HP 8116A Error 42 (E42) Repair - Page 1
Forum section: electronics / repair

author: henmill
author_rank: Contributor
timestamp: October 18, 2024, 05:18:03 am
content: Hi y'all,I recently bought this 8116A function generator off eBay hoping to 1) fix it 2) learn something in the process and 3) gain a function generator for my bench!The listing was scant on details but said that it powers on but little detail beyond that. On first inspection of the outside, there is a concerning amount of what looks like corrosion and corrosion-esque residues all along the bottom 1/2 inch of the enclosure, front panel and all over the connectors. Not looking so hot so far. (you'll see in the pics)But, I was delighted to find it more-or-less working! I don't get a whole lot of time to work on my projects during the week so didn't get to let it really warm up until a couple days later, when I decided to crank it up into higher frequencies, ended up letting it run at 100kHz 75% duty 1V amplitude and would periodically check on it.Lo and behold after a couple hours maybe, the negative half of the waveform had gone all wonky. Positive side still doing fine. Okay, now we're getting somewhere. Upon reboot it greeted me with E42 but would still output, just nothing below 0V pretty much.I should mention, soon after those early tests I cracked it open to look around, but not much stood out from the top side. Except it does look like somebody has worked it over a time or two, probably even replacing the pesky HP ASICs I've heard about (evidenced by discolored flux residue underneath). (yay! maybe?)Upon getting the error code 42 and a quick google, this does seem to indicate a problem with the negative half of the output amp, nice!Last night I went ahead and paid somebody for a scan of the manual, and happy to have it now. Today I've taken a closer look at the output area and oh boy there's lots of corrosion. My mind jumps to leaking electrolytics ...but maybe not so, as you'll see later. *suspenseful music plays*I cleaned it up and to me it looks as though the Unknown Past Servicer may have already replaced the output diodes and big electrolytic. I'm judging this a couple ways: 1) the amber flux residue on particular components 2) the slight difference in color of solder joints compared to rest of board and trimmed leads that gives it awayI think there's a story here. Imagine, if you will... brilliant engineer working into the night finally gets the thingamabob to work, and in a moment of ecstacy smacks his beer over, spilling right onto his trusty 8116A (which he just had repaired). He quickly turns off the unit and does a hack job cleaning it. It sits unused while engineer achieves great success and ultimately sells off his old gear for new kit and he lives happily ever after.But seriously, could the leftover flux and proximity to the chassis in a corrosive environment (humid + degrading battery on top PCB) contribute to this type of failure?At any rate, next I need to actually go through checking voltages at all the nodes in the negative half of the output amp I reckon. All the power rails checked out good.I really appreciate any advice you folks can give!I'll be adding pics here, trying to keep it chronological:https://imgur.com/a/Tkf4w8F

author: stevopedia
author_rank: Contributor
timestamp: October 18, 2024, 03:42:16 pm
content: Given how far apart the battery and the corrosion near the front of the outputs are, I'm doubtful... but you never know. I've heard that long-term exposure of rosin flux residue to a humid environment, such as storage in a non-climate-controlled area, can encourage corrosion as rosin flux residue attracts moisture, but usually rosin flux residue is Mostly Harmless™ when left in place.Humid storage conditions do seem to be a factor generally, especially given the appearance of the instrument's frame and front panel--all that crusty stuff screams "moisture exposure" to me. Maybe even "covered in condensation." The white deposits on the board in picture #9 might just be a result of that and not corrosionper se. If you clean it off and the board is undamaged underneath, then it's just the salts/whatever left behind by the moisture exposure.Either way, get rid of that crusty old battery ASAP and clean off the corrosion as well and thoroughly as you can. Be sure to check whether or how much the traces the battery connects to have been eaten away. You may need to carefully remove some solder mask to get a good look or if an ohmmeter doesn't give good results. Battery corrosion has a nasty habit of spreading if unchecked, but it's usually only areas that directly contact the corrosion or which the leaking battery juice falls onto.

author: henmill
author_rank: Contributor
timestamp: October 19, 2024, 02:11:45 am
content: Yeah, I think the corrosion down underneath the output is unrelated to the battery. But my thinking is it must have been stored in a not so great environment, maybe after breaking just sat for a really long time. The way the salts or "corrosion" as I'm calling it travelled all around the frame makes me think that. My first thought when I saw it was I hope it wasn't in a flood or something because of the gunk all along the bottom. But I don't think that's the case.Btw the battery is still holding a voltage. Front panel will recall last used settings. Although it came to with defaults when I turned it on first.But I've removed it anyways to clean and inspect the PCB. Will replace eventually.I've read water + baking soda and/or vinegar can neutralize the battery corrosion residue. Any other tips welcome!

