Electronics > Repair
Keithley 197 - jitter/jumping last 2 digits
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corehbola:
Thank you,
And apologies for having hijacked your thread but, but your description of the offset voltage when applied to a 10M ohm resistor was (and is so far) exactly the same as mine, so I really thought there could be a common root cause.
I`m not a collector and I only want a meter of this league, working as well as it possibly can so, I`m not much worried about swapping parts (if necessary) for the sake of troubleshooting, or lifting some component leads to rule out possible causes.
And also, because the guy who I purchased this meter from, has quite a decent lab at home, with a reference voltage generator and a good and recently calibrated 7 1/2 digits HP multimeter, and he told me already, should I chose to chase the minor flaws this meter has, that after I`m done, I`m welcome to take the meter there again and he will re-calibrate it for me.
So, as it stands, I`ll take my chances a little bit further at least, because the worse it can happen is losing its calibration and I can have it calibrated again.
If, in the process, I find some flaw that can be useful for your meter... it will save you from the troubleshooting.
This meter, it's in general good shape and doesn't have signs of extensive rework. The soldering of pretty much all electronics is its original wave soldering (or was until I start touching it).
But it does show it has had its switches replaced (because some pads are in bad shape) and there's one broken trace underneath the switches (like if it has been fused, suggesting some pretty nasty overload) and this broken trace has been bodged wired on the bottom side of the PCB.
From the looks of it, whoever did it, was some real service center (Keithley or else, I can't tell) because they used teflon insulated wire to do the bodge and not some random piece of wire, and the switches are original replacement parts for what I can see from pictures of other 197s.
I`m convinced that it's a good opportunity to try to improve it, and in the process gain some experience in servicing this kind of meters. Some components may be difficult to source, but it has not a single proprietary component, and this is a very good thing.
I`ll let you know.
trobbins:
Fabio, I have no concerns at all about this thread being used for any similar 197 faultfinding - that is the benefit of threads as a means to collate info and make it easier to find :-+
corehbola:
Thank you TRobbins,

I didn't find the culprit yet, but matters have advanced somewhat.

As I mentioned earlier, this unit has signs of rework in the gang switches (desoldering pump marks on several of the PCB traces and pads) and, as it also had residues of solder flux cleaning (or a failed cleaning attempt as the underside of the board was gross), I thought that, although risky, I would never be sure of the state of the switches and the level of contamination on the PCB, under the switches.
And, as it goes, where some tens of pA is enough to produce errors in hi-impedance readings... I decided to remove the whole switch assembly and clean the board and clean the switches thoroughly (by taking them all apart and, physically cleaning them inside and out and re-lubing).

While I'm at it, I took some pictures of the board, under the switches, and post here for future reference.



Studying the manual, I think I have a solid plan, that will allow me to spot what is the origin of the input leakage current that is causing the 1~2mV readings when I ground the leads through a 10Mohm resistor (just like in your unit).
Using machined round socket pins, I`ll socket the four MUX FETs (Q110 to Q113) and R116 (the 200k resistor that goes into the Input Buffer AD542) so that I can isolate any of them, together, or one-by-one.
Then, using the troubleshooting mode and selecting between modes 0u2, 1u2, 2u2 and 3u2, and connecting an external hi impedance meter to the 197's input (where the voltage is building up) I will be able to identify the source of the leakage current and, with some luck, source the parts that are needed to amend it.
Finding a replacement to the AD542 nowadays is not at all difficult and there are many alternatives that can match or out-spec this old part.
The MUX FETs... Those may take a while to arrive and are not exactly cheap, but they're still orderable from DigiKey so I think I will order a few of them even if it's for the sake of it (or for the future).
As for the attenuator range switching FETs, those ones are not yet so clear to me if they can be obtained and which current part number they would be but, on the other hand... there's no way they can be the culprits (at this point at least, where I`m only testing on 200mV and 2V range), because they can't possibily be the source of ANY positive (outgoing) current, as they're not connected to any positive voltage source....

And, by the way, I`m doing all these testing now with all the switches removed. I soldered jumper wires to emulate the switches in the positions Power ON, DC, Volts, 2V range to elimintate any extraneous source of leakage.

I'll keep you posted.
Kleinstein:
The switching JFETs should not be that critical. Likely most of them are some PN4393 / J113 or similar as the typical switching JFET. Many others should be OK, as the voltage is still relatively low and this is only a 5 digit meter.
corehbola:
Thank you Kleinstein...
I`ll keep those FET P/N in mind... As you well said, TO92 FETs are not that common nowadays and it's always good to know some suggestions.
The MUX FETs on the 197 are of type PF5301 and are still available from Linear Systems and for the likes of it, their main "selling point" is the low leakage, at max. 5pA.
I don't have enough experience with FETs and with this level of low current to be able to even tell what is the typical leakage of a "vanilla" switching FET, but I assume that it's probably bigger than that 5pA, otherwise they wouldn't bother to highlight this particular spec.
They don't have particularly low channel resistance. I measure the ones in my 197 at anything between 7kOhm and 12KOhm.
The datasheet of the PF5301 suggests even worse than that, with Idss from 30 to 500uA (@Vds = 10V and Vgs = 0V) suggesting an Rds ON between 20k and 300k (not very "on" to my taste, but in these hi-Z application, it shouldn't matter anyway).

I wonder if you own yourself a 197 (or 197A). I noticed mine is probably from a very early revision.
Besides many small (and probably insignificant differences) I noticed one particular difference between mine and the only available User/Service manual there is, which is about the Guard Rings around sensitive components.

The generally available manual mentions basically 2 major guard rings... one around the range switching FETs, connected to the bootstrap buffer U101A output (pin7), and the other around the MUX FETs + Input Buffer OpAmp, the AD542 U102 and its gain switching FET, connected to the other bootstrap buffer, U101B (pin 1).

It happens that on my 197 in (and I don't know if in newer ones it's like mine or it's like it's shown in the newer documentaiton), the guard ring around the MUX FETs is not connected to U101B pin 1, but it's actually connected to ground (so it's not really like a bootstrapped guard ring as the schematics suggests, but it's more like a ground shield).

I would like to know how is it actually in the 197 newer models... if it's connected to ground (and the schematic then is just plain wrong) or if they really changed it on the PCB and made it follow the guard ring of the input buffer (instead of ground as it is in mine).

Rgrds,
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