Author Topic: Repair : 94' Keithley 2002 8.5-digit DMM , good ol' cap leaks destuction derby..  (Read 14351 times)

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Offline TiN

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Quick video of this 2002 repairs:


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Offline TiN

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Just a little follow-up on whitening FP on this 2002..

First thing - remove PCB, rubber pad and current input jack/fuse:



Then tape logo, model number, jacks with tape to avoid contact with chemicals



Put panel into proper sized box, pour some warm water with TAED (better not, see later) and puke some Vol.40 peroxide over the top. Make sure everything covered evenly and put it for few hours on direct sun for some UV magic



And....3 hours after if no one stolen your panel out there, it's done:



TAED seem to eat black silkscreen a bit, i'll try without it next time.

Also gold plated terminal pins getting nice shine as well now:



And happy (or not) whitened crowd smile to camera again:

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Offline Le_Bassiste

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aaand another question that i hope belongs in here:
found a small area on the analog board beneath Q241/Q250 JFET pair  that is covered by a conformal coating. it's transparent and flexible, starting to peel off at one of the solder joins. (see picture below, i added the blue tint to the picture to make the area in question more obvious)
any advice on what substance would be best to apply as new coating?

« Last Edit: June 09, 2016, 08:33:33 am by Le_Bassiste »
 

Offline TiN

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On my older 2002 there is nothing there, and on newer (07') there is thermal sponge pad there. Might be better just to carefully clean the area and leave it alone, if you don't have fan blowing under the cover hood.
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Offline Le_Bassiste

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thanks, TiN.
interesting, seems that they had to improvise some protective measures after the first units. may be a leakage problem? my analog baord is a 1994 rev. D, serial A45xxx
i'll be giving this one a try, ordered the 15 ml bottle:
http://www.electrolube.in/products/conformal-coatings/fsc/modified_silicone/

 

Offline TiN

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I'd be reluctant to add any coating over guarded circuits, as that defeat guarding purpose in a way.
Let us know your results though.
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Offline Le_Bassiste

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understood. my unit has the thermal pad, too. that pad seems to have "sweatened out" some silicone, i noticed some fine oily film on that  pcb area.
 

Offline Towger

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'Silicone' absorbs moisture overtime. It would be much better not to have any. I have see equipment where it was used to protect xtals and the build up of moisture/capacitance over the years would stop them from oscillating.
 

Offline TiN

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There is another sick 2002 on fleaBay for $1.5K take-me-now in case someone want 8.5 digits  :-DMM

Seem to be in fairly decent condition, made in mid-90's. Usual treatment with capacitor replacement and PCB inspection is mandatory.
« Last Edit: June 14, 2016, 04:34:24 am by TiN »
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Offline TiN

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Attempted calibration yesterday of this fixed Model 2002. Had once gotcha though.
During calibration of 200uA current range, I sourced 200uA from Time Electronics 9823 MFC (range 200uA) into 2002 but I gave error.
Second 2002 indeed show Overflow on same condition, whem MFC sourced 200uA on 200uA range into meter.
If I switch range on MFC to 2mA and just source 0.2mA - 200uA range on DMM works and calibrates OK.
3458A had no problem measuring current on 2mA range as well.

Shot youtube vid covering this:



Why is it so?  :-DMM :-/O

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Online Kleinstein

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The problem could be due to some rather high ripple on the current from the calibrator (so a unnoticed defect in the calibrator).

Another possible trap could be if the calibrator does not like the impedance of the DMM as a load and starts to get unstable. Adding something like 10 Ohms in series or about 10 Ohms + 1 µF in parallel might fix this.

 

Offline TiN

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3458A reading is steady to +/-2nA, on same range, so I'd expect ripple is not the issue.
HP's impedance on 2mA range for current is 100 ohm, while 2002 on 200uA range is 1KOhm.
Output compliance on 9823 per specifications is 15V with 10G output impedance on DCI.

I'll try with RC network and get back to you.


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Online Kleinstein

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It's not low frequency ripple that can be a problem, but more like short spikes. So the reading can be stable but during peaks the current might be well over the nominal 200 µA and thus cause an overflow as some of the current might flow through protective elements. So I would look at the current with a scope.

With 100 Ohms of 1 K impedance (e.g. classical shunt with 200 mV drop) this should not be a problem for the current source. It could be a problem, if the meters were using a TIA input - some meters (e.g. HP3457) do that with low currents. The impedance in this case may be not so well behaved and cause oscillations. Just like a voltage source does not like a pure capacitive load very much, a current source can get trouble with a highly inductive load. If the is the case, I would consider this more like a defect of the current source.
 

