound. Any attempt to use copper wire for that failed. While solder bridge solution is far from ideal I would like to hear some opinions.

Any advice on how to solder to micro vias? If I cannot solder to micro vias, 3 traces that were broken will have to be repaired by wires stretching half the way though entire board. Thanks.

[WastelandTek]

First things first; great work!  The effort you have put in really shows.

As far as getting the transformer wire to bond; what are you using for flux?  Are you able to tin the wire first?  It seems odd that it is refusing to bond.

[TheSteve]

Is the solder not sticking to the via or the wire? The enamel on the wire can be a pain.

And I agree with WastelandTek - great work so far, I really appreciate the post detail. I have no doubt someone else will be helped by it in the future.

[nikonoid]

I scraped and tinned the transformer wire first. I used Kester 951 low solids flux pen.
I used same flux on the via.

Via might be corroded with electrolyte or maybe it is just too small and surrounded by solder mask that repels solder. Maybe the gauge of the wire is too big for the via? I just know that I had been trying for a while without good results.

[WastelandTek]

You might try some no-joke RA flux, I use MG chems 835, pretty aggressive stuff.

PM me your zip, if you're local I have a litre of it...lol

[nikonoid]

Thank you. I will give it a try.


Sent from my iPhone using Tapatalk

[ManateeMafia]

How about some soldering tip tinner? That may help remove any oxide on the via.

[TiN]

Why even bother with transformer wire? Just get piece of regular multi-strand UTP cable (LAN RJ45 cable) and take a strand from there. It will be bare copper metal, which solders like a champ.
When I was fixing my first 2001, I just heated via from the bottom side with the iron set to +240c, and poked a copper strand from top side till it goes thru and sticks from the bottom.
Then you just cut the top remaining wire and solder it down on top as well. Takes just a minute once you practice a bit. if via on either side peels off, you just use same strand to extend to the PCB trace track and connect there.
Don't use solder only bridges, as solder != copper tempco (%0.4/C!) which might be important for analog traces!

[martinr33]

Another good alternative is wire wrap wire - solid core, 30 AWG, silver plated, Kynar insulated.

If your solder is not taking well, you probably have contamination on the exposed traces. The capacitor electrolyte has combined with the copper to create a compound that is hard to solder. Your solder bridge is probably making contact in tiny areas, which is no good. I would clean the solder off, and look to see how much of the underlying trace is tinned.

This is a precision instrument and the board is sensitive to contamination from cleaning products. Or finger oils. Or just being in open air. So whatever you use to clean it, pick something that is easy to wash off. I'd start with white vinegar, being very careful not to get it on the rest of the board - use a damp rag. On really bad corrosion, I use MG Chemicals tin plating solution. As well as dissolving junk, it plates a thin layer of tin onto the copper - but you have to clean it off carefully. The plated tin makes the trace easier to solder.


On your question about standards and the 732a - 8.5 digit meters are quite noisy in the last digit or two. A 732a gives you a stable, quiet voltage so you can see how much noise your meter has.

My K2002 has an SD of anywhere from 300nV to 1.2uV on the 10V range with shorted inputs, depending on NPLC and filter settings, and other variables like time of day. My Advantest units do 200nV - 700nV.I'm bringing up a 732a now - it is nice to see similar noise from the 732a 10V output as I do on a shorted input. And the two Advantest units I have hooked to it will differ by anywhere from 1 - 5uV. Now, I am using rubbish cables but the temperatures should all be stable by now.

Good luck with sorting out that board - it's worth the effort, the K2002 is a good meter. You also have one of the first run of units there.  Post if you need help with the EPROMS.

[macboy]

Another good alternative is wire wrap wire - solid core, 30 AWG, silver plated, Kynar insulated.

+1 on this. It should be the first choice, not an alternative. The strand of cat5e cable would be an alternative to this. You can straighten it after untwisting by grabbing a length of it and pulling it until it just gives a little. The copper will then be perfectly straight.

[nikonoid]

Guys, thank you very much for suggestions. It took me few days to find time to try them.

First of all I got a cheap iron tip thermometer from ebay, calibrated my iron and used it at 250C.

You might try some no-joke RA flux, I use MG chems 835, pretty aggressive stuff.

I had MG chemicals 835, both older bottle (stuff is more like a paste by now) and fresh bottle. It helped in few places, however did nothing for vias affected with electrolyte. Tip cleaner worked better there. Used it very sparingly with a tip of a toothpick. It looks cool under microscope.

I bought the silver plated Kynar insulated 30awg wire and I am very impressed by it. I removed solder bridges and made all the trace repairs with this wire. My multiple attempts to get the wire to go through the hole did not work. Maybe a wire is too large for microvias. Nonetheless I was able so  to solder it partially going in. Pictures are coming.

I will have to try TiNs "going through the via" suggestion another time.

In the end I had to do two trace repairs and 3 bodge wires. Luckily they all are power related, not measured signal.

It took 8 hours to repair the board and solder all ICs and capacitors back. Electrolyte compromised areas are not fun to work with.     

After this I partially reassembled the meter for a quick power up test. It came up without a hitch. As I was shooting a thermal image of it, I noticed a diode CR119 getting really hot. It was shorted and somehow I missed it on initial check.
 
I did not have a replacement handy for this BAV103 250V switching diode. I had a 600V zener diode in a similar package (BYD57J), so I used that for a quick test. Proper part will be in next week.

With this zener unit powered up with thermals looking much more reasonable. Small U108 regulator (78L05A) was getting to 180F, but apparently it it is specked to at least 250F.

This time I did the open inputs test and got errors:
306.1, 306.2 ohms current source
308.3    2M Ohms overload
400.5    +4.48V DAC output
401.1    Regulator (not sure what this is yet).

My next action item is to check all voltage rails before tracking these individual errors.
Thank you everybody for your your help this far.

[TiN]

That's it, no more errors? Looks too good to be true

[Kleinstein]

The DAC  error and the 401.1 regulator error might be hiding more to come. AFAIK the DAC is used extensively in self-tests (that't the main purpose of it) so some self test might not really work right now.

The list is rather short and the first two errors might even be related. Still I would start with the other two errors.

[martinr33]

For corrosion repair you want this stuff:

http://www.mgchemicals.com/products/prototyping-and-circuit-repair/prototyping/liquid-tin-421

I use it for battery contacts. First application dissolves off the crud,  rinse, apply again for tin plating. The plating is rough  ( not shiny) but it works well, and readily takes solder.

Well worth having a bottle to hand.

[nikonoid]

Shorted diode replaced. New round of testing shower problems with power rails.

Repair manuals does not clearly define some testing points. They refer to some diagrams like 2002-100 but I cannot find any of these diagrams available. If someone has it, please let me know, otherwise I might have to produce my own. There are lots of unmarked test pads in the [power supply section, but no description. Attached is a very rough draft. I am not very sure about difference between COM, Isolated Common and Digital common.

As per power supply testing procedure (also attached) step 3 (U108) gave me 15 volts instead of 5. I think I installed a right part (LM78L05ACZ) and in proper orientation. By the way this is also a part that is getting really hot. What could it be?

Step 4: U110. I am getting 1.4V instead of +5V

Then +15 and -15 are just fine and bootstrap is a bit high 38.9 and -38.8.

U105 is producing something completely wacky with with 1.4V and -7V

And last two steps (11&12) again completely fine.

U108 was replaced by a new part, the rest of regulators are originals. What would be my next thing to check? I am especially puzzled by getting 15V instead of 5.

Thanks a lot.

[alm]

Maybe the silkscreen is wrong? Check the voltages on the pins of the 7805 and compare with the datasheet.

[nikonoid]

Another update.

For power test 9 and 10, Voltage on the U105 is actually fine. Before I used a wrong point as an Isolated Common (it is not in the manual). This time using a negative leg of a bridge ratifier CR104 did the trick.

Power test 3 is actually a mistype in the repair manual (yes, another one). Sink screen is fine, but the manual was supposed to refer to Pin 1 not Pin 3 for 5V measurement on U108.

After these measurements I reran the self-test and only saw errors 306.1 and 306.2. Somehow other errors disappeared after diode change!

However this was only an open inputs test. I reconnected all the inputs and ran the full test. This time I got more variety of errors.
306.1/306.2
500.1/500.2/500.3/500.4/500.6/500.7     basically current measurements are shot
501.2
600.2

Regular spot checking of measurement functions showed that voltage works reasonably well, 2W/4W ohms work on everything but 2MOhm range, Amps do not show anything resembling input signal. It is almost like the amps path got blown somewhere. 


PS. I just realized that I forgot to recheck Power Test 4 results. This will be a task for another day.


