Electronics > Repair
HP 3456A voltage offset
<< < (8/10) > >>
Wallace Gasiewicz:
I thought you had already replaced Q 110???
stevopedia:
I did. In my excitement I got it wrong--the one I actually did just now was Q105.

Also, the new part produces the same offset as the old one. Still, this is a powerful clue, and if nothing else I'm fine with sacrificing 4-wire ohms for an otherwise working instrument.

Edit: I can't even say that much. The offset quickly increases (decreases?) to about -5.0 μV over the course of a minute or so after power-on even with Q105 removed. So much for that.
stevopedia:
So I replaced the PN4392 as Q105, and I think my pessimism over that not working was a bit excessive: the offset is definitely reduced with the new Q105 in place vs. either the old part or no part at all.

I think the noise on TP303 (input amplifier output to ADC) is responsible for a lot of what I'm seeing now, both the offset and the noise in the measurement; the least significant (100 nV in the 100 mV range) digit is quite unstable/noisy even with the input shorted and generally always has been. I checked all the power jumpers by the ADC section (JMPR403 and JMPR503-507) and the +12 V and -12 V rails had the same noise that I saw on TP303. That certainly won't help, especially given the -12 V rail sets up the constant-current tail on the input amplifier's input JFETs Q310 and Q311.

Looking at the schematics (mine has the LM399-style reference of schematic 5B), there's shockingly little in the way of filtering on those rails. I count exactly one cap on -12 Va, a 0.1 μF wet tantalum (C401) which won't do much to reduce high-frequency noise. -12 Vb has a 2.2 μF wet tantalum, C404. But the +12 V reference has no filtering at all!
Kleinstein:
Not much decoupling on some of the voltages is not good. However the problem is that some voltages, like the +12 V,-12VB or the +30 V (at the amplifier) are directly driven from OP-amps and thus can not have much capacitive load. So there is no easy way to improve on that situation. Part if the signal on the supply is likely normal and that way by design.  Instead of a normal decoupling capacitor one may have to include a series resistance to get at least some dampening.

At  leat the +12 V part seens very little load and should not see transients - so here capacitance is not needed.
The problem is more the -12 V.

I agree that the "noise" / high frequency background on the amplier signal can be a problem. It may be enough to get some rectification at the JFETs and additional bias this way. If all supplies and the signal seem to have the same noise, maybe check the ground that is used for the scope. It could be just a bad ground point.

Not having reference filtering is a common point in many DMM designs. The LM399 ref. has quite some white noise that can effect the ADC. This part is missed in other DMMs too.  Adding some ref. filtering could be possible, e.g. at the input of U501 and between pin 2 and 6 of U505. It could be worth the relatively moderate effort.
stevopedia:
For the moment I'm focusing on the slope-switching noise/hash I'm seeing on the +/- 12 V references--it doesn't show up on any of the other power rails. Of these, -12 Va is probably the most important, since the others are generated from it, as you noted. I think the noise is coupling in there and U504 and U505 are faithfully duplicating that noise on their outputs.

Yesterday I did a little further investigation. I found that the switching hash was present on pins 2 (- input) and 6 (output) of U501, but not at pin 3 (+ input). I connected a 470 pF ceramic cap directly across U501 pins 2 and 7 (ground, conveniently) on the bottom of the board and this quieted U501's output beautifully. The noise was still present on the collector and emitter of Q501. (It was much stronger at the collector--I think Q501 is acting as a common-base amplifier for it in this case. Not especially important, but I thought it was an interesting observation.)

U402 looks like it could be the source of the switching noise. I'll try putting a 470 pF cap across its power inputs (not coincidentally -12 Va and ground) and see what happens.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod