Electronics > Repair
Keithley 197 - jitter/jumping last 2 digits
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Kleinstein:
The PF5301 is indeed a very high impedance JFET, about equivalent to the 2N4117 / 4118.  This may be useful at some positions, but not that well suited at other positions in series with tha divider resistor, due to the high on resistance.  Chancs are they should used different fets for switching. The exact type should not matter that much, unless the leakage currents get too high. There is an PF5103, that is quite similar to the PN4393 / J113.
The fets  Q105, Q106 , Q109 and Q116 should be more lower resistance types. Q109 may even be more something like PM4392 for even lower resistance at the cost of more votlage needed to turn off.

With the leakage of the FETs, the actual leakage is often quite a bit better than the specs - which are more like test limits. Testing for very low currents takes time and is thus expensive. So with many types the spec limits are not very tight and typical parts can be way better. This is one reason why selcted parts may be specified to get parts screened for tighter limits.

The on resistance is not equal to Vds / IDSS. If not directly given a better estimate is  1/gfs. So for a gfs of 70 -500 µs this would be 2-14 Kohms.
The measurement should be some with a relatively low testvoltage, a higher test voltage can effect the result.
MathWizard:
I have a 197 and used MG chemicals contact cleaner (legit brand), and the performance was horrible after, it left a resin in there, I had it take it all apart and clean it, which was a nightmare.
 
Does anyone know a good replacement relay for these? In the schem. it's the K102, custom, Coto 3500-0021 instrumentation relay. I've searched a lot online, and I can't find info on the custom model#, or replacements.

It's some Low Thermal EMF, High voltage, w/ Hg option available. So IDK what the contact materiel was either. It's single coil, normally open, shielded/gnded. It uses 5V w/470R coil. I'm not worried about the driver logic, I just need a recommendation for the spec's, or any similar relay or suitable replacement for a 5.5 or 6.5d meter.

I haven't looked at the schematic in a long time, but I made a sim of most of it. I never understand the slope-integrator section good enough to get it working. But the rest of it worked ok.
Kleinstein:
It should be relatively rare that the relay fails. If leakage gets a problem, a good bake out (some 120 C ? for quite some time) could help, by removing water. It would at least be a chance.
Low thermal, high voltage reed relays are tricky. To some degree a 2 pole relay with both contacts in series helps: it gets the higher voltage rating and the termal gradients tend to be symmetric to give some compensation of the thermal EMF. With Hg wetted contrcts the choice of contract material is very limited - it is effectively Hg. Many of the normal contact materials react with mercury.

The ADC differs a bit from the normal multi-slope ADC in that the run-up part is combined with the input before the switch to the integrator and the rundown part is with a single slow slope only. So it is multi-slope run-up and just 1 slow slope rundown. The supply to U117A is the main reference.
MathWizard:
I burned out the coil somehow switching the diagnostic modes, and then I really broke it trying to get it out of the can, I should have heated it up, or cut the top of the can off.

I've only looked at ideal op-amp integrators, next time I get deep back into this thing I'll know a bit more. Hopefully I haven't damaged anything else I can't easily replace.
here's a pic of the NI of U108, when the DMM input shorted, the yellow trace


Swainster:
Just to add, my K197A suffered from a significant offset in AC volts (didn't check AC current but was probably the same). This offset caused the 2V range to read something like a volt, and the millivolt range to overload even with a shorting plug fitted I.e. it was pretty easy to spot. I traced this to the output of U104, where it's DC level went further and further from AGND as the ranges were switched, hitting the rail at the millivolt range (5x gain mode). In my case, at least, this did indeed turn out to be caused by faulty CR102/CR104. According to another K197 thread on this forum, these are just 1N4149. All I had to hand was some off-brand 1n4148, but these fixed the issue and got the lower AC ranges working again. Having said this, I've since stocked up with some quality 1N4148, 1N4149 and low leakage BAS45A. Is it worth me going back in and swapping out the dodgy 4148s for the BAS45As?
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