 

[TiN]

Stupid questions: Is the current fuse ok, and did you short HI input to AMPs jack before going with current test?
Btw, what's the FW rev?

[nikonoid]

Yes, I shorted Input Hi to Amps and I did check fuses. 
I also checked both front and rear connections in case of a switch problem. For the front I was shorting directly into solder pad on the board as my white wire for current is still de-soldered.

I got exactly same list of errors for front and rear connections. It sounds like some sort of protection on amps input is blown. There is also another possibility: when meter is going through tests I can hear that one of relays is making a strange sound rather than regular click. Maybe a have a bad relay that is responsible for switching in current?

Also I tried to measure resistance between input low and amp as I am going through different current ranges. It did change I think from about 2 Ohms to 90 Ohms, but was fluctuating. Tomorrow I am going to compare it to resistances I get from my other 2002. There is definitely a benefit from having multiples.

The firmware in the meter is A5. If you like a copy, I have a programmer and can load it in before upgrading the firmware.

[TiN]

My older 2002 has A05, thanks. I'm still after miracleous B-version firmware for 2002 (saw once at ebay unit with B02).
Resistance from DCI jacks on my 2002 is:
200uA = 1.0194E3
2mA = 119.636
20mA = 14.5467
200mA = 1.2568
2A = 0.3491

[nikonoid]

Thanks, TiN.

I actually got somewhat very similar numbers on both working and broken 2002. The only exception is that on the broken one they seem to fluctuate rapidly by 1-2%. It was a very quick test and I might have to redo it.

Overall resistance switching seems to work ok. Then in a pinch I used a different meter in a resistance mode to serve as a current source. When I was pocking around all of the sudden current started to work then stopped.

So far it appears that proper voltage is not reaching Pin 3 of U225 (unity opamp LT1097S8, incorrectly marked in the manual as V425 instead of U225).

My next stop is to check for compromised vias or solder pads on the way to U225, especially considering that it was heavily compromised by electrolyt and replaced by me.

As promised, here are photos of the unit after repairs. I am considering shortening bodge wires later, however I will have to get a shield first before I decide on the final route for them. I am considering hot melt glue or E6000 glue to semi-permanently hold them to the board.

[Le_Bassiste]

gents,
i need some clarification on your current (Amps) input findings.  were all those measurements taken from outside, i.e., using AMPS  and LO input jacks?
if so, then it's little wonder that measurements are fluctuating. the internal spring loaded white cable "pigtail" connector on the fuse holder is just f* crap.
some intensive IPA cleaning of the fuse holder interior and fuse itself helps alot.

[TiN]

In my case, yes, outside AMP + LO jack. I'm not taking apart 24/7 online 2002's anytime soon, unless they go kaboom  .
I didn't see much fluctuation (random 3458 in reach was used to measure resistance), but again I don't ever remember using 2002 to measure current, other than during calibration  .

[nikonoid]

Le_Bassiste, for rear testing I was using rear terminals (from outside). For front I was using common (Input Low) from the outside and the hole on the PCB where the white AMPS cable goes to. I do not have it soldered yet. So I was bypassing the fuse and fuse holder.

This "fluctuating reading" behavior comes and goes, so I cannot yet pinpoint what is going on. One possibility is something going wrong with control signals that switch in different resistances. Maybe I get a behavior when specific resistor is not completely turned on.

I found what looks like a bad via where the signal after chain of resistors is traveling to pin 3 of U225. I may have to learn how to copper stitch vias. After a did a quick fix to bridge that via, the situation improved just a bit, but not much.

As I am pocking around the miter it behavior seems to change. It could be a mechanical effect or thermal, as I had with a previous K2002. There I had a bad IC switch that depending on a temperature would sometimes be partially closed.

With me fixing that one via the test 500.1 is now passing most of the time, while others still fail. After the fix, the current function seem to work to a degree, it is couple of %s off on 200uA range, little bit more off on 2mA range, and so on with 1A range being about 40% off.

I was tracing the U234 chip that controls 2mA and 200uA switching and found that while 3 channels would get 0V or 5V on their control inputs, one channel got -3V on it, so now I will have to check where control signals are coming from.   

So we are basically down the rabbit hole....


 

[nikonoid]

Today I found another bad via. After fixing it, I am down to only TWO test errors 306.2 and 600.2   

Current passes the self test, but it is working strangely. With shorted inputs I get:
200uA range: -000.0461 uA
2mA range:   -0.013751 mA
20mA range:  -00.03802 mA
200mA range:  -001.7962 mA
2A range:   -0.018315 A

If you take this offset into consideration, then DCI reading works pretty well.

I will trace the problem with Ohms next and get back to current later.

[Kleinstein]

Usually the test for zero current would be with open inputs. For a classic solution with shunts this should not make a significant difference though - it can be a big factor for a configuration with TIA (for low currents).

The data look like quite some offset - it might be just a question of adjustment / calibration, but could also be a problem with amplifier offset or similar.  Before doing any adjustment / recalibration, the rest should be working of cause.

For the Ohms source, one could externally measure the test current.

[TiN]

Any update? Can't want for next episode of 2002 repair. 

[nikonoid]

TiN, thanks for checking on me. Yes. it had been a long saga.

I had been stumbling for a week, trying to figure out a problem with 2M Ohm range (Test 306.2). This is also the same range that I had a problem repairing first 2002.   

What I know so far is that on 2M Ohm range I get between 0.5uA and 1.8uA of current between Input High and Low, instead of 1.92uA. In many cases it starts at 0.5uA on warm up and within minutes would get to 1.5uA, then to 1.7uA and fluctuate from there.

When I check voltage drop across R271, I get 1.42V instead of 1.44V in manual - close enough and R271 being 750k it corresponds to correct current.
When I do the same thing across R322 - 33 Ohms I calculate the current that corresponds to what I have on the "Input High".

Somewhere in between we seem to lose some current. From reading the manual in between these resistors we have Q211, Q212 and Q213. My best guess is that either one of them is at fault or a circuit that controls one of them. I am having hard time tracing the circuit in that area.

Any suggestions are welcome. Thanks.

[TiN]

I'd suggest to reverse ohm schematics, for our all enjoyment. Then it will be obvious what currents are expected.

[nikonoid]

The process is slow. Attached images are as much traces as I was able to redraw so far.

Meanwhile I have some additional information. 306.2 error seems to be highly temperature dependent.
I plugged 100K Ohm standard resistor and used 2M Ohm range. When unit is cold it would start reading at 20k, then in minutes gradualy rise to 65k, in an hour the reading went up as much as 95k. At that point when I reran self test every OHM test passed successfully, including 306.2.
If I let the meter cool off a little bit is goes back to reading 80k, if it cools a lot is goes back to 20k.

So the issue appears to be stable, repeatable and temperature related.

I compared voltages to working Keithley 2002. The performance is identical for all pins on:
U220  DG411
U221 MC14094B
U223  MAX326
U228  AD711
U231  AD706
U232  MAX326
U233  AD705

Where I get the difference is:
U238 pins 2,3,5 and 6, respectively
0.787  0.787  0.788  0.859  on the bad meter
0.476  0.476  0.476  0.539  on the good meter

Q210
10.159  0.785  0.785  on the bad meter
10.005  0.475  0.474  on the good meter


Q211
12.792  10.198  0.787  on the bad meter
12.694  10.174  0.475  on the good meter

Q212
12.809  12.792  0.785  on the bad meter 
12.707  12.698  0.475  on the good meter

Q213
10.155 0.786  0.784  on the bad meter
10.011 0.475  0.469  on the good meter

Q214
0.000010  between (63mV and 125mV) 0.004  on the bad meter
0.000009   0.001  0.004 on the good meter   

The major difference is the Q214 transistor and 0.786 voltage instead of 0.475 voltage.

It is almost like performance on one of these transistors (Q210-Q214) is changing with temperature. I do not yet completely understand the role these transistors are playing.

Can someone please suggest a next thing to try?

Thanks a lot.

[TiN]

I'd remove DG411 near diodes, remove two diodes (big ones at right), remove 1Meg resistor in metal case, clean everything thorougly with IPA.
Once you done cleaning, dry it. Clean again. Then carefully put new DG411 in, clean 1Meg resistor, clean diodes, put them in.
Test again If same, buy JFETs and transistors to replace Q210-Q214, and iterate again.

[saturnin]

Before starting replacing parts using "try and error" approach (which you can use as a last resort anytime), I would suggest to analyze how ohms circuitry works actually...

In K2002 Repair manual, you can find quite detailed ohms circuitry description (see info on tests 304.1 and 306.2). You have already tracked multiple connections on the PCB, so put these information together and try to draw a partial schematic of ohm circuitry. Then, it should be much easier to understand how it works and find error...

