Electronics > Repair
Keithley 197 - jitter/jumping last 2 digits
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Kleinstein:
The JFETs for switching are one part of the leakage only.  I don't think the JFETs are that special - just TO92 fets are getting rare in general.
Replacing the FETs with a CMOS chips would really change the circuit - so you would essentially design you owns DMM. Usually JFET leakage can be quite good - compared to CMOS more like higher performance, but more tricky to use.

Another part of the leakage is from Q103 and Q127. These 2 would be prime candidates for leakage, both from the parts and also from damage (e.g. ESD).

A way to hunt the leakage current is to use local heat and see which parts react much to a little heat (e.g. +5 K). Most semiconductors double there leakage with about a 10 K temp rise. This also applies to many partially broken chips with excessive leakage.
corehbola:
Thanks...
Hmm... Q103 and Q127... Those were the first ones I originally thought could have been damaged from prior abuse...
And after your comments, as a second thought now I might have ruled them out too soon.
I mean... since this DMM has a 10MOhm input resistor connected directly to the input, and switch-in the low side of input voltage divider, I thought - at first - that in the 200mV and 2V range, the signal path would always come through the 10M resistor (for DCV at least) (as it's the case with many Auto-Range designs).
But later I learnt (and was quite surprised) that in the 200mV and 2V range, signal goes directly (via K101, R107, C109, R106) and that's when I stopped thinking of Q103 and Q107 as a possible source altogether. (I even thought of Q114 and Q115, but I removed them altogether and it didn't change anything).

But Q103 and Q107, even though behind the 10M Ohm resistor, and out of the direct signal path, may be slowly charging  C109.
What puzzles me most is that, when the inputs are left open, the voltage slowly builds up (at about 10mV/sec), up to about 0.58V, and then it stops building up (suggesting there's a one diode voltage drop kicking in somewhere, limiting the voltage to increase any further...), but I can't see anywhere a diode in parallel to the signal path (apart of Q114 and Q115, but since they're bootstrapped, the voltage in them never reach anything near conduction).

I will follow your hint and take out Q103 (which is easy to remove and will be more conclusive).

Thanks... I`ll report back here.
Kleinstein:
The input stage is different from mainyl modern handheld meters with a 10 M resistor in series for everything and than an inverting amplifier at the input.
10 M in series would add quite some additional noise. So for 5 digits and more this is no longer a good option.

The 0.58 V seen in case of an open input could be just coincidence - it is not unusual to have an open input to float all the way to overload.
If one has a way to record data, one could look at the speed a capacitor charges / discharge from a lower and higher voltage and see if the picture is more like a constant current plus resistance or more nonlinear with the bias current changing faster when above 0.6 V.


Other contributions to the input current can be U102 - what type of OP is this ?
corehbola:
Hi,
Well, first of all, Q103 and Q127 are not the culprits... I removed them altogether, and it didn't change the behavior whatsoever...
Spot heating components didn't bring up any conclusion, maybe because after it hits about 0.58V it stays there so, there's no room to see it worsening... This already smells like the source voltage of the leakage, whatever it is, is around that value (and not that it's being "clamped" at that level).
Spot cooling components, didn't help either, I tried cooling one by one all the multiplexer FETs, U102 (AD542 dated 8429), U101 (the bootstrap/ guadring buffer)... Of course that, at times, when I cooled them too much it would go all over the shop... but then I cannot rule-out some condensation. Everytime I cooled them carefully, I couldn't see anything outstanding.

But one thing did make A LOT of difference.
I lifted the input side of R107 and two things happened:
1. Leaving it opened, the build up voltage slowed down more than 20~30 times. Especially past 0.3V (granted, maybe this slowing down past 0.3V was already there, but since it was going relatively fast before, I didn't notice).
2. When I connect the 10Mohm resistor directly at the tip of R107 to ground, it still shows a predominantly positive voltage, but it visually averages at much lower levels, (most of the times the display is showing values around 100uV).

Both of those above can be very well from U102, but I would be very happy with that already.

With R107 disconnected as it is, it leaves very little (component wise) as possible culprits.
a. The P.C.B. (surface contamination under the switches because everywhere else is quite clean).
b. The Switches themselves - most likely -  |O .
c. Leakage from the relays coil to the contacts (I know, very unlikely, but easy to rule-out at least).
d. A handful of range switching FETs connected after the 10Mohm resistor. Again very unlikely because there's no positive voltage there for the leak to come from (only negative voltages at the gates, at least in DCV ranges).

Other than that, I can only think of RFI rectification, that is finding its way into C109... but from what is left of components, who would be the one rectifying? The switch contacts? The FETs?
On that hipothesis... I wonder if it would help to add a ferrite bead at R106 and R107 leads?

More to come... I`ll let all know.

Thanks for the insights so far.
 
trobbins:
Hi Fabio, thanks for pursuing your 197 issue and discussing it.  I haven't as yet pursued further assessment of my own 197 issue, but may well get keen to do that.  I'm not keen on removing/swapping parts if I don't have to, but I would take on board Kleinstein's comment about warming individual parts to see if anything was noticed.  The use of cooling seems a bit more complicated if taken too far in step temperature, or contributing its own leakage.
Ciao, Tim
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