Electronics > Repair
HP 3456A voltage offset
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stevopedia:
It didn't work. Nothing changed.  |O |O |O |O

I am so close to giving up on this thing again. I don't know why I'm suddenly so demoralized--perhaps because I thought I'd finally cracked it after ten years--but the fact remains.
Kleinstein:
The collector of Q501 should not have much AC signal and there is already a capacitor to ground (C506). One could try adding extra capacitance there. Different from the amplifier output there is no problem adding capacitance there. With not enough capacitance there the transistor can not drive the -12 V fast / strong enough as it should. Maybe a problem with the ground connection ?

There is already a 100 nF capacitor (C401 on the ADC page) at the -12 Va. So 470 pF more does not make a big change.
From the scope pictures it looks a bit like ringing as the interference signal. Some RC combination e.g. 22 or 39 ohms and 100 nF could help in dampening the oscillation more than added capacitance.
Also just a little more load (e.g. 5 K to GND) to the -12Va could help, as the output resistance of the transistor gets lower with more current.


The loading of the reference switches is a principle tricky part for this switch configuration. So don't expect the -12 V to be perfect, there is some good enough. After all the HP3456 normally works OK and is usually considered a good meter for it's time, depite the weakness with the -12 V ref. / supply.
The question is only how much AC part is normal acceptable for the -12 Va.
stevopedia:

--- Quote from: Kleinstein on October 17, 2024, 07:02:08 pm ---The loading of the reference switches is a principle tricky part for this switch configuration. So don't expect the -12 V to be perfect, there is some good enough. After all the HP3456 normally works OK and is usually considered a good meter for it's time, depite the weakness with the -12 V ref. / supply.
The question is only how much AC part is normal acceptable for the -12 Va.

--- End quote ---

You're absolutely right, of course. I've been letting myself get carried away. I think it's time to take a deep breath and a step back. I also have the luxury of access to a working 3456A I can take reference measurements from--a capability I haven't taken advantage of.
stevopedia:
I took the opportunity today to measure the gate leakage current of all the switching JFETs I've recently extracted from the 3456A with a Keithley 485 picoammeter. For most of the FETs it was too low to measure (less than 100 fA!). But for the one that was on the analog filter circuit, it measured about 50 pA with 18 V reverse bias on the gate-channel junction. On a whim I replaced Q102, Q103, and Q104 with some of the verified-good original 1855-0298s, but as expected this made no difference.

With the Volts inputs shorted, the meter's offset is about 6.5 μV. I tried shorting the lower input node (Q102, Q103, Q104, Q105) to the low Volts terminal with a probe lead and the displayed offset went to zero, as expected. But when I shorted the upper input node (Q109, Q116, Q115 etc) to the low volts terminal, the offset shot all the way to -1.1 mV, a full three orders of magnitude higher! I don't know what to make of that, though it would seem to me that if the instrument was working perfectly then shorting the upper input node to ground should also result in a 0 V reading... though I'll test that on a known-good unit tomorrow.
Kleinstein:
Soemthing like 50 pA at 18 V is about what is expected from a somewhat bad JFET. Less than 100 fA is surprisingly good for the good ones.

Getting the zero readign with the lower input not shorted is no surprise, as both reading of the AZ cycle would see the same voltage and thus zero difference. There much be something badly wrong to get a different result.

Connecting the upper input node (Q109,Q115,Q116,...) to the input should not make that much difference. It could make a difference for some self test / gain check, that tries to read the internal -12 V reference. To protect Q112 one may want some series resistance (e.g. 10K) and not a hard link to the input terminals. A wrong ref. reading would result in totally off readings - maybe the factor 1000 to high, though it should trigger an error message too.
The test with the upper terminal ground could still show a small offset, e.g. from amplifier input bias acting on R104 and the offset from R149/R150 that would still be active for the zero reading.

A point to check could be if the offset changes between 1 PLC and 10 PLC setting (100 mV range). A difference can come from addition input current from the switching / compensation ciruit around R147/R148 / Q102.
Too much offset of the Q120/ U105 combination could still be an issue here, though I would expect only a small error. If needed one could adjust R126 or R127 to trim the offset. This may be easier than exchanging the dual FET.
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