[Kleinstein]

Even if the voltage reading seems to work, it might be worth checking for input bias, as too much input bias could be an issue that can also effect the high ohms readings.

Having it working better when hot, somewhat points to a problem with dirt / electrolyte / flux residue. This would be one of the very few cases where current leakage would decrease when hot (as humidity will go down).

The suggestion from TIN is thus not so much about replacing parts, but cleaning in a area that might be influenced be leaking caps. A new DG411 is just more convenient / easier than a really good cleaning of a part that might already be damages from unsoldering.

A little more understanding of the Ohms circuit would really help. Having an idea from the manual could help a lot here. One would at least know if the control of the FETs should be OK.

As the 2 M range is off so much, a leakage current of that size should be visible in the 200 K range too. Not that obvious, but still well higher than uncertainty specs. One could at least check if the missing current flows through the resistor for the 200 K range.

[nikonoid]

Guys, thank you for the feedback.

I think D411 next to metal 750K resistor is actually part of current measurement, not resistance. I will try to verify it. The actual board is much cleaner now than what you see in the picture. I even have a replacement part for it. However it was so close to precision network, I was not sure if I can remove it without accidentaly touching it with soldering iron. I may have to try if everything else fails.


I just ran few more tests. My resistance standards are not the greatest, some are +/-100ppm, some are +/-20ppm.
2k range off by 30ppm
20k range off by 150ppm
200k range off by .25%
2M range off by 2.5% this is after 1 hour warm up. It is much worse before warm up.
20M range off by 30ppm
200M range off by 30ppm
1G range off by 40ppm

Seeing that performance improves at 20M, I doubt it is leakage that is responsible for the problem.

I also checked bias voltage with 10M resistor and got -0.3mV

I see the pattern with two parallel diodes pointing in opposite direction is used in at least 3 places:
CR210, CR211   (by that DG411)
CR212, CR213   (by large rad COTO relay)
CR214 - actually 2 diodes in one case (by large rad COTO relay)

I think it is used for some sort of voltage clamping. Would someone know what exact use is? Thanks.

[saturnin]

Both 200k an 2M ranges use the same setup of the ohm circuit. The only difference is 7.2V appears across R277 (=750 kOhm) for 200k range, which gives 9.6 uA and 1.44V appears across R277 for 2M range, which gives 1.92 uA (see description of 306.1 and 306.2 tests). According specs these currents have absolute tolerance of 5%. R277 has 1% tolerance, so voltages across should be 4% close to nominal values (this is a rough estimate).

I think the cause both 200k and 2M ranges are way off is the same. Do 200k measurements drift upon warm up as 2M measurements do? Since high ohm ranges (20M and above) are basically spot on, I would not suspect surface contamination by electrolyte (it would have to be very localized to affect only these two ranges).

At first, I would check stability of voltage across R277 for 2M range  (has to be very stable). Is it stable or does it change? If it is changing, does the change correlate with change in 2M readings? From R277 I would follow path of the constant current and draw a partial schematic to get ideal where the current could be leaking...

[macboy]

Just thinking... Monitor the control register outputs (which drive the various relays, analog switches, and FETs) while manually switching between 20k, 200k, 2M, 20M ranges. You will be able to determine which switches or FETs are used differently for the different ranges, and hopefully narrow in to specific FETs or switch input-output combinations only used for 200k and 2M ranges. Then look for leakage in or around those components.

[nikonoid]

Today I measured 200k and 2M ranges for cold meter.
For 100k resistor on 200K range I got stable 7.11670V across R277 (750MOhm) or 9.49uA - within 1.1% off the target - good.
I got 295uV across R322 (33Ohm) or 8.9uA - 6.2% off target not so good. Resistor read at 96.8k

For 100k resistor on 2M range I got stable 1.42343V across R277 (750MOhm) or 1.9uA - within 1.1% off the target - good.
I got 35uV across R322 (33Ohm) or 1.06uA - almost 45% off target horrible. Resistor read at 55k instead of 100k.

So far it looks like we have a proper current on R277 and loose it by R322. There are only few transistors inbetween.


I also liked the idea with similarity of 200K and 2M ranges. Today I retested the meter cold instead of hot, here is what I got:
2K range off by 160ppm
20K range off by 2200ppm
200K range off by 3%
2M range is off by 40%
20M range is off by 30ppm

So really the problem is not on the 2M range, but on every range below it too. It just gets smaller on lower ranges.
It is just small enough on 200k that self test error is not catching it.


Now this also reminds me of the current problem I have. When measuring 0 current I get:
200uA range: -000.0461 uA
2mA range:   -0.013751 mA
20mA range:  -00.03802 mA
200mA range:  -001.7962 mA
2A range:   -0.018315 A

So far I can see one similarity all failing modes including current should have U221 pin 4 set as 1, and 20M, 200M and 1G have it set as 0. I previously could not trace what that pin controls, so there is more work to be done.

As always suggestions are very helpful and welcome.

[saturnin]

Today I measured 200k and 2M ranges for cold meter.
For 100k resistor on 200K range I got stable 7.11670V across R277 (750MOhm) or 9.49uA - within 1.1% off the target - good.
I got 295uV across R322 (33Ohm) or 8.9uA - 6.2% off target not so good. Resistor read at 96.8k

For 100k resistor on 2M range I got stable 1.42343V across R277 (750MOhm) or 1.9uA - within 1.1% off the target - good.
I got 35uV across R322 (33Ohm) or 1.06uA - almost 45% off target horrible. Resistor read at 55k instead of 100k.

So far it looks like we have a proper current on R277 and loose it by R322. There are only few transistors inbetween.

It seems at least the core of the constant current source is working properly.

So far I can see one similarity all failing modes including current should have U221 pin 4 set as 1, and 20M, 200M and 1G have it set as 0. I previously could not trace what that pin controls, so there is more work to be done.

If U221 pin 4 is not set according the manual, I would definitely check what input data go to U221. U221 might be faulty. Btw. Don't you mean rather pin7 (Q4 signal)?

[TiN]

Is there any reason why you not willing to reverse engineer related circuitry schematics? I'd see that much more beneficial, than guessing correct/incorrect voltages/currents around parts.
There are not lot of parts involved, so I'd expect that to be just few evenings effort. You don't need to trace all back to 4094's, just up to LM339's would do just fine.

[nikonoid]

Is there any reason why you not willing to reverse engineer related circuitry schematics? I'd see that much more beneficial, than guessing correct/incorrect voltages/currents around parts.
There are not lot of parts involved, so I'd expect that to be just few evenings effort. You don't need to trace all back to 4094's, just up to LM339's would do just fine.

TiN, the reason is simple.... I simply do not know how to draw circuits. I have never done that. It is on my TODO list, but so is many other things.

I know how to use Photoshop, so I used that to reverse engeneer connections to the best of my abilities.

If someone skilled at drawing circuits can work together with me on that that would be great. Anyone interested?


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[TiN]

Piece of paper with squares and pencil will work just fine.
Then scan it and I'll be happy to digitize it for community happiness.

[Kleinstein]

For reverse engineering the first drawings are often on paper, especially if the circuit is rather simple. There are a few programs to draw schematics, like LTspice, Kicad or Eagle (e.g. free limited version).

 As the Ohms circuit uses extra diodes, chances are the circuit is similar to that of the 3458. Also the Keithley 2001 might be similar from the general layout, though it is different from the usual path. The current sources are usually not that different. There are a few choices, like using CMOS MUX or a set of JFETs. A second point is the question of switching the resistors at one side only or at both side - for a high quality meter I would expect switching both sides, especially because they use the extra diodes.

[nikonoid]

Here is what I have so far as Ohms schematics is concerned.

I have traced it all the way from R277 resistor to Input Lo and path to Input Hi (R322 and on).
I found what is controlled by U221: Q1, Q2, Q3 and Q4.
You will see that some connections are going into nowhere, that either means that I have not traced them yet, or they don't seem to be relevant for Ohms understanding.

It is collector of Q212 that is at .4V on working meter and .75V on broken one. Let me know what you think.


TiN, let me know if this is enough for you to start working with. The layout can definitely be improved from what I did by hand. Thanks.


PS. I have also been compiling a file with all mistypes/mistakes I found in the repair manual so far.

[Kleinstein]

There are still a few open points in the partial schematics. So it is a good start, but it needs a little more.

One interesting area would be the upper end of R277 (750 K). Another part would be how the other current setting resistors (one more would be enough) are connected.

There also needs to be a kind of path from the lower side of R277 to an OP ( could be U233 Pin 2)

There seems to be a connection missing from the emitter of Q210 to something like the source of Q213.
I am a little confused there is no diode in series with Q213, though is might be somewhere following R322.

