Electronics > Repair
Keithley 2002 repair help
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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.
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