Electronics > Repair
HP 3456A voltage offset
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Kleinstein:
There is quite some high frequency background at TP303. Also for the case with the filter off it does not look like a clean signal, but more like something oscillating.  An oscillation can also cause an offset. At least it is not a good sign to have that much background.
A point worth noting is that the  background changes between the normal input reading and the zero reading in the AZ cycle. This makes me think of some unwanted coupling to the input / switching part or filter circuit and not so much the supply to the amplifier.
stevopedia:
I had my scope in peak detect mode as I normally do, which has the side effect of accentuating noise. But your point stands: the noise is greater than I'd like. What's most interesting to me is that in the filter off case the waveform at TP303 is identical for both the input measurement and the zero measurement, and for some reason turning on the analog filter, which should remove this kind of noise, instead adds it in.

In hindsight, I think I may have made an error when I replaced Q120 and U105 (gate bias amplifier #1) quite a while ago. I made the same mistake I almost made with U307: substituted a slower part (OP07 again) with the thought that this would improve stability and performance. I think I'm going to replace Q120 with an LSK389D (it is currently an LSK489, which has a transconductance about an order of magnitude lower than LSK389) and the op amp with a TLE2071.
Kleinstein:
For Q120 it is not about low noise, but reasonable low low offset and low input current. Especially a positive offset ( > 20 mV)  could lead to addition input current. One could consider replacing Q120 and U105 with a low input bias MOSFET - possibly just a selected TL071H / TL031 or OPA145. So only a low bias OP-amp as a buffer (these were rare when the 3456 was designed and Thus the dual JFET plus OP-amp as the standard solution at that time).

For U105 a slow OP-amp could be OK, as here the OP-amp speed sets the overall BW of the amplifier. The FETs just work as source followers and thus little effect of the transconductance on the BW. The limited slew rate of the OP07 may be more an issue than the GBW - not so much with the shorted input, but possibly with a higher voltage.
Wallace Gasiewicz:
I fixed an episode of buzzing on the amp. I think this is what you are talking about..... I do not know if this will help but here is a rendition of my notes from earlier this year:   

Had buzzing on the square wave output from amplifier. tried increasing the mica cap and this helped but noticed more square wave distortion on low range-----putting my finger on Q303 stopped buzzing, replaced Q 303 with 2N4117     
This fixed the buzzing but did not help the offset. I was still not able to adjust out the 100mV range to zero, it remained 120 uV low.   

I know my notes are not really complete but maybe check Q 303.that was not helping.     

EDIT:  I also had trouble with a noisy 33 Volt Supply It was fixed by replacing caps.


stevopedia:
I GOT THE OTHER ONE!!

Removing Q105 rewarded me with an input offset voltage of 0.0 μV!!! I replaced it with my very last PN4392, and with the instrument cold/just powered on I had an offset of about -2.0 μV, which is very similar to that for the 3456A that's on my bench at work and which also has a current cal. I'm letting mine warm up now to see if the offset settles out to zero like it does on the working instrument.

(Edited to correct the part changed)
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