Electronics > Repair

Keithley 197 - jitter/jumping last 2 digits

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trobbins:
My unit hasn't been serviced for decades afaik.  Display levels were jumping around at lower end quite noticeably, so recently had some time to open it up and with the unit vertical I could carefully flush the front panel slide switches using a contact cleaner and excercise the switches, and let it dry overnight.

Jitter/jumping is much reduced, but still consistently there between display refreshing on lowest two digits on all ranges and all modes (V-ohm-amps) including AC, with inputs shorted, and after a number of hours operation and with switches being excercised. Jitter reduces to just last digit typically on 200k and Megohm ranges.

Same jitter/jumping when testing a stable DCV like 5V.  As ranges increase to 1kV the 5.xx display is jittery on last 2 digits.

With a 10Meg resistor across the inputs, and 0.2VDC range, the display jumps around between 0.5xxmV and up to 2.xxxmV.  With a 2VDC range, the display jumps around between 0.0005xV and up to 0.0018xV.  That indicates an input leakage of circa 50-200pA, so perhaps not gross.

I guess that may suggest a slightly noisy common part of the circuitry from U102 onwards (ie. through Q120, Q121, U110, to U114 pin 17, and the phasing/clocking signals of the single slope A/D circuitry), as the jitter is common to all ranges and modes.

The closest relevant thread topic appears to be https://www.eevblog.com/forum/testgear/sick-keithley-197a/msg1139739/ .

Any advise appreciated on whether this type of fault has been troubleshooted to a particular region or parts.

Tim

Kleinstein:
For an old Keithley meter one of the first points would be to check the filter caps. It may not be as urgent as in K2001 / K2002, but one may consider a change before they leak. At least I would measure the supply ripple.
The ADC uses the 5 V(A) as the reference signal, so this supply has to be really clean / low ripply.

I don't think the clock signal would be a problem - this is more the digital domain.

Some of the switches ( V , A ) can also effect all ranges.

trobbins:
Thanks again Kleinstein for responding.

My unit has the GPIB accessory board, so I removed that assembly as the simplest first-in change to loading the power rails.  Luckily that pcb assembly is the root cause of the jitter/jumping, so I can easily troubleshoot that aspect (the main 2,200uF 16V bulk filter cap on that pcb is just showing a slight bulge on the X vent).

But like rocks in a river as the water level drops (or in Australian rivers its the 'snags'), I can now easily discern a small offset in ACV readings with the input shorted.  The 200mV range is showing 62uV; the 2V is showing 0.00011V; the 20V gives 0.0033V; 200V gives 0.020V; 1kV gives 0.11V.  There is no offset display for shorted DCV except 0.0003 on 20V range. 

So I guess that indicates some drift in operation related to the AC measurement circuitry around U104 and U106 (there is no change with AC/DC switch wiping). 

CR102 and CR104 are both 197-604 in the BOM.  It is a pain to visually identify the actual part number for those two diodes, but along the lines of a previous comment from you, a likely swap out would need a low leakage BAV199 (which I don't have to hand so can't easily test that scenario).

There is a 15uF polarised coupling cap between U104 and U106.  I could also add a temporary short on Q118 output to see if that is an influence.  I could also add a temporary short on pin 13 of U106 to local ground to see if that shows a non zero display, or confirms the issue is upstream.

Tim
 

Kleinstein:
I doubt CR102/CR104 are the problem. those small diodes are normally rather robust. If needed for a test one could use the base to collector junctions of small transistors (e.g. 2N2222, 2N3904/6). The BAV199 would be OK, but it is a smaller case.

Excessive leakage with C113 could cause some offset problem as an DC offset of U104 would be visible. For a test one could use a smaller (e.g. 1 µF)  film cap to see if the offset disappears.
A small offset when in AC mode may be normal - there is noise and the RMS converters may not work that well at near zero. 

trobbins:
I just shorted the U106 (AD637) input to gnd, and the display for 200mV range is 27uV; 2V range shows 0.00013V; 20V range shows 0.0014V; 200V range shows 0.013V; 1kV range shows 0.13V.   The AC Amp mode shows consistent last two digits as 28.  So it looks like that is down at the noise floor end.  I would have thought that offset was managed by calibration offset, although I don't see the 0V end being calibrated for ACV in the cal procedure, only near FS levels are calibrated.  I have Rev C, so have the calibration enable switch on the rear panel. 

The U104 (LF411CN) has a programmed gain of 5x for the 200mVAC range, so that seems to be the origin of the relatively large 62uV offset, compared to the other ranges being pretty much the noise floor of the AD637.  I quickly tried adding a 10k offset trimpot but it didn't reduce the 200mV range offset, so I think I will just have to enjoy having some offset (as I'm not keen on swapping out the 15uF tant cap, or any other surgery for now).

I had to replace the three main filter caps on the GPIB/Analog output board to eliminate any last digit jitter, although the biggest cap with the slight bulge was the dominant contributor.  All caps were Nichicon 85C, so perhaps could have done with the 105C range in hindsight.

PS. I can see now that the ACV specification is +/- 100 counts, so it seems the meter is within spec.

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