The voltage at the collector of Q212 depends on a few non critical parts (e.g. the diode mentioned above) and also on the input connection. So I am not sure this would be a significant difference.

[nikonoid]

There are still a few open points in the partial schematics. So it is a good start, but it needs a little more.

One interesting area would be the upper end of R277 (750 K). Another part would be how the other current setting resistors (one more would be enough) are connected.

There also needs to be a kind of path from the lower side of R277 to an OP ( could be U233 Pin 2)

There seems to be a connection missing from the emitter of Q210 to something like the source of Q213.
I am a little confused there is no diode in series with Q213, though is might be somewhere following R322.

Thank you Kleinstein. I am working on version 2.0, so feedback like this is very helpful.



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[TiN]

What is Q211 device? You sure it's 4-terminal FET?
Also mark all precision resistors and route them both ways, that would be great.

We would need to know how resistors are selected/switched on various ranges, to make it all meaningful.
All that happening thru pair of MAX326 low leakage switches U223 and U232. I'd expect arrangement is similar to one in 2001, just with better parts.

[nikonoid]

Yes, Q211 is a FET with 4-terminals. Two of them are shorted together. So I guess it is happening on outside of the can rather than on inside.
I started tracing precision resistors. It is a little tricky. They connect to all 4 channels of max326 and also to four diodes above them. Then these diodes connect to all four channels of dg411, next to max326.
I cannot visually see these connections in my "X-ray" photographs. So tracing them one by one is very slow.

I also so far cannot find where the output of u228 goes. It is likely finding its way to these resistors. I just cannot find it yet.

TiN, what software are you considering using to draw this?


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[Kleinstein]

connecting the resistors to diodes + DG411 and max326 makes kind of sense. This is one option to do the switching: The DG411 and the diodes to carry the current and the max326 to connect a sensing amplifier.

So the overall type would be rather similar to the circuit in the HP3458 and not very similar to the Keithley 2001. Only the protection part would be more similar to the K2001 (using high voltage FET and PV opto-coupler instead of cascaded PNPs).

p.s.:
Possible leakage paths to cause the current to be to small could be:
1) the diodes Cr212/SR213 , it might be worth checking the guard potential for this.
2) the guard amplifier - easy to check via voltage across R294.
3) The P-channel 4 pin MOSFET - might be visible at R290
4) U233 (OP) - that is likely connected to the 750 K lower side.
5) backwards through other precision resistors (for some on could check at the upper end).

[nikonoid]

Kleinstein, I did find the connection from U233 pin 2, missing link from Q210 and also diode after resistor R322. All of your comments are right on the money. New version of schematics are coming in a day or so. Thanks.


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[nikonoid]

It took quite some effort and now I have a fairly comprehensive schematics (attached).

I did not bother to draw power to any ICs here. Power is fairly obvious.
I traced Q1-Q8 bits of U221 and also found some things controlled by U203. This is far from perfect, as I still have few points that I cannot trace, like pin 1 of U231 of few resistors next to it. There is also some connection from input divider to Q226.

Still, I think I got most of the functionality. I am asking for help to see if I made any mistakes. Maybe even compare schematics to some clues from repair manual, to see if I got them correctly.


On the topic of fixing my meter, I also measured voltages across many resistors, like Kleinstein suggested. Current is correct on R277 and wrong on R386. It appears that some current is leaking through precision resistors, especially R272. It is loosing .5uA there and that is close to the total amount lost. I am still not sure what specific component would be responsible for this leak.

See the second attachment for voltages and associated currents. My measurements were done on a cold instrument with 100K resistor in the 2M Ohm range.

Thanks a lot.

[saturnin]

It is much better now! Good job Although there are still some missing connections to important nets/pins (U231-3, U231-5, U228-3...), it is not necessary to track them completely - it is enough to measure voltages at these points. It will tell you a lot about circuit function...

As concerns the leakage, check U228 works properly, i.e. there are same voltages at its pins 3 and 6. If so, CR208 or the switch connected to R272 may leak some current. (When 2M range selected, there should not be any significant current flow though R272, definitely not 0.5 uA...)

[Kleinstein]

The circuit plan so far looks reasonable - At least I don't see an obvious mistake. There is some connection missing around R276 / U232 pin 2. probably a connection to a fixed reference voltage (around 7 V).
The still open connection of  3 of U231 mit be towards pin 5 or 7 of U231. At least this would be a logical target.

Leakage through R272 has only 3 ways to go: dirt on the board (e.g. below U223), the diode or the CMOS switch (U223). Checking the diode path it likely easier, up to the point of de-soldering the diode. U228 is likely OK since otherwise there should have been even more leakage trough the other resistors. It would be more like the 100 K resistor open or U221 Q6.

[nikonoid]

Kleinstein,

You are right on the money again. I found Pin 3 to Pin 5 connection for U231.

I did not have much time today to troubleshoot the circuit. I did check all 4 100k resistors in the vicinity of U220. I got readings of about 98k for all of them. For 100k resistor connected to Q251 I got 83k. I guess there is some parallel circuit somewhere.

I also checked how much current is flowing through R270,R271,R272 and R277 on working meter and it is quite a different picture. I only got between 0 and 0.003uA there.

I measured all four diodes (in circuit) and got pretty analogous readings between meters: 0.66V in one direction and 2.5V or 2.6V in the opposite direction.

Not sure what to check next...

[Kleinstein]

The 100 K resistors don't need to be accurate, they are just there to cope with any leakage from CMOS switches above. One should measure the voltage on both sides of CR208 - ideally it should be very close to zero voltage over that diode, unless the resistor is chosen as the active one. The voltage should also very close to the point where the 4 precision resistors join together. As there is some extra current through the resistor, the polarity of the voltage over CR208 should tell it the current flowing this way (e.g. leakage in the diode) or the other way (through the other CMOS switches). In the 2nd case is would be de-soldering the switches U223: as it would be either that chip, or maybe residue - likely under it. Likely a new chip for U223 is than needed - as there is quite some risk it is damage (either from the beginning or when removing it).

If it is current through the diode, one could for a test remove CR208 - trouble is than more like with the other CMOS switches (U221-Q6).

[TiN]

Prettified nikonoid's handiworks just a bit. Time to get back with pensil again

PDF-version.

EDIT: Bugfix ver3.1

[Kleinstein]

I think TIN got a mistake around Q213 / Q210. The hand drawn circuit looks much more plausible. I also doubt that Q213 would be a VN0605 - it should be more a high voltage (e.g. 1200 V) MOSFET, as it is for protection against high voltage in ohms mode.

VR216 should be either the other way around or a zener diode.

[TiN]

Fixed a bit. I think "wrong" direction for opamps cause the perception rejection  . That thought crossed my mind...
Usually it takes 2-3 redraws for clarity once more blocks filled in. Anyhow still hopefully better than K2001's schematics, which are pure joy.

[nikonoid]

TiN,

Thank you for doing drawings. It is a lot of work.



Q213 is a "2SK1413" not "25K1413". That is another mistype in the repair guide.
I am attaching a list of errors I found so far.

[nikonoid]

The 100 K resistors don't need to be accurate, they are just there to cope with any leakage from CMOS switches above. One should measure the voltage on both sides of CR208 - ideally it should be very close to zero voltage over that diode, unless the resistor is chosen as the active one. The voltage should also very close to the point where the 4 precision resistors join together. As there is some extra current through the resistor, the polarity of the voltage over CR208 should tell it the current flowing this way (e.g. leakage in the diode) or the other way (through the other CMOS switches). In the 2nd case is would be de-soldering the switches U223: as it would be either that chip, or maybe residue - likely under it. Likely a new chip for U223 is than needed - as there is quite some risk it is damage (either from the beginning or when removing it).

If it is current through the diode, one could for a test remove CR208 - trouble is than more like with the other CMOS switches (U221-Q6).

Kleinstein,

I measured the voltage across diodes (100k resistor on 2MOhm range)
CR206:   .41V
CR207:    50mV
CR208:    .1mV
CR209:    -4.6mV

On a good meter I got:
CR206:   .41V
CR207:    -0.26mV
CR208:    -0.20mV
CR209:    -0.13mV

Broken meter seems inconsistent in terms of both magnitude and sign.


I am not sure though what these numbers are telling me, though.

PS. I have a spare DG411. I am ordering MAX326 now.

[Kleinstein]

The positive (though small) voltage over CR208 tells us, the 0.5 µA leakage current is not flowing through this diode (would be the other direction). The 50 mV for CR207 indicates there might be a problem it that path too (or maybe the diodes / readings are interchanged: 50 mV across the diode at the 75 K resistor would make more sense). Anyway this would point towards a problem with U223(max326), or maybe some leakage on the board around that part.