Offline smoothVTer

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seemingly not entirely on topic, but still...
there are a lot of tear down pictures of KEI2002 and KEI2001 out there, but only a very few give a clear view on the top black plastic shroud that covers most of the analog circuitry. what i've noticed so far:
the shrouds for KEI2002 and KEI2001 are different:
KEI2001 shroud doesn't have any air vents at its circumference, thus protecting the analog circuitry from any "busy" air.
KEI2002 shroud has a vent at the fan-side and an exhaust vent at the opposite side, somewhere in the area of connector P1027. also there is a transparent air duct installed over the fan.

and here is my question: what is the correct placement of the transparent air duct in a KEI2002?
is it like this, allowing for some airflow onto the mains controller V105?
https://xdevs.com/doc/Keithley/2002/photo/Chuck/img_0113_fan%20baffle%20detail.jpg

or is it like this, where the entire airflow is directed into the shroud?


I recently scored a 2002, totally dead, power supply issue.   Opened up the top analog shield to have a look at cap's condition.  Looking good, not bad, all Nichicon 105C,  see zero evidence of leaks.   Looks like IC date codes are 1999. 

Confusing that the 2002 would have the design decision to divert/force air along through the analog section.  It is bad to have metal junctions at different temperatures cause by air currents, is it not?   Maybe it was getting too hot under the shield and those caps were dying?   Regardless this has at least one problem:  very fine dust collecting over all the sensitive analog circuitry.   Either fan filter was never replaced or it was removed totally... but some dust gets in and over the years it does collect.   

Also wondering how to place this transparent air diverter,  now that I have the entire unit open.   To divert the air away from that analog section is my preference ... but then again I don't know. 

 :-// What's the best way to clean this PCB?   PC air spray can?     Wash in  IPA / methanol?     
« Last Edit: August 13, 2016, 07:49:15 am by smoothVTer »
 

Offline plesa

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I recently scored a 2002, totally dead, power supply issue.   Opened up the top analog shield to have a look at cap's condition.  Looking good, not bad, all Nichicon 105C,  see zero evidence of leaks.   Looks like IC date codes are 1999. 

Confusing that the 2002 would have the design decision to divert/force air along through the analog section.  It is bad to have metal junctions at different temperatures cause by air currents, is it not?   Maybe it was getting too hot under the shield and those caps were dying?   Regardless this has at least one problem:  very fine dust collecting over all the sensitive analog circuitry.   Either fan filter was never replaced or it was removed totally... but some dust gets in and over the years it does collect.   

Also wondering how to place this transparent air diverter,  now that I have the entire unit open.   To divert the air away from that analog section is my preference ... but then again I don't know. 

 :-// What's the best way to clean this PCB?   PC air spray can?     Wash in  IPA / methanol?   

I will not clean the PCB in current state. Rather connect  external power supply to check if meter is working and perform self test.
 Can you post some pictures, because the complete analog section is covered by top and bottom covers to prevent dust. LTZ has its own cover.
In your case compressed air is way how to clean it. IPA will dissolve the dust and after this cleaning you can have more issues.
In general is safe to clean PCB with IPA mixed with water and after that second batch in IPA followed by baking @ 50°C/2h. Avoid rinsing.
 
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Offline TiN

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+1 on photos.
I'd go with dry compressed air first and then leave it as is, if it's not looking too bad. If you can avoid liquids on board - better to avoid them.

Capacitors... they always look good before it's already to late. Replace them. I don't quiet understand why some owners of 2001/2002 would want risk their meters over 10$ worth of caps...
I posted number of horrid photos of what are the consequences, but this question still arise every time. ( :rant: over).
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Offline smoothVTer

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+1 on photos.
I'd go with dry compressed air first and then leave it as is, if it's not looking too bad. If you can avoid liquids on board - better to avoid them.

Capacitors... they always look good before it's already to late. Replace them. I don't quiet understand why some owners of 2001/2002 would want risk their meters over 10$ worth of caps...
I posted number of horrid photos of what are the consequences, but this question still arise every time. ( :rant: over).

I will change out all the electrolytic capacitors for sure;    the big 15000uF cap on the digital board I wanted to replace with a 105C rated cap, but unfortunately I cannot find this same axial cap in 105C version on Mouser, Digikey, or Newark.   Might just replace with a 85C like original.

OK SO Pictures that may explain:

Reference image with the cover in place above the analog board


Cover off now


Cutout in black cover showing the clear plastic fan air diverter and the cutout right across from the fan.  When received, it seems like all the air flow was directed AWAY from the electrolytics/regulators outside of the black cover, instead forcing air in and out thru cutouts in the cover:


Cutout from side view, close proximity to analog section caps & regulators:


Dust over a portion of the analog section;  representative of dust under the black cover in almost all the analog PCB.  Probably will blow this off with canned compressed air.


 :-DD   I'm not 100% sure but ... it looks almost ... as if a prior owner or service tech took a dremel tool to the sides of the cover and cut vent holes into it?   Nice move, Mr. Prior Owner guy.    Can anyone post if their 2002 cover is the same, different or does it lack this air-flow-thru ventilation completely?   






 

Offline plesa

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You are right, the two vent holes are not present on 2002/2001 units. The most recent unit on this forum has TiN, mine one revision older. None of them has these external holes. Cover can be purchased from Tek for $50, If I remember it correctly.
The contamination on picture I will remove with plastic brush ( ESD) and compressed air. It is around AC circuit, so not so critical.
Did you power it up?
 

