Electronics > Repair
HP 3456A voltage offset
FrodeM:
What is the reading at TP104?
I had a 3456A with a front-end switching problem once, and when measuring TP104 with another known good 3456A, it mirrored the exact same offset as my unit had. From that I was able to realize that the unit was measuring the gate bias and not the shorted input terminals. Turned out a previous owner had applied some sort of oil(!) to the connector between the two inboard PCBs, and in turn random switches were stuck.
stevopedia:
Thanks for your inputs, everyone.
FrodeM: With the input shorted, TP104 is steady at about -17.5 mV (yes, I know that's technically out of spec, but I don't think that's my issue). It does not move with the 3456A's displayed offset.
Wallace: Thanks for the reminder to adjust the charge balancing circuit--I've probably done enough under the hood to warrant that. This particular instrument has been beguiling me for about ten years now, on and off. I started thinking of it as "the basket case" a while ago for a reason!
Since I couldn't shake the feeling of going in circles chasing my tail in this troubleshooting, I've done some sanity checks on my 3456A using the HP 3458A and Valhalla 2701C I'm lucky enough to have access to at work. I've verified that the ADC and input amplifier are both working extremely well, so the problem absolutely must lie within the input switching. I also verified with the 3458A that there is in fact no difference in voltage across R103, so I'm not dealing with stray current paths--the low-range DCV input impedance on the Keithley 196 is simply low enough to have a significant impact on the circuit.
I also tried recalibrating the DC V ranges just to see what happened. I was able to bring all the ranges into cal at full scale, but they all read substantially low (some 20 counts with a 6-digit display) at 10% scale--for example, with 1 V input on the 10 V range the instrument showed about 0.99980 V. Since I'm quite certain the ADC and input amp are both good, this means my apparent linearity error is actually an offset arising within the input switching that is proportional to the voltage at the high Volts input. That would align nicely with a leaky gate somewhere, I think. And I just realized that gate leakage implies we're looking for FETs that are off in the DC V ranges. Now I understand even better why Q115 is particularly suspect.
I tried a test for 4-wire ohms: I shorted the Volts input terminals together and to the Ohms Sense Low terminal, and connected the Valhalla 2701C across the Sense terminals. I found that the offset was still present in that configuration, if I was reading the results correctly. So given a FET with a leaky gate must be turned off, I think I can rule out Q115. I have ideas for more things to try, but I do have to get some Real Work™ done today!
Kleinstein:
A FET with a leaky gate can still turn off OK. There can be separate leakage from the gate to source and drain (no need to be symmetric).
I point that hints to a bad Q115 is that the 4 wire ohm case showed very little offset. 0.8 mohm correspond to 0.8 µV and thus way less than some 20 µV seen in the DCV case.
In 4 Wire mode the input path switches between sense H and sense L. So Q115 is still off half the time, so an external meter could still detect leakage current at sense H, though not necessary the same as to the meter input side.
Current through R103 would be the input current of the amplifier. Even if there would be some input current there this would see a resistance of some 100 K in all paths and thus a rather constant offset for both readings in the AZ cycle and thus less (like 1/10) offset from the same current.
The likely culprit is gate leakage at one of the FETs (Q115,Q109,Q112,Q114,Q120,Q110, Q118) that than causes a voltage drop at the 4x27 K for the protection or R106 or the 100 K from the divider. For the other reading in the AZ cycle Q103 is off, and thus the current not reaching R104 for the zero reading. Leakage from Q104 would be off when the FET is on. So it could also cause and offset voltage and measurable input current.
Gate leakage from Q103 would cause a positive offset as the zero reading would be negative.
stevopedia:
I found one of the culprits!
Earlier today I found that turning on the analog filter caused the input offset to drop substantially--it seemed to have been the primary source of offset when the instrument was fully warmed up. So putting that together my realizations I replaced Q114 with a new PN4392. And--it worked! The offset with the filter off is now the same as with the filter on! I'm going to let it warm up again and see how the offset changes then. I've got four new 4392s left--hopefully that will be enough.
I hadn't considered that Q115 was actually switched off during half a 4-wire ohms measurement cycle. In that case, it definitely could still be a problem.
stevopedia:
I tried replacing Q115 earlier today with another PN4392 and it didn't seem to have any effect... offset is still sitting at around -60 mV after warmup, same as before replacing Q115.
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