A slightly different voltage over the diodes for different meters is not a problem. A main contribution would be from the offsets of U228 and U233. Ideally the drop should be the same one all 3 non active paths. Differences would be due to leakage of U223 and less critical U221/U220.

[nikonoid]

This sounds right. Thank you. I mixed up the sequence of diodes. It should be the opposite sequence. The max326 chip is in the mail. The surgery is scheduled for Wednesday. 


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[nikonoid]

Today I replaced the Max326  (U223) with a new one. The situation did not get worse.... and also did not get better. The board underneath U223 looked clean. I cleaned it more anyways.
I am still getting same errors: 306.2, 500.6, 500.7, 600.2.

I guess next step is to desolder diodes. Any other ideas?


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[Kleinstein]

The direction of the leakage current is in a way that it can't flow through the diodes (higher voltage at the other side), unless there is massive photocurrent. So unless there are other ports connected, what I somewhat doubt, the current must go through the max326 or maybe leakage on the board.

As the current is through the turned off 75 K resistor, it can't be the OP following the max326. It is slightly strange that the 20 K range (using the 75 K resistor active) also has a current more on the low side - so leakage is not between the two sides of the switch in the max326, but more like towards the negative supply.

One might want to check the supply of the max326 - a wrong supply might cause trouble. One might need to use a scope here to check for AC trouble. A wrong DC supply level should give different effects.
Also a borderline / oscillating / out of bound (e.g. to negative) signal at the control input could be a cause. Oscillation in the current regulation part is another possibility.

It would make sense that the 2 ref voltages for the ohms circuit are generated from the charge pumps (LTC1043) - so AC ripple there might be a possible source of errors too. To understand which voltages are needed for the supply the value of the two ref level would be interesting too (could be 14 V and 7 V or 21 V and 14 V).

[nikonoid]

Thank you, Kleinstein. I will recheck power on the chips this weekend.

I am now looking to get an insulation transformer, so I can safely do oscilloscope measurements.

[Kleinstein]

Measuring on the analog side of the DMM does not need an isolation transformer. The analog part is isolated from ground anyway. Even with the digital part one can get around the transformer, accepting slight possible noise from a ground loop.

[nikonoid]

I did a number of measurements comparing good 2002 that I have and a Bad 2002.

I used 100k resistor on a 2MOhm range. Most measurements were done relative to Input LO.
On Bad instrument ohm reading was at least 10% low, sometimes more depending on how warm the instrument was.
Please keep in mind that all values from good 2002 were stable, while many values on bad 2002 were drifting with temperature and since readings were taken at different time they may not correspond to each other precisely.

I compared values and highlighted any place where I found significant difference. Please tale a look at scans attached and let me know what you think. Pardon my horrible handwriting .

Some most notable differences include Q251, Q226, VR204, Q210 - Q214. Thanks a lot.

PS. I have not had a chance to get to measurements with oscilloscope. It will have to be sometime this week.

[Zucca]

 

PS: Excel next time?

[Kleinstein]

It would be much easier to have to voltages from both meters on the same sheet.

The voltages around Q251 look a little odd. One thing is the rather high supply (collector or Q251) with the bad meter. So there might be a slight problem with VR216. However this should not cause this problem it would be more a problem causing higher temperature at Q251 in the low resistance ranges. Still worth checking VR216.

Ar the pins in the same sequence ? It is a little odd to have the base slightly below the emitter. This might an indication that there is some AC super-imposed. If swapped the two values would make more sense. Similar a base voltage around 15.3 V instead of 14.3 V (this would also match U231 pin 1)

The difference in the voltage around Q213 and so one could be a minor difference in CR217 ( different type of diode, with higher voltage drop on the bad unit. This normally should be of no consequence - just in case one could temporarily have a second diode (like 1N4007) in parallel.

So far the only part really indicating a fault is the voltage over CR208 and thus R272. The polarity of the voltage indicated the false current can not flow through the diode. If at all the diode would add a few pA to the output current. So far I only see U223 (pin 10) for the current to flow through - somehow leaking toward ground.

The DC voltages around U223 (max326 - the one that got replaced ?) look good. Still a little odd they supply +-15 V to the chip, as there is no need for a high negative supply and this would only add to leakage. But it seems to be the same way on the good one.

I would really look for AC trouble (like a control loop that oscillates or maybe spikes on the supply).
One might learn a little more about the missing current, with testing in the 200 K range. This would change the voltage at the critical node.

[nikonoid]

Thank you. I will remeasure everything on transistors and make note of pins in case I made an error before. I also prepared an oscilloscope to do AC measurements.
I did notice before when working on power supply that bootstrap voltage was a bit high. As per manual it was supposed to be +34V to +38V and I was getting something like +38.6.

This comes to be the most difficult repair I ever attempted.

[TiN]

This comes to be the most difficult repair I ever attempted.

I bet you learned a lot already as result. 

If you have more schematics works, I'd be happy to digitize as well., 

+38.6 sounds fine to me, I saw up to +42/-42 few times. It depends on line frequency/voltage a little.

[Kleinstein]

The slightly annoying part is not having a schematics. But this is now there for the Ohms part  .

The only thing that might want a recheck would be the voltages around Q251. The protection part so far looked good. And just for the DC part, it can't be the reason for the current through the 75 K resistor.

Worst case Q251 would see some 7 mA (don'T know the extact max current) at 24 V , thus about 150 mW. So it could get a little warm, but no too bad and it won't help that much moving some of the heat to VR216.

[nikonoid]

This comes to be the most difficult repair I ever attempted.

I bet you learned a lot already as result. 

If you have more schematics works, I'd be happy to digitize as well., 

+38.6 sounds fine to me, I saw up to +42/-42 few times. It depends on line frequency/voltage a little.

I added few small details to schematic. I am trying to collect more of them before passing them to you. Meanwhile, if you or anyone else has any pictures of 2002 analog board with components off it, please send me these pictures. Even if it is not OhmS section, if would still be useful. I am putting together a drawing for analog PCB, and sometimes what is happening under microchip or even resistor is hard to decipher.

If I find time, I may also draw something for amps section, as it has problems on my meter too.

PS. The design of Keithley 2002 reminds me a way how some software developers write code. They managed to squeeze performance that is close to 3458 into a space that is less than a half of it, if you take space for multiplexing cards into consideration. It is Smart, highly optimized and agile, yet very difficult to understand and troubleshoot.


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[nikonoid]

Another small report. I remeasured voltage at all the pertinent transistors. I did it twice, once on a cold meter and second time after a bit of a warm up. It seems consistent with numbers I posted previously: Transistors.JPG is attached.

It seemed that behavior of cold unit vs hot is very consistent, so I measured the progression of Ohms measurements as meter warmed up. It was on 2MOhm range measuring 100kOhms. See KOhms_Drift.jpg attached. Horizontal axis is seconds.

Then I measured amps on 2A range with open inputs, again starting from completely cold meter. See mA_Drift.jpg.
It looks like these two problems could related. After all, the electrolyte spill from a location point of view affected both ohms and amps circuitry.

Seeing that performance improved with a warm up, I let it run for 1.5 hours and then I ran self test and it passed completely this time, where before I got some ohms and amps related issues.

I do not see much circuitry in common between amps and ohms. Do you think it is leakage on the board itself rather than bad components? I am considering replacing U234  (DG411) to thoroughly clean everything around and under it with IPA. Under similar considerations are diodes and 750k resistor.

What do you think? Any other suggestions?

[Kleinstein]

Leakage due to the spill is still a possibility. However this would be relatively localized around R272 and the U223 (the max326). I doubt that critical node would extend very far. It is more like there should be a guard trace around it - so maybe check that guard. Guard line are sometimes rather thin and might got damaged despite of the gold coating.

The other point to look for would be AC noise - this could be a shield missing / disconnected and thus just 60 / 120 Hz hum or maybe an OP oscillating or to much noise on the supply. Reading the 20 mV of AC at Q214 - which is essentially a high impedance node, suggests that there is some AC floating around. The reading of cause depends a lot on the probes - so hard to tell if 20 mV AC is a lot or little.

Except for the close proximity to the spill I see not much in common with the amps part. Especially the 2 mA range should not be that easy to disturb by leakage. So it might be worth looking at the amps part first - it might be easier to find the trouble there. An oscillating amplifier in the amps part might cause trouble with the ohms.

[nikonoid]

Kleinstein, I did a quick check of guard traces. They look Ok. I will spend a bit more time on it tomorrow. By the way, my traces are led covered - no sign of gold flash. Maybe too old of a unit.

