Electronics > Repair
HP 3456A voltage offset
stevopedia:
Hello all,
I've got an early (serial 2015A04414; has a fan) HP 3456A DVM that's been beguiling me for a while now. Ultimately the fault is that it displays a DC offset of more than 100 counts with the input shorted when cold, increasing to 400 counts or more as the instrument warms up. It passes all the self-tests and I've followed the procedure for input switching troubleshooting (8-C-26 in the op & service manual) at least twice now. I got to the final step of "replace Q102 and Q112" both times, and replacing them (once) with new PN4392s (by Linear Systems, sourced from Digikey) had no effect. The offset is real, in that it follows range adjustments and can be measured with another hi-Z DVM.
Here's a list of all the tests, changes, or other work I've done thus far (all parts are on A20 unless otherwise specified):
* Replaced all the electrolytic caps in the inguard and outguard power supplies
* An error in the procedure under 8-B-10 in the OSM (steps c. and d. should be swapped) lead me to replace U405 with TLE2071 and Q408 with PN4392. As it happens these were unnecessary because autozero has always worked in this instrument, but as the comp changes haven't seemed to cause any problems they have been left in place
* Following the corrected procedure under 8-B-10 shows the ADC is working perfectly (checked with Valhalla 2701C and HP 3458A at work)
* Found input amplifier oscillation; cured by replacing U307 with TLE2071 (fixed self-test failures on 4-6)
* GB2 has an offset of about 17 mV--OSM says to replace parts if offset exceeds 5 mV, but I doubt this is really an issue
* Tried replacing Q120 with LSK489 and U105 with OP07 (C108 was removed) but this made no change. GB1 output offset is about 10 mV now. This was unfortunately done before I started taking meticulous notes, so I have no record of how it performed before the change, but it having made no difference is enough for me in this case
* Noticed C701 (on the unregulated +33 V line) is bulged, but don't yet have a proper replacement
* Jumpering across K103 at R09 & R110 has no effect on the displayed offset
At this point I'm completely out of ideas. Admittedly I'm far from an expert in these instruments, no matter how thoroughly I've read (parts of) the OSM. What am I missing?!
Kleinstein:
Is the offset a fixed 100/400 count, or a fixed voltage (e.g. 10 µV) ?
Where is the offset measurable with another meter ?
A point to check would be if the problem is really an offset voltage or maybe way too much bias current.
One could trim the offset of the GB2 buffer via the resistors R320/R321. It is not ideal (may have more temperature drift) but easier than changing a dual JFET. I still doubt it would make a big difference for the overall offset.
There is an offset adjustment via R150 ( resistor choosen to compensate an offset) - so maybe check how much offset this really is.
stevopedia:
The offset is a variable voltage. The numbers given are approximate.
By memory, the offset appears somewhere between the switching input at R102 and the output at J19, and can be observed at all points downstream (TP303, TP401, etc).
Bias current is a new one for me. How do I go about checking that?
The OSM has a procedure for determining the correct value and placement of R150 as part of 8-F-18. The values it gives are for offsets of 2.0 μV or less. But the offsets I'm seeing are 100 counts or more on the 100 mV range, equating to 10.0 μV or more. The OSM simply says: "If the offset is >2 μV, calibrate and/or repair the instrument."
Kleinstein:
Ideally the meter should have a very low input current (e.g. < 20 pA or so) with zero input voltage and maybe a little more from the input resistance with high voltage. When dirty or with some bad parts the current can be higher and the input resistance lower.
A simple test is comparing the reading with a short and some 10 M across the input (may be the meter internal divider and thus 10 M input resistance mode). A test also for other voltages can be done with a low absorbtion (e.g. C0G, PP or PS type capacitor of some 10 nF) at the input and abserving the rate of voltage drift. 10 pA of input current would than cause a drift of some 1 mV per second, that can be observed at the meter. With the capacitor one can start at different voltages and this way check the input current at different voltages, nor just near zero.
It is normal to have some offset at TP303, TP401 and so on. The question is if the offset is also there in the auto zero mode.
Is there also an offset in the 4 wire resistance mode ? There may be an extra zero compensation in software, that could make this test useless. However ideally there not be an extra constant.
For some odd reason old LM339 comparators have a tendency to fail. So it may be worth checking of the gate signals for the input switching are all working, especially the gates of Q104 and Q102.
stevopedia:
Remember, I'm seeing this offset with the inputs shorted (my shorting "bar" in this case being some 2.5 mm desoldering braid, saturated with solder and just long enough to be securely clamped in the terminals). So while I don't think input current is the culprit, I'll try the procedure you described when I get home. (It's both clever and effective--I like it!)
I should have said sooner that I can bring at least the 10 V range into cal, using the Valhalla 2701C and 3458A I mentioned in my first post.
I'm aware of the offsets generated by the input amp and the correction for it that AZ provides. If you're asking whether the 3456A displays the same offset if AZ is on, the answer is yes.
Now that you mention it, I haven't checked any of the ohms ranges yet, being so focused on DCV. I'll test them and see what happens.
I also haven't actually checked the gate signals on the input switching FETs either--another good catch! I'll probe those too.
Thanks again for the pointers! :) I'll report back with what I find. (That will be at least five hours from now!)
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