| Electronics > Repair |
| HP 3456A voltage offset |
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| stevopedia:
To be clear, my experiments with directly connecting the input nodes to the low Volts input with the Volts inputs also shorted together were only temporary, using a regular "pokey" multimeter probe, just to see what happened. The meter never does a check of the -12 V reference (via Q112) unless it's in the self-test mode, and I had it in the normal DC V mode. I'd have to repeat it to be sure, but I don't think AZ made a difference. The effect of shorting the high node to the Volts input terminal is to bypass the section from R103 through Q116. What doesn't make sense to me is why that should result in the meter's displayed offset being almost a thousand times greater than without the short, and AZ having no effect. GB1 (Q120/U105) is working fine; offset is about -6 mV. Another thing I noticed: with the Volts inputs shorted, I connected one end of a meter probe to the low Volts terminal and touched the other to GND at TP101, which should have literally no effect (the schematic shows the terminal shorted to ground). The displayed offset went from about 7.0 uV to about 4.0 uV and returned to 7.0 uV when the probe was removed. I think that indicates at least part of what I'm dealing with is that I need to implement the one service note HP put out for the older 3456As that involves drilling out the rivets originally used for grounding the outguard case parts and replacing them with screws. The purist/preservationist in me is sad that I can't (easily) get the prescribed Pozidriv hardware, but such is life. |
| stevopedia:
It's been a while since I last posted an update here. I've tried a bunch of things, and now I know a lot of things that don't affect the offset issue! * Implemented the service note where the grounding rivets are drilled out and replaced with screws: No effect. * Replaced Q101: No effect. * Replaced some of my replacement FETs with verified-good original parts: No effect. * Carefully and scrupulously cleaned the top and bottom of the A20 board in the vicinity of the input switching circuit, separately using 99% isopropyl alcohol, acetone, and soapy (Triton X-100, a nonionic surfactant, in the form of Photo-Flo) water: No effect, or maybe a few single-digit counts on the 100 mV range (tenths of a microvolt). I was messing around with it last night, probing with my oscilloscope and another DMM, when I noticed that the offset had dropped to -1.5 μV or so, or well less than half its normal value of about -5.0 μV, and I have no idea why. I also found that turning on my desk lamp, which as an LED bulb and is connected to the same outlet (an isolation transformer) as the 3456A, consistently reduced the magnitude of the displayed offset by about 1.0 μV (10 counts on the display). I turned it off for the night, and this morning I turned it back on and the offset was back to its usual value, though I was able to reproduce the lamp's effects again. I wonder if I'm dealing with common-mode noise on the power input generating this offset somehow. If so, that raises the question of why the autozero doesn't cancel it out--that's the whole purpose of having an AZ system! I dunno, man. This thing is stubborn. :-BROKE |
| Kleinstein:
An EMI effect is a real possibility. EMI (e.g. from a noisy LED lamp or SMPS) can cause some extra offset, that the AZ mode is not allways able to correct. The AZ mode is not made to correct EMI effects. When the 3456 was build there were a low less SMPS and cell phones around and no Wifi. I don't really think it is common mode noise, as the power supply is still via a classic mains transformer and not a separate DCDC converter. A CM filter at the mains side could still have an effect (could be part of the line filter already). It can still be tricky to improve the EMI suppression, maybe some ferrites at the input side. In many other aspects at least the circuit diagram looks good - could still be a layout thing for the really high frequencies. For a quick check one could test click on ferrites with the mains cable or a the analog inputs. |
| stevopedia:
I agree. I don't doubt that's at least a significant portion of what I'm seeing, especially with respect to noise. After all, I live within line of sight of an FM broadcast transmitter tower! It gets weirder, though. I left the 3456A on during the day with its volts inputs shorted and the outer upper cover sitting on top of the instrument. When I got home from work the display read about -7.0 μV. I took the cover off and set it aside. A few minutes later, the display was bouncing around +/- 0.2 μV! I rested the top cover back on the DVM and the indicated offset slowly began to rise again. With the cover on, I tried to reproduce the effect my desk lamp had, and--nothing! I knew I saw what I'd seen then, so I took the upper cover back off and bingo--instantly reproducible. I realized that I had the desk lamp pointed directly at the instrument so I tried blocking the light, and the effect was the same as if I'd turned the light off and with immediate effect. So either I'm seeing something thermal that's ridiculously sensitive, or there's something photosensitive in the input switching area. As it happens, C104 on my instrument appears to be some kind of ceramic capacitor in a transparent glass or epoxy housing (not sure which; haven't touched it to find out). The parts list says it's just a 470 pF, 100 V DC capacitor, but for some reason HP saw fit to use a different component here instead of the identically-rated polypropylene caps specified for A20C101-103. Of course the part number and mfg code point to an HP custom part (probably a custom spec) so it's impossible to know for sure what the difference is, but it's worth at least trying a replacement. Edit: I just tried moving my desk lamp right up to C104, and I could make the offset go as strongly positive as it went negative! I wonder if these caps were always light-sensitive, or they became more sensitive with time. Second edit: It's not C104, but there's definitely something either directly photosensitive or something phenomenally sensitive to temperature on that board somewhere. |
| Kleinstein:
Some parts are indeed light sensitive. This was however less an issue in the old time with THT parts, but more with small SMD parts like TSSOP and similar with not much plastics. In the old days it were diode (and transistors) in a glass case. For some odd reason they still make low leakage diodes in a glass case. Thermal EMF can produce an offset of a few µV with some parts. For the FM band it can make a difference of the cover just rests on the meter or it is screwed down. For me RF effects are still a bit unpredictable. |
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