Today I did a pretty thorough IPA cleaning with ESD safe Q-tip equivalent. I covered both amps and ohms section. Interestingly ohms did not improve at all, while amps improved significantly. See attached before/after charts.

As the AC noise is concerned, it went down a bit on Q214 to 11mV AC. I am measuring between input LO and collector.

I also measured AC noise on U225 (OpAmp in amps section) where I have a pretty ugly bodge (attached) and found them to be under 0.7mV AC.

By the way what would be a proper point to attach ground of oscilloscope to?

[Kleinstein]

The logical ground point for the scope is the negative (com) input. This could be either the plug or maybe a connected point on the board (it should not make a big difference at lower frequencies).

Unless very dirty a leakage in the 0.5 µA is quite a lot for just dirt. So it would need way more than just a missing guard. It would more like a dead short towards a guard that could cause that much leakage.
Even this would not match the measured voltages.

So I would either check the amps circuit first of look for AC (with the scope) on the supply and the critical points (e.g. outputs of U228 and U238) of the Ohms circuit. The control lines (e.g. U223 Pin 9 ) might be interesting too.

The strong drift in the amps part looks odd and could be easier to find. The circuit for the amps part should be relatively simple compared to the ohms part.

[nikonoid]

After the last post, I tried quite a few things. Let me list them somewhat chronologically:

I did detailed review under microscope of guard traces and also checked continuity of them and possible shorts between them and microchips - nothing suspicious.

Then I did measure a Drain on Q214 for AC component with oscilloscope. I placed scope ground on Common test post that sticks up next to voltage reference under ADC. I see some low frequency and high frequency components, but I am not sure if this is significant, or I am just picking up some unrelated spikes. See attached.

I let meter to warm up for 3.5 hours and remeasured voltages at att transistors involved. See the Transistors1.jpg attached. Interestingly the meter drifted closer to be in spec, yet some voltages are almost double of previous measurements of good 2002 meter. Is it possible that from 1994 to 1997 they changed not only some microchips to their equivalents but also changed operating modes?

Today, being out better options, I replaced U234 (DG411) and two diodes next two it. When I removed U234 I noticed that R277 was pushed to a side and down (not by me) and basically laying on some traces and vias under it. I measured about 10 to 20MOhms between either lead of the resistor and its metal body. I did not like that for a 750k resistor. I used a small screwdriver to push the resistor higher. Afterwards the resistance between leads and the body became greater than 100M. I thoroughly cleaned the area around and reinstalled fresh U234 and diodes.

After U234 change and this last round of cleaning around U234, performance of the meter improved quite a bit in both Ohms and Amps. Another positive is that it now passes all self tests, even when cold!

If comparing 1 hour warm up readings, it used to read 98.6k and now is reading 99.95. The phantom current on 2A range after 1hr used to be -950uA. After initial cleaning it went down to -337uA and now I get -23uA.

After 2 hours the meter is very close to be fully functioning. I get 99.985k that is no longer improving, +6uA reading on 2A range.

I am still not sure if I am dealing with bad component/components or dirty board. When I got the meter the board was dirty to the point that I was initially removing mice excrement and dead insects from it.

The temperature dependence of performance is quite puzzling. I am considering putting a gloved finger on microchips one by one, trying to find one that reacts to temperature the most. Also I can use cold canned air to cool different areas of the board.

What else can I try? How can I distinguish components trouble from board trouble?

[Kleinstein]

The drain of Q214 is not a good test point: it is an essentially floating point in normal operation. So the readings are essentially noise picked up by capacitive coupling and DC is determined from non critical leakage currents. I don't really understand why they have Q214 at all - it should have very little influence. I would not be surprised if the meter would even pass the self test if it is missing or broken (shorted).

A suitable point to check for a superimposed AC signal would be more like drain (or source - should be essentially the same) of Q213. Another interesting test-point would be the emitter of Q251. Checking the supplies (e.g. +-15 V for the DG411 / max326) for spikes would be a good idea too.

For the current part it might be interesting to see if there is some AC residual at the terminals when in the lowest amps range. Also the measured DC voltage in this range might give an indication. There might still be some cleaning needed in the path (e.g. upper end of highest value shunt) of reading the voltage at the shunts.

I still can't see a path from leakage around R277 towards the R272, were the odd leakage toward ground was initially measured. So far I don't see a path coupling the the ohms and amps part for something like an automatic adjustment or just an extended self test. So it is odd to see an error in both parts that is so close coupled. For me this somewhat point's to an AC problem, of some noise / oscillation that couples to both areas.

Much of the different voltage readings for the good and bad meter around Q213 can be due to a different type of diode for CR217 (higher forward voltage for the bad meter). Such a change would not have a significant effect on the meter operation. Also slightly different levels of the reference voltage can change the readings (e.g. around Q251).

In TiN's circuit drawing, I just noticed the guard connected through R370 to U238. This would be rather odd. The more logical guard potential for the resistor part would be the output of U228. I would more guess this is an error in the plan: the output of U238 would be good for the guard around Q213, CR217 and connected, but not for the range resistors. There might be a second guard for the resistor part too.

[nikonoid]

Kleinstein, I did a number of AC measurements today, as you suggested. Please see attached.

I do see spikes and sometimes even combination of several frequencies but I am not sure what is important and what is not. I noticed one strange thing, though: the power on U220 is +20 (pin 13) / -15 (pin 4), while U223 has +15/-15, as expected. Not sure if this is important. I checked and working 2002 has same power on these +20/-15 on U220 and +15/-15 on U223.

I also used K2000 to measure voltage between Amps and Lo inputs of broken K2002. I did this as both AC and DC voltage. They are in the last attachment here. Please note that K2000 has a 40uV AC offset with inputs shorted, so supposedly 40uV AC should be subtracted from AC measurements in attached table.

I do see at least two guards around ohms circuitry. I will look into tracing them in a bit.

Thanks a lot for taking a look.

[nikonoid]

Kleinstein, you were spot on about the guards. There are at least 4 different guards in the ohms circuitry. I see one guard going from output of U238 through 10k resistor, I also found another guard from U238 though 100k resistor. That one covers Q213, Q210 and other nearby elements.

I also found that output of U228, as you predicted, through R371 (10k) resistor drives guard covering majority of OHMS switching including U220, U223, U232, U233 and precision resistors. and some legs of Q211, Q212. I am making some changes to my drawings and I am planning to pass them to TiN.

I also found a bit of circuitry that is conected to Q226, including precision divider R335, R339 and also involving compactor U239 and register U203. I am not sure though if they could be considered part of OHMS circuitry.   

[Kleinstein]

The AC signals don't look very good, but also not that bad. So it is hard to tell if this could be the cause of the problem or just normal.

Due to the charge pumps (LTC1043) on the K2002 board (1 should be used to generate the 14 V ref level for the ohms circuit, not sure what the second one is good for), there is a chance that some spikes are normal for the K2002. However to much could cause trouble in precision DC circuits. One should be able to see which spikes originate from there, by checking the frequency / timing.

At that level, some could be a problem of where the ground is connected - there might be comparable background between the com terminal and corresponding ground on the board. It somewhat depends on the environment and scope, how much "noise" is picked up. Finding out if such small spikes are real of an artifact from the measurement is a little difficult and where experience makes a difference.
However it looks like the noise spikes at the amps part are larger and thus very likely real - possibly the source of the spikes. I see no legitimate reason to have them there, except if there is a chopper amplifier used for the amps circuit.

It might be worth comparing at least the measurement at the amps terminals to the working K2002. At least the large spikes look suspicious. The large spikes could cause some current in protection diodes as a bootstrapping/guard amplifier might not follow that fast. A possible cause could also be the guard/bootrapping amplifier in the amps section oscillating. Also the DC offset looks a little high, especially in the 200 µA range. 62 µV at an 1 K Shunt would be 62 nA (about what the DC reading is).

Have you checked the AC reading of the "bad" meter ? No need for a throughout test, but is it at least reading near zero on something like AC amps or AC volts with a shorted input. The AC circuit could be another possible source for higher frequency spikes (e.g. the rectifier circuit switching hard between positive and negative).

[nikonoid]

Here is an updated OHMS schematics, version 3. Changes to V2 are fairly minor and include Q226 transistor. By the way I still cannot figure out what that transistor is. It seems to be a matched pair with Q227. If someone knows, please post that info.

In trying to troubleshoot AMPS functionality, I also traced most of AMPS circuitry. It is very similar to K2001 schematics with addition of an opamp. So I used 2001 schematics as a basis and just changed what needed to be changed.

Let me know if these look ok.

TiN, if you have a chance please incorporate changes to the nice drawings you did. Thanks.

