Help me to repair Keithley 168 - can't zero the input

[miceuz]

I've recently got a Keythley 168 multimeter off ebay. In general it works ok, but there is one problem - the input can not be zeroed out - when I turn zero pot the value just changes sign at some point and starts increasing.

Say, input shorted, multimeter shows -0.0045V, I turn the pot and the value starts to go towards zero, but when it reaches -0.0035, it suddenly flips to 0.0035 and starts increasing again.

The multimeter works ok - values seem to be measured pretty ok plus annoying offset (didn't do an extensive test yet)

Upon inspection I've noticed that the device had an incident related to 1k current shunt - the resistor is replaced as are J-fets Q102 to Q105. There was pretty crusty flux residue around QA103, but it seems to be the same device (opamp in a can). I've cleaned the residue. The corpse of the old resistor is taped inside - it's charred.

I'm attaching a manual for it, can anyone suggest some points to start looking at?

[SArepairman]

Are you sure that it is not a problem relating to the potentiometer adjusting zero? I noticed a similar error in my keithley electrometer, which also has a zero adjust pot.

It is an interesting story because it had a similar problem, where when I got to around 0, it would get rather erratic and before I knew it it was at a non zero value, I thought maybe that there was dirty in the potentiometer so I took it apart, had a look inside, it looked great, well lubricated, no problem. WHen I put it back together the machine worked much better. I noticed this only happened around zero, if I left it some where else (at a non zero offset) then it would be pretty stable.


I realized the problem was that the knob on the device was not mounted properly, it was off at an angle, and it just so happened to be that at the position where 0 normally is the knob was pushing against the housing, exerting a torque on the potentiometer (I had noticed earlier it did not rotate very smoothly, like it would be "scratchy" at the point right where zero should be, I took this scratchyness to mean the internals of the pot were bad, not that the knob was scraping against the meter housing).

Obviously when I reassembled the meter the zero adjust worked much better because I was more careful in tightening the knob.

If this is un clear


meter    | |              -this is what it should be, the bottom face of the knob should be parallel to the meter and seperated by some distance.

What I had:

 |/  - the knob was attached non parallel do the housing and it was too close, so at a certain point in its rotation it would scratch.


Anyway, but defiantly check that potentiometer for dirt and possibly mechanical problems.

[miceuz]

In my case it's just a trimmer pot, from action it does not look like erratic pot behaviour - too consistent.

I've digged thru manual yesterday, it appears, multimeter has a "display zero" trimmer - will try to go thru calibration procedures and see if I can nail that down.

[SArepairman]

In my case it's just a trimmer pot, from action it does not look like erratic pot behaviour - too consistent.

I've digged thru manual yesterday, it appears, multimeter has a "display zero" trimmer - will try to go thru calibration procedures and see if I can nail that down.

are you sure? I have seen trimmers jump pretty consistently.