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| HP 8116A Error 42 (E42) Repair |
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| MarkL:
If it is one of the diodes at the output as you mention, it would have to be a shorted VR504. You could measure the anode of VR504 to see if it's around -10V, which would say it's ok. It could also be an open somewhere on the negative side in the output amplifier in that it's not pulling down at all, or possibly on the positive side before the inverting stage at Q505/Q506. The distortion you're seeing at TP4 is likely because of the output amp's feedback, which is trying to compensate for the bad output waveform and some of that feedback is getting back into the input at TP4. You could tell for sure by changing the amplitude to 100mV to see if the distortion at TP4 improves. (At a lower amplitude the attenuator switches in higher values and hence more isolation.) Earlier this year, I was helping someone with an 8116A also with a E42 error. I documented many of the voltages in the output amplifier section from a working unit under static conditions. It might be useful to do a comparison with your unit, although it's possible your problem might only surface when outputting a signal: https://www.eevblog.com/forum/testgear/8116a-code-42/msg5359664/#msg5359664 |
| henmill:
Yes, I definitely did read most of that thread you were helping on, and even already saved that annotated output schematic you shared haha. So thanks for that too! I got to comparing your numbers to mine, and there are some differences. I checked with same settings you indicate on the schematic, as well as trying to drive the output negative and did not see much difference. 1) I am not seeing the voltage across VR501 and 502. They are more like 1V across either of them (your schematic shows ~10V) 2) Only seeing <1V across either Q501/502 I also checked the output diodes VR504/505 and measured about +/-18V at either node. Which makes sense to me given the 18V designation on the zeners on the schematic. But leaves me stumped because I was hoping for an easy out given the crazy corrosion stuff that was directly beneath them (see picture attached). What confuses me is not the values I'm seeing but that they are symmetrical about 0V. I'll admit that although I have an EE degree, I do not know exactly what is going on in the analog amp design world haha. About the picture, I pretty much had to scoot the front panel off in order probe those diodes anyway, so thought I'd get a good shot of the corrosion that was below. I could DEFINITELY see a spilled drink shorting the output rails (~48V!) to the chassis. The bottom terminations rest just over the front panel frame, probably about 2-4mm maybe. Anywho, it's gonna be slow going on this bad boy as I only get an hour or two a night usually to tinker. So thanks for staying interested! |
| MarkL:
Sorry, I meant 18V on VR504/505. Just glancing at the schematic the "8" looked like a "0". But the presence of 18V says that they're ok. That part of the circuit is only acting to clamp the output to approx. +/-18.7V. It's telling that there's low voltage across VR501/VR502, and I assume when you say across Q501/Q502 you mean Vce. I would focus on that area to start. It would be more helpful if you could measure around the Q501/Q502 area with respect to ground. While of course the devices only care about what they see across them, it doesn't say as much about the rest of the circuit. The first thing I would verify is if the Q501/Q502 bias voltage (Vb) and Vc are sane and continue from there. Also, half of this amplifier appears to be working, so one troubleshooting technique is to look for major voltage discrepancies between the positive and negative halves. They should be mirrors of each other. The corrosion looks nasty, but also looks like you have it cleaned up on the PCB. Maybe you'll find a trace got invisibly eaten away, especially if the liquid was conductive. Slow is fine... I don't always get a chance to respond right away either. |
| stevopedia:
--- Quote from: henmill on October 23, 2024, 03:56:32 am --- The documentation in these old manuals is SO GOOD! --- End quote --- Yeah--the older HP manuals are stellar. They're not always perfect, though; I've seen some procedural errors and typos creep into them. Just wanted to add: in case you weren't aware, you can almost certainly plug a USB drive into your scope and save direct screenshots onto it. Much easier than a more literal screenshot! :) |
| henmill:
--- Quote ---It's telling that there's low voltage across VR501/VR502, and I assume when you say across Q501/Q502 you mean Vce. I would focus on that area to start. It would be more helpful if you could measure around the Q501/Q502 area with respect to ground. While of course the devices only care about what they see across them, it doesn't say as much about the rest of the circuit. --- End quote --- Yes, sorry I should be more precise with my language. I have been taking measurements with respect to gnd (using the gnd test point right next to TP4 btw), but doing the math in my head. So when I say I am getting low voltage across VR501/502, I mean I measured at the cathode and see something close to 24V, then anode is around 23ish, so I said there is about 1V across it. Same goes for the transistors, I took measurements at collector and emitter and did the math. 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. I was hoping for the easy out with a failure at one of those output protection clamps. But I guess just shorting the rails there (my "spilled beverage theory") would not necessarily kill those diodes, but maybe fry something upstream in the amp. --- Quote ---Just wanted to add: in case you weren't aware, you can almost certainly plug a USB drive into your scope and save direct screenshots onto it. Much easier than a more literal screenshot! :) --- End quote --- Fair point, and yes I definitely have the ability to grab screens directly from the scope. Whether I will do that next time, we will see haha ;) Thanks again folks, and hope to update again tonight! |
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