[nikonoid]

Back to troubleshooting. It appears that problems with AMPS and OHMS are NOT related to each other, but both seemed to be related to temperature.

When starting a cold meter it would start reading -6.3uA with open inputs. It quickly comes down to -2ua, -1ua and so fourth as meter is warming up. If I blow some cold air in the general direction of the APMS components the reading goes back up. To me that sounds like a clear proof of temperature dependency.
I measured voltages relative to Input LO while meter is still cold. Please see attached file.

 
For OHMS, I used hot air soldering station at its lowest setting of 100C and medium airflow. Blowing hot air focused around U232 for about 5 seconds fixed 2MOhm range readings (+/-40ppm of perfect). Blowing for 5 seconds more did not change reading from being good. I am still not sure if one of components is temperature sensitive or contamination on the board is responsible for this behavior. While hot air fixes OHM issue, AMPS reading problem appears unaffected, so they seemed like independent issues.

Klienstein, seeing this temperature dependency should I still further pursue AC analysis?

[Kleinstein]

The Ohms section working only when warm is kind of odd. Usually leakage gets worse with higher temperature.
Also the leakage current is really high (like up to 500 nA) this is a lot for leakage due to contamination.
Just looking at the DC readings would strongly point to the max326 already changed. So the 2 main suspects are leakage due to contamination/burned board and some AC effects. Checking for AC trouble might be easier, at least a quick check. There should be very little AC visible around the Ohms circuit, especially at Q213 (with a 100 K or 1 M resistor at the output), output of U233, emitter of Q251, U223 pins 9 and 11. No need to measure around Q214 this is close to having the probe in the air.
Instead of directly the output, one might use the low value 4 wire shunt as ground. How are the old measurements done (e.g. Probe). the old measured 25 mV_pp at Q213 might indicate a problem, but could be just an artifact from poor ground / probe. For a precision instrument I tend to prefer a quieter signal.

As the leakage problem got slightly better with cleaning, there is still a chance it could be just contamination. Residue from the caps might leave behind rather thin layers that might not be so visible. For cleaning DI water (usually plenty in several steps) might be more effective than IPA or similar. The main suspect would be the area around U223.

The extra transistor (Q226) seems to be part of the high ohms part. I don't think it is really related with the problem, though it might be nice to have the Ohms schematics complete. It is kind of separate from the normal ohms source, more like having the 14 V reference (or ground) connected through a large series resistor.

The amps section look rather conventional, though leakage in the µA range would be a lot even for the DG411. One suspect would be U234, especially the 2 gates connected to the DCA line. There might also be leakage from the DCA side. It is likely going towards some precision OP or maybe the main mux before the gain stage. Again a lot of leakage from the board might be the culprit too.  For surface leakage it would be more like relative humidity than temperature that is the culprit. So cold and really dry should be similar to warm.

[nikonoid]

I already replaced U234, U229, U225, and other and cleaned everything under them, so I had to look elsewhere.

I was just checking the board again and Q217 caught my attention. The source to drain resistance was about 4Ohm in both directions and gate to source and gait to drain measured between 52k and 54k. Not sure is this is OK. Does it make sense to you guys?

The solder on leads of Q217 looked dull and pitted. Since one of the legs is sitting at -15V, I decided to clean between the legs of this transistor.

Immediately after cleaning the AMPS performance improved to almost perfect state:
On cold start:
200uA    +000.0087uA
2mA       +0.000121mA
20mA     +00.00116mA
200mA    +000.0183mA
2A           +0.000103A

After 5 minutes, readings stabilized around:
200uA    +000.0037uA
2mA       +0.000043mA
20mA     +00.00044mA
200mA    +000.0050mA   (this reading was a bit strange, it would go as high as +000.0095 when turning this range on and then go back to 000.0050 fairly quickly)
2A           +0.000042A

These reading did not change after additional 30 minutes of warm up, so calibration might be able to take care of this offset.

This seems like a proof that the contamination was the reason for trouble with AMPS. I will try to do more cleaning with DI water and IPA.

Kleinstein, I will try your suggestions tomorrow. Thank you.

[TheSteve]

Sounds like you're making great progress.
FYI I've had a power supply from an HP network analyzer before with caps that leaked. No amount of alcohol would remove the contamination film on the PCB. I couldn't see the film, but an ohm meter could measure it. I had to scrub with water etc and then finish with lots of alcohol to bring it back to life. And the power supply was likely nowhere near as sensitive to contamination etc compared to your K2002.

[Kleinstein]

The critical of the max326 used in the ohms section is U223 according to the plans (TiNs and the last one below) shown.

If even the rather large amount of leakage in the amps range seems to have been due to leakage, chances are the the ohms part leakage is also due to contamination.  So my suggestion would be a little more cleaning, especially around U223 and the 75 K ref resistors. Even if the first try on cleaning would not solve the problem, any significant change would give an indication that contamination is the culprit.

[nikonoid]

Thank you very much, Guys.

Today I did a bit of DI cleaning followed by IPA and now it is time for some baking    at 130F   (see attachment)

Should 2 hours be enough?

[floobydust]

That's great news. It was a lot of work.
After an IPA wash, a hour in heat should be fine. It's just the water (if not 100% IPA) needs time to evaporate underneath IC's.

I've never seen leakage on a PCB like you were observing.
Unless they used the incorrect flux/wash, or creep corrosion is occurring. Not to be confused with tin whiskers.
Creep corrosion is usually blackened copper in dendrite-like patterns.


I have Keithley 2001,2001 service manuals but they contain no schematics, just long flow chart of parts to replace, kind of gross.

[Zucca]

I have Keithley 2001,2001 service manuals but they contain no schematics, just long flow chart of parts to replace, kind of gross.

Here the schematics K2001 and tons of info about the K2001

https://xdevs.com/fix/kei2001/#fix_2001

Back to the K2002 repair, nikonoid go on like this... you will win for sure!

[nikonoid]

Yes, Guys, I used TiN articles a lot.

This saga is slowly coming to the end. After cleaning with DI water, IPA and doing some fine baking, I gave meter a day to rest and cool.

Testing today showed that almost surprisingly OHMS function is working now.   It reads about 100pm high on a cold start, but then in just 5 minutes gets to good readings. At the same time AMPS results got a bit worse after this round of cleaning, but not much. After 30 minutes warm up the meter is pretty usable, considering it is running without top shield, metal case, etc.

So now I am pretty confident that everything that had to be fixed is fixed. CONTAMINATION is the major problem that is left!

This meter was missing case and top shield when I got it and was literally full and dust, insects and mice litter. The fact I could get it to this point is somewhat unbelievable. What else can I do to clean contamination, short of removing all the components?

Thanks.

[Kleinstein]

Congratulations 

If cleaning worked so far, it might be worth a second round. Dissolving contaminations is sometimes slow. So it might take quite some time and DI to get rid of some stuff. One could try a rather dilute acid - it can help with some salts / deposits. With now not much lower leakage, there should not be very much dirt left - so maybe just a few more steps of dilution.

[Zucca]

My .02$: Soak it in a bath of 50% distilled H2O and 50% IPA... leave it there for a day or two... then rinse it with fresh distilled water and finally in oven again. repeat if necessary.
Maybe an antistatic brush could help.

[nikonoid]

I have done special oil free air dusting and anti-static brush before using liquids. I wanted to minimize dirt flowing under microchips.

As the bath is concerned, could some elements be damaged by it? Like mechanical relays, voltage reference, large capacitors and maybe others.

Is shallow bath, right to the top surface of pcb, a good option?


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[Kleinstein]

Relays could have a problem when the full board is in a bath.  It depends on the types - some are washable, some are not. Especially reed type relays could be a problem due to increased leakage if the glass gets contaminated. The caps should not be a problem. The tricky part could be leakage under chips or relays. There can also be residue moved from the dirty power supply part towards the more sensitive parts (Ohms and volts input). So I would prefer more local cleaning.

One could use the liquid locally, like a few drops or small spill, so it would only effect the area of interest. Local washing would be adding a few ml of water and remove it with a syringe or similar. So more like a shallow local bath, that is repeated several times. For the parts between chips the brush could also be good option for an initial cleaning.  One can hardly avoid the part below the chips to get wet. Compressed air to blow the liquid out can be tricky, as some will end up as small droplets in other areas. It might still be the best option. The idea is usually to use enough water to dilute the dirt so much that there will be very little left. IPA or similar is than to a large part to dilute the residual water and allow better drying.

[nikonoid]

With input from the forum I did a "sponge bath" for a meter. I would saturate small section with liquid for a minute or two, then wipe it out with dry q-tips. I did it over and over again working one section at a time.

