Electronics > Repair

Keithley 2001 repair log and Heat Sink Clip request

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vsmith:
Hi all,

I see that there is quite a bit of info regarding repair of Keithley 2001, especially on TiN's epic thread "Restoration Glory of Keithley 2001"
I have read it through several times, and still have a few questions/comments, and hopefully can contribute knowledge as I go along.

First off, the 2001 I purchased (eBay, naturally) came missing the heat sink clip for the power FET and Resistor on the left rear of the chassis/analog board. It is also missing the rear bezel, handle, and the long screw which holds the analog board shields to the pcb. I can probably scrounge up the last 3 items, but need your help with that Heat Sink Clip: do any of you have a spare parts unit and could spare that clip for me? Willing to pay reasonable price and shipping of course.

Now for the details. The unit is the M model, serial number 1316678, and the FW rev is B17 A02. I think the unit may date from 2010 or so. But maybe I am reading the date codes on components wrong?

The unit was described as working but failing numerous self tests. And I received it in that condition. Display is good, digital/processor function seems good. But the errors, which began with 200.1 and went all the way up through the 300s (I stopped recording at that point) indicated a problem with the A/D and Reference sections. I checked the power supply voltages and saw that +15 was +18, and the regulator appeared to be shorted. All other voltages checked out OK.

I took the shields off and immediately realized that while the electrolytic caps had been replaced (with good 105 degree caps), whoever was doing the repair stopped and closed the unit back up LEAVING THE UNCUT LEADS OF C101 and C104 TO SHORT OUT WHATEVER THEY COULD TOUCH ON THE BOARD!

Unbelievably, the only actual short circuit that resulted appeared to be the +15v regulator, meaning that the +15 rail was sitting at about 18 volts. Once I cut the leads from those caps, the 15 volt regulator came to life and all the PS voltages were correct.

(By the way, aside from the uncut leads, the board rework was done well, and the board itself showed no signs of corrosion or damage or even leakage)

Re-ran self tests, and this time the errors began at 201.2, (7 volt reference) continued thru 201.3 (1.75 volt reference) and then 300.2, 302.2, and 303.2, plus all the resistance range checks (304.2 thru 304.7). As before I stopped there, knowing there was a fundamental problem that needed attention before any higher level stuff got done.

At this point I discovered that several of the op-amps in the reference and a/d mux/buffer circuitry appeared to be fried. Evidently that +15 rail going high wreaked havoc. I think the lm399 is OK. It's companion op-amp u328 is bad tho, so can't be sure. I have ordered a bunch of replacement parts (there are at least four different op-amps that took a hit) and now have to wait for those to come in.

When I get those circuits working again, I plan to post dc readings at the key points for each of the 201 series of test steps, which I hope will help with future repair attempts.

I'm a little sad that this unit's recent calibration is now useless, I was hoping that a simple power supply repair would allow the unit to come back to life cal intact. But I guess I lost that gamble. I am still very happy to have a 7-1/2 digit DMM, if I can get it repaired!

Question: I quickly realized that without the A/D daughter board plugged in to the analog board, the reference switching/routing circuitry on the analog board did not work. So reading signatures/bit patterns on the various ICs can't really be done, since the A/D card obscures that part of the analog board circuitry when it's mounted, and if it's taken off, the bit patterns are not valid. What have other repairers done to probe that area of the board while troubleshooting? Has anyone made an extender for the daughter board? It's a really annoying mechanical design!

Comments, please!

Vic

vsmith:
Update: I finally got the parts I ordered and had time to do the rework on the Analog board that I hoped would resolve the problem. I replaced U327, U328, U324, U331, U333. All of these were opamps, and all had inputs with large differences in their voltages, which I interpreted as them having been damaged by the +15v regulator being short circuited.

Unfortunately, nope. U328 (the +7v reference circuit) still has inputs 7 volts apart (7v and 0v) and it still gets hot when the meter is powered on. I probed around a little more, this time looking at u330 (the precision switched capacitor reference divider) and found that the drive voltage at the Cosc input was only 150mv p-p. Meaning that the output of the Mosfet driver U535 was only 150mv, rather than the 5 volt square wave I would expect to see. I dont fully understand how the reference circuit works, but without u330 switching properly, I think that the voltages in the circuit would not be correct.

Without breaking the connection between the driver IC and the switched capacitor IC, I cant be sure which device is causing the problem. They are both supplied by that same 15V rail, though. I have ordered both and will try replacing them next.

Have any of you with experience repairing K2001's ever found and troubleshot problems with the reference circuitry? Any thoughts on theory of operation of this circuit?

openloop:

--- Quote ---What have other repairers done to probe that area of the board while troubleshooting? Has anyone made an extender for the daughter board? It's a really annoying mechanical design!
--- End quote ---

My speculation is that the ADC board can be plugged upside down and to the left (it has holes for that), but only when the analog board is out of the case.


--- Quote ---Have any of you with experience repairing K2001's ever found and troubleshot problems with the reference circuitry? Any thoughts on theory of operation of this circuit?
--- End quote ---

It is very simple.
-Vref is coming from the ADC board and sets the operating current for the LM399. Check that -Vref is there (about -10V). ADC board has its own zener - that's how Vref is made.

U330 should not load U328 pin6 (op amp output). Moreover, K2001 probably will work without U330, so you can remove it for testing - just some ranges will be dead.

Also, bad COSC on U330 is a possible indication that something's wrong on the ADC board where the clock is made. Pray it's not U808
Or simply U330 is so dead that it loads everything...

Zucca:
If you need some IC I have them.

vsmith:
Openloop, Zucca, thank you both for your replies! I have read your previous K2001 related comments, and I value your input!

As far as the bad (low amplitude) COSC waveform, the input to U535 (Mosfet driver) labelled "600Hz" is a full 0 to 5v square wave. So I think that things up to that point are OK.

Thanks for the tip on -Vref...I will check on that. And removing U330 will definitely help pinpoint the problem IC, will try that.

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