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| HP 8116A Error 42 (E42) Repair |
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| MarkL:
--- Quote from: henmill on October 24, 2024, 05:24:29 pm ---... When you say "it's telling that there's low voltage across VR501/VR502" are you thinking it could mean Q501/502 are not turning on? Seems like one side should be working at least, but I'm pretty sure the values were mirrored. I'll focus in on this area for sure. If I get the chance tonight, I'll do a more regimented documentation of the voltages throughout the amp and see what we've got. ... --- End quote --- If you're saying both VR501 and Q501 are up around +23V or so, and both VR502 and Q502 are around -23V, that puts approx. 46V across R518 and R519 which are in series (both 59R). That's dissipating more than 11W each, so they would be smoking and open by now. Vce on both Q501 and Q502 should be closer to 10V on both, so they could be overly on (bias issue), shorted, or no load is being presented to them. I'm not coming up with any good scenarios just yet, which also includes why it's half-working. I think I'll wait to see your more precise readings. I don't think you need to spend time measuring/comparing all the voltages on that sheet. I think we should explain what's going on in the VR501/Q501/Q502/VR502 area first. |
| henmill:
Alright, I was allotted almost zero time last night to investigate, but I did get to check the Q501/502 stage out. And I must have been mistaken about the Vce values before. There certainly is not such a large potential between the emitters. Rather it is the opposite which makes a lot more sense. See my scratch pad schematic, hopefully you can read the values ok. Perhaps I will spend more time on it tonight, until then, cheers! |
| MarkL:
Interesting... It appears that Q501 is not on at all, given the negative Vbe and no positive pull from the emitter on R518. My initial guess is that R516 is open, or perhaps there's an open trace (maybe from corrosion?). Q501 should pull up, and be balanced against Q502 that wants to pull down. But because Q501 is not being biased on by R516, and we're looking before the inverting stage, it could explain why there's no negative part of the the output waveform. Also, I think you meant -0.63V (minus) on Q502's emitter, right? |
| henmill:
It's possible I got the -0.63V and +0.63V values swapped... that would make a bit more sense. If R516 is open, then I'm not sure how current would flow through the diodes CR501/502. Which the voltages seem to indicate they are on as there is about a 0.7V drop across each. Unless Q501 is failed short from Collector to Base? But the voltages disagree with that idea.. We'll try again tonight and see. Would it also be helpful to put the scope on some of these nodes to look for the signal? Btw I was running a square wave at I think 2.5V. I've noticed I have to provide some positive offset in order to get anything on the output. Which is odd because I would have thought I could just get the positive half of the waveform with zero offset, but that does not seem to be the case. |
| MarkL:
You have a good point there that the voltage drop on CR501/CR502 is correct. The node between CR501 and CR502 is being driven by opamp U502. Perhaps we should poke at the "Offset Voltage Control" section a bit. What are you seeing at pin U502.6, which is the output to CR501-+-CR502? And how about the node R528-+-R529 which is one of the inputs to this section (and it's also the feedback path from the amp output)? And the other input which is labeled "Offset Vernier Control"? You've already measured the third input at R518-+-R519 as 0.001V. And what is the amplifier output voltage under static conditions? You can measure this at the BNC as long as the output is not disabled. Maybe there's an issue downstream and the feedback is trying to correct it, which results in Q501 being off. In this amplifier with the static settings shown, all transistors should be biased on, except for Q514 and Q515. Vbe should be around 0.7V. You could give the scope a try too with a failing signal. Similar to the static probing, you could look for an area where it's not mirrored correctly between the positive and negative halves. You can concentrate on the main signal path which is drawn with a heavy line in the schematic. |
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