Two days later the meter is now in close to PERFECT condition. It passes all self tests even on cold start.
Current still have a bit of a drift on start and stabilizes after 10 minutes. Ohms seems to read right. Again about 20ppm drift from cold to warmed up. I also noticed noise during OHM measurements of ranging from +/- 1ppm to about +/-5ppm. This might be as good as it gets considering I am running meter without top shield and case.

The plan now is to finish recaping, buy shield and case from Keithley and do full evaluation of the meter. I can take it through few more rounds of cleaning, if becomes necessary.

Thank you everyone for staying with me on this and specifically thank you Kleinstein for your continued help and comments.

[saturnin]

Although it is definitely great progress (considering the initial state) and passing self test may be good indication there is no critical failure, I would still hesitate to declare the unit anywhere near to "perfect" condition. In such old units there still might be hidden serious issues (intermittent glitches in readings, slow long-term drifts...). I saw multimeters which were passing built-in self test with flying colors, but after detailed verification of their performance it was obvious there was an issue. It could take weeks or even months to find and solve these ones...

[TiN]

Well, that to be determined by full low-level calibration and verification (which in turn require access to 5700A + booster) .

[Le_Bassiste]

Although it is definitely great progress (considering the initial state) and passing self test may be good indication there is no critical failure, I would still hesitate to declare the unit anywhere near to "perfect" condition. In such old units there still might be hidden serious issues (intermittent glitches in readings, slow long-term drifts...). I saw multimeters which were passing built-in self test with flying colors, but after detailed verification of their performance it was obvious there was an issue. It could take weeks or even months to find and solve these ones...

so, how would you actually declare this unit to "anywhere near to "perfect" condition"? what are your criteria to do so?

[Zucca]

With input from the forum I did a "sponge bath" for a meter. I would saturate small section with liquid for a minute or two, then wipe it out with dry q-tips. I did it over and over again working one section at a time.

Which liquid? 50% distilleld H2O and 50% IPA?

Congrats Sir nice job! 

Although it is definitely great progress (considering the initial state) and passing self test may be good indication there is no critical failure, I would still hesitate to declare the unit anywhere near to "perfect" condition. In such old units there still might be hidden serious issues (intermittent glitches in readings, slow long-term drifts...). I saw multimeters which were passing built-in self test with flying colors, but after detailed verification of their performance it was obvious there was an issue. It could take weeks or even months to find and solve these ones...

Yes you are right, but I believe nikonoid will not give up  easly.  Moreover the probability a SELF PASSED unit has more problem inside is cam'on small. It happen to me too, but it was a single event.

[saturnin]

I am sorry, but "self test passed" is necessary but not sufficient condition to say that K20xx units work perfectly. As I wrote performance/stability verification needs to be done. This is especially true about nikonoid's unit which was affected by severe leakage due to surface contamination (0.5 uA!). Ideally, F5700 or similar calibrator should be used, but even hobbyist can reveal a lot issues by using a decent set of DYI standards.

If I were at nikonoid's position I would check long-term stability of all DC and ohm ranges (because they have tightest specs) before declaring the unit to be close to perfect state...

I would like to add it is not so rare a DMM has an issue although it passes its self test. Let me mention two examples I solved some time ago:

1/ K2000 - very clean unit, never repaired before, it passed all built-in tests, at first glance everything was ok. During stability tests, I found out there were glitches in readings (see K2000_noisy_test3.png). Reason? A zener in an auxiliary power supply was dying...

2/ K2010 - very clean unit, never repaired before, it passed all built-in tests, very stable and quiet on DCV ranges. After some time, I realized there was a drift on all <10 MOhm ranges. (see K2010_ohms_drift_issue.JPG). Reason? Slowly drifting TF-245 resistor network...

That's why I don't exaggerate relevance of built-in tests...

[Zucca]

Thanks saturnin, very interesting, yes you are right.

Now it looks like you have more experience than me, quick questions: how many devices you saw passing self test and have no problem then? More/less then 90%?

Thanks!

[nikonoid]

I might have overstated the "perfect" state.


Saturnin, K2002 self test conditions are very wide. I think some of them are as wide as 3% accuracy, so I am completely aware that passing self test is not guaranteeing anything. My assessment of the condition was based on measurements outside of the self test. I got it to provide believable, relatively stable readings, considering absence of shield and case.

Questions of calibration, time stability and especially temperature coefficients still need to be addressed. My other K2002 is @CalMachine, for low level calibration, so it will be fun to compare them against each other when it is back.

TiN, I wish I had 5700....     However I can check stability and consistency even before going for low level calibration. I am also quietly experimenting with a DIY temperature controlled chamber.

Zucca, I did use 50/50 mix. My previous attempts with 99% IPA were not as successful. By the way, my original K2002 was sometimes failing self test. In the end it was MAX326 chip that sometimes worked properly and sometimes not. So I actually share the saturnin's concern. Self test, especially if it ran just once, does not guarantee anything.

[saturnin]

Saturnin, K2002 self test conditions are very wide. I think some of them are as wide as 3% accuracy, so I am completely aware that passing self test is not guaranteeing anything. My assessment of the condition was based on measurements outside of the self test. I got it to provide believable, relatively stable readings, considering absence of shield and case.

That's my point exactly. Sorry, it seemed to me you relied on results of built-in tests only. Thanks for posting updates on the actual status of your repair project! Hopefully, you will get fully functional and reliable unit in the end.

[nikonoid]

After fifth, yes fifth, round of cleaning and baking I did measurements for two modes that gave me problems before:
1) AMPS with open inputs and
2) 100k Ohms in 2M and 200k ranges.

Both measurements were done from cold meter (off for 24 hours). I did measurements without top shield but I borrowed the metal casing from a donor K2001.

Amps took 45 minutes to an hour to completely settle. After that it would fluctuate from time to time by about 3ppm of the range.

Ohms took a bit longer - 60 to 120 minutes to settle.

Can someone try something similar with fully functioning K2002 from cold, so we can compare? Thank you very much.


Today I am getting a parts 2001 unit that will donate the case and top and bottom shields. I got it for $150 - cheaper than replacement parts.

PS. The score so far on this K2002: Two blown capacitors, one shorted zenner diode, 3 traces killed by a hole burned in the board, two traces eaten away to electrolyte, two bad vias, and LOTS AND LOTS OF CONTAMINATION!

[WastelandTek]

looks like it is settling down nicely

[nikonoid]

I finally have an automated logging going. Here are first few logs recorded for the newly repaired meter, covering warm up performance from completely cold.

In general it looks like it takes about 1 hour for readings to be very close to stable and 2 hours to completely stabilize.

10V log will be coming next.

[TiN]

Sounds right to me. I would try my units, but I has no available GPIB dongles, it's all busy at 3458s doing measurements.
When I need to use meter, I power on them to warmup 24 hours ahead. You can watch internal temperature to calibrator sensor reading (TEMP function - Internal sensor).

[nikonoid]

Thanks, TiN.

I also measured 10V produced by 732A for warm up (3 hours) and also 14 hours run after 24 hours warm up.  I am happy with 3 hours warm up performance. Calibration manual suggests 4.
 
Voltage variation withing 14 hours run is definitely better than 24 hours spec and somewhat withing ballpark of transfer spec. I may need a tighter clamp on environmental noise before judging transfer specs.

I do not yet have temperature logging synchronized with voltage, but temperature variations were about +/- .5 degrees C during the run.

Are these consistent with what I should be expecting?

[TiN]

Yes, all good, 0.3ppm pk-pk, well within expected normal.

[nikonoid]

Thank you, TiN. Time to move on to new projects...


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[TiN]

Need a calibrator? 

[nikonoid]

What calibrator?


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[Samogon]

I think he meant, for Your next project and it should be fluke 7500 or Datron 4808

[nikonoid]

5700a and 4808? These are a tad too expensive. I almost bought 4708 few months back.

I have K2001, K6517, K237 and K2601 on my to do list...


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[branadic]

The softkey pad on our K2002 was nonfunctioning. I found the conductive elements of the softkey pad completely worn out, no conductivity at all. Curious enough, Keithley has originally tinned the corresponding contacts on the PCB with HASL, which is weird, they should be gold plated, shouldn't they? I'm confident it is original, as all vias are tinned too.
I today received a CW2605 repair kit made by CHEMTRONICS.

https://de.farnell.com/chemtronics/cw2605/repair-kit-elektr-leitend-1x3gr/dp/128790?ICID=I-RP-STM7REC-0

However, the process was easy and the material is already drying until monday, as they say it's dry within 24 h at room temperature and fully usable after 72 h.
I felt I should share the experience in case someone else has the very same issue.

-branadic-