Electronics > Repair
HP 3456A voltage offset
Kleinstein:
Some -2.8 mV at 10 M indicate an input current of 280 pA. So too high in the input current. The specs should be like < 20 pA.
If the leakage is in the pos side part the effective resistance are some 104 K and 280 pA would result in some -28 µV offset.
Balancing with R104 would only fully work if the leakage is more at the amplifier input or other side of Q103. So the input current could be the culprit for the offset.
Reducing R104 would increase the imbalance in the resistance. With the extra switch Q103 the resistor balance may not be that important anyway. So not such a good idea.
Due to the heating effect I would suspect Q115. For a test one could see if a voltage (e.g. 1.5 V battery) applied between the sense H and Voltage Lo terminal would effect the offset reading. If there is quite some effect this clearly points to a bad Q115. The test procedure for Q115 in the service manual should also work, but needs to open the case. For a first test one could check the function even with Q115 removed (obviously no 4 wire ohm then).
The test procedure for Q104 looks a bit odd to me. Chances are it would show a change even if a different FET is leaky. It adds a 2nd path to ground reducing the resistance to about half and thus expect half the effect of input current.
The 100 V range shows relatively little offset (-0.8 mV compared to -2.2 mV expected from 100 x the 1 V range offset). This make Q109 a little suspect - maybe worth retesting with some heat.
stevopedia:
After reading your reply I took a closer look at the input switching schematic. I admit I half-glossed over it before, but something about what you said made it click for me--thank you!
I'll try your idea to apply a voltage to the 4-wire sense high terminal as soon as I can.
I should mention that in the course of my testing last night I found that there was a substantial voltage across the switching output resistor R103. The main DMM I've been using in the course of this diagnosis is a Keithley 196, which is rated to > 1 GΩ input impedance in the 3 V and 300 mV ranges. Using that meter with its low input connected to the 3456A's low Volts terminal, I found that the input switching side of R103 was at around +1 mV and the J19 side of R104 was at around +0.02 mV. That condition persisted even after disconnecting the input amplifier from J19. That would imply 100 nA flowing through R103 via some parallel path. That seems substantial to me now--I should have investigated that more closely. I'll do that too, when I can.
I'm fairly certain Q104 is a major part in the autozero system, so in hindsight I completely agree that with your assessment that the manual's procedure for Q104 is flawed. If the two input nodes are for some reason not at ground even with the inputs shorted, then deliberately turning on Q104 will force a change in the measurement whose direction depends entirely on the polarity of the charge on the upper and lower input nodes (as the OSM refers to them). That's not the first problem I've found in the manual either, unfortunately.
By the way, are discussing drain-to-source leakage or gate leakage with these FETs? Looking at the schematic now, it seems to me that shorting the gate of Q115 to the low volts terminal (which I did try, and per the manual it tested good) with the high reference terminal open would serve only to test Q115 for gate leakage or perhaps gate leakage of diode-connected Q117 since the fail condition is the displayed offset moving in a positive direction or to zero. Then again, since the high ref terminal is normally not connected, a Q115 that was leaky drain-to-source wouldn't cause any problems if Q108 and Q117 were both good.
stevopedia:
Applying +10 V to the high ratio ref/sense terminal with the Volts terminals shorted had no effect. Same with applying -10 V. :palm:
Wallace Gasiewicz:
I know you already replaced Q 102
I had a very similar problem where I could not get the 100 mV range to adjust high enough on my 3456.
After switching multiple FETs and doing lots of things that did not help, I eventually replaced Q 102, the pre charge FET. Everything works fine now.
I believe that that FET was the problem although I changed so many FETs that I cannot prove it. Anyway that was the last change I made and before that the 3456 would not adjust high enough on 100 mV range and after that replacement it now adjusts easily..
I do not recall what the shorted value was I was using a newly calibrated volt source.
I do think that I had problems on the other ranges, but I was able to adjust them out, so I was fixated with the 100 mV problem.
The alleged troublesome FET tested OK when removed.
As I remember this my repair went on for far too long. Good Luck!!
Kleinstein:
The main suspect is gate leakage. With open ohms sense inputs there should be little voltage on most of the other JFETs that may have drain - source leakage.
As the 4W resistance case shows little offset (1 µV range) and with the strongest heat effect at Q115, I would still suspect Q115. This is even though the external voltage did not have an effect. The test for Q115 in the service manual should work better and should also work with the meter in AZ mode (less noise and drift). One could also repeat the offset test in the 4 Wire mode (100 ohm range) with an added resistor (1 M or 10 M) in series to the sense H input. This would measure the input current for the sense H input. If Q115 is the culprit the sense H input should have less input bias current than the main input.
There is a chance that the leakage can be from more than 1 souce, as the 100 V range showed less (like 1/3) offset than expected, though in this case there is still a chance for some thermal EMF at the divider and trimmer.
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