Offline Le_Bassiste

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as the others already mentioned: avoid cleaning if possible. i'd also suggest to wear gloves while handling the analog board. the repair manual mentions "dry nitrogen" for cleaning. pressurized "air" from cans may be ok (but is most likely not air but gas), while i'd not take pressurized air from compressors (even with oil / water separators installed) to dust off the pcb.
as for the shroud, the specimen on my KEI2002 looks the same as yours, having the same port holes, without the "handicraft" marks, though. i had the same concerns as you wrt the airflow going over the analog components, so i took my KEI2002 out for a wild ambient temperature ride and measured offset variation on the 200 mV range over several days. 
see attached diagram, with 1000 points on the x-axis being three hours. y-axis reading is temperature in degC, 2nd y-axis is offset deviation with low-thermal input short on 200 mV range, in ppm of 200 mV. measurement took place on fully assembled unit, PT100 sensor for temp sensing located slightly off center and 2 cm away from rear fan intake.
would really like to know whether others' KEI2002s have similar offset drifts. at diagram start, KEI2002 was already on for several hours.
« Last Edit: August 13, 2016, 07:05:01 pm by Le_Bassiste »
 
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Offline smoothVTer

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You are right, the two vent holes are not present on 2002/2001 units. The most recent unit on this forum has TiN, mine one revision older. None of them has these external holes. Cover can be purchased from Tek for $50, If I remember it correctly.
The contamination on picture I will remove with plastic brush ( ESD) and compressed air. It is around AC circuit, so not so critical.
Did you power it up?

Thanks everyone with your responses so far.  I am going to use the conservative route and do as little touching of the PCB as possible, relying instead on compressed air to get some of the dust out.   

Certainly did power it up, yes.   No voltage on any power rails on secondary side of T100.   I annotated some measurements and voltages from the primary/secondary when I first applied power to the unit.  So far here's what I've found as problems:

Optocoupler that drives the big power MOSFET in the primary pre-regulator circuit is totally blown - open circuit on the input LED.    Testing S-D of Q101 for a short;  no short present, good news, tested gate drive too;  Q101 appears to be functional.   


For the optocoupler replacement, I could find only  the  very similar TLP591B on Digikey.   Same as the TLP590B except that it has a shunt resistor across the output pins "for optimized switching speed" according to the datasheet.   Don't have much experience with opto replacements ... any idea if the TLP591B will work as a drop-in here?    Was also considering a similar pinout but different topology opto like a transistor output opto but again, not certain this will work as a replacement.


On the digital board, I found a single diode short on rectifier CR622 ( red arrow points to the shorted diode )   CR624, CR625 tested good.   Applied 7.5V to U619 and U629, received 5V out.   Appear to be good, I will replace anyway just to be safe.   Could not test comparators U627, U628, U629 yet.   Obviously all caps on the + output of CR622 I'm going to replace as a matter of course.     Also will probably replace Q608 which might have destroyed the opto's input. 


My guess at what happened is probably a voltage spike on the lines coming in from J1032 caused the rectifier CR622 to blow, also perhaps sending the spike further down the line on the +5VC rail and blowing the opto. 


« Last Edit: August 14, 2016, 03:27:17 am by smoothVTer »
 

Offline Vgkid

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You might not have to worry about that 15000uf cap. In a bunch of my gear, it is often the physically smaller caps thst tend to go bad.
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Offline TiN

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My newer 2002 has the vents holes in cover and air deflector for airflow. Older 2002 does not have that, so i was under impression that only newer units got this "tweak". Which maybe still true, perhaps someone with unit made in other dates can confirm it?

I did not see DCV performance difference on 20V range either with vent hole or without though (tested within 0.6ppm uncertainty vs 3458A).

Rest of questions I'll reply later, waiting for boarding in LAX atm :)
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Offline TiN

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Beefy 470ohm resistor is usual blown too, if you FET dead. And 20k 1210 resistors on bottom side bbq the pcb, often taking zener nearby out too. Check all that.
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Offline smoothVTer

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Dead relay coil found, K101.    Seems like it was permanently stuck to 220V tap.

Relay K102 was working,  however the flyback diode CR108 ( BAV103 ) was reading 0.4V Vf ... both forward and reverse polarity.  :palm:  Ah, think it is toasted.   All other BAV103's read at 0.6 Vf.   

As you mentioned in previous posts TiN,   its not worth a few dollars to risk some other power supply component being bad or overstressed due to a transient overvoltage condition.   Better to replace entire power section.  That is my plan now.  Need to learn to use a desoldering pump ( no experience with these )  for the relays and power switch removal ( VR101/102  & Q102 hidden underneath -  sneaky sneaky )     








 

Online Kleinstein

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Q608 can hardly do any damage to the optocoupler. If at all the 150 Ohms resistor could.
The TLP591 should work as a replacement - it's rather similar. R104 is maller than the extra internal resistor.
 
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