author: stevopedia
author_rank: Contributor
timestamp: October 21, 2024, 02:32:58 pm
content: It almost certainly got very wet at some point. The fact that the white stuff seems to have cleaned off very easily points to waterborne salts or whatever. That's the only explanation for all the residue I can think of. But for all that it's inreallyfantastic shape! I think you lucked out on this oneRegardless of whether that old battery is holding a charge, the fact that only the battery leads seem to haveactuallycorroded is a sure sign that it's leaking electrolyte. If you leave it in, a good portion of the whole board will look like its leads do. Old rechargeable batteries just like that one arenotoriousfor absolutely wrecking boards beyond repair.Edit:I had another look at your pictures, and the second to last shows some clear evidence of battery leak damage. See how some of the pins on R11 have a dull appearance, the same as the two vias next to the battery's negative terminal? That's early-stage corrosion. You can see some marking on the traces from those vias too. But the biggest warning sign of all is the green corrosion on the one leg of the resistor pack next to the little blue capacitor on the bottom left.You caught this one early. The only place that old battery belongs is in the trash.The good news is that NiMH cells/batteries are still readily available and are usually drop-in replacements for the original (probably NiCad) batteries.

author: MarkL
author_rank: Unknown
timestamp: October 21, 2024, 06:34:44 pm
content: Quote from: stevopedia on October 21, 2024, 02:32:58 pmThe old battery is NiCad and is 2 cells, HP part #1420-0251. Possibly 2x1/3AA. What battery did you find to replace this?

author: stevopedia
author_rank: Contributor
timestamp: October 21, 2024, 07:57:01 pm
content: Digikey has many options for NiMH and NiCd cells, and there are a few options for premade two-cell batteries also.https://www.digikey.com/en/products/filter/batteries-rechargeable-secondary/91?s=N4IgjCBcoCxgTFUBjKAzAhgGwM4FMAaEAeygG0QBmADgAZaB2CIm%2BgNgFYQBdIgBwAuUEAGUBAJwCWAOwDmIAL4KibJCEkATYQFowtRPyGQQIIgICefPMI05USoA

author: henmill
author_rank: Contributor
timestamp: October 22, 2024, 03:11:31 am
content: Thanks for the very helpful replies! Firstly, about the battery replacement options. I figure the info is out there, but I really appreciate getting right down to it in this thread.And your comment about the corrosion near the battery, I had to do a double take myself. I tried to capture it a little better and also look for more while I was at it. But your right, things were not looking good for this unit if it sat for more years. I really hope I can get it to 100%!I checked across the terminals with the worst corrosion and was reading about 10k, along with its neighbor. So I think things are ok there.Corrosion close up pic:Now, about this Error 42. I've only poked around a little bit, and it's slow going while getting to know the circuit and poking at it from below. So I made this helpful overlay to save myself some time going back and forth between pages in the manual, see attachment.If anyone has any experience chasing down the E42 fault, I'd love to hear it! I hope it is only something to do with the output amp negative supply or offset control as opposed to the shaper IC or something.Thanks!

author: MarkL
author_rank: Unknown
timestamp: October 22, 2024, 04:19:06 am
content: If you haven't done it already, I would check the power supply rails as described in the beginning of the adjustment section. I wouldn't adjust anything yet if it's out of spec; just see if each rail is in the ballpark.If power is ok, a good place to probe is A1TP4 which the is signal after the shaper and before the output amplifier. Set up a signal that is failing, and take a look at A1TP4. A good or bad signal here could point you in the right direction for more troubleshooting towards the shaper (signal generation) or towards the output amplifier.It could be helpful if you would post a scope capture of what you're seeing on the output and A1TP4, along with the settings you're using.And it also may be important to know the serial number on your unit. In the service manual I have, there are 76 changes made to the circuitry over time organized by serial number (see the "Backdating" section). If you don't want to reveal your serial number, at least indicate the change implementation number from the manual.And for the forum, it's usually better to embed the pictures as part of your posts instead of linking to an outside service. If in the future your imgur archive becomes inaccessible for some reason, the discussion here without illustrations will likely have much less value to future readers.

author: henmill
author_rank: Contributor
timestamp: October 22, 2024, 09:34:51 pm
content: Hi MarkL,Thanks, I did quickly check all the power rails early on in troubleshooting. All seemed well there (I did check on the side downstream of the 0R's and fuses btw).Tonight I will absolutely be checking out TP4, thank you so much for this tip!Serial number is: 2708A05093. I paid somebody 10$ for four different revisions of the manual because I wasn't sure which one I needed. But all of them have the G in the serial indicating made in Germany. I wonder if there will be a huge discrepancy? One thing that's a little fuzzy that I realized last night, is my A1 board has those diodes near the output, where there was a lot of corrosion underneath. But the manual thatshouldapply to my serial number does not show them in the assembly drawing/schematic, and thus they are missing from my overlaid picture. I tried redoing the overlay with an assembly drawing from the next version up, but the components do not line up nearly as well as in the one I posted. Maybe this is just due to different styles of drafting the drawings or something, idk.And that's a good point about embedding vs. linking pictures. I will make sure to do so from now on, and might edit my first post if I still can.Will follow up later on with some results of probing that test point, thanks again!

author: henmill
author_rank: Contributor
timestamp: October 23, 2024, 03:56:32 am
content: Good news (I hope):I had a chance to probe TP4 and it seems the signal coming from the shaper IC to the output amp might be OK.In the attached pics, the filenames show the relevant settings on the function gen, and you can kinda see the scope settings and measurements. Basically I started with the default settings of 1kHz sine, 1V amp, but added 1V offset to show the good output alongside good input.Ch1 Yellow trace is function gen output, fed into 50ohm passthrough to scopeCh2 Blue trace is TP4 aka shaper IC output signalYou will see that the output from the shaper is mostly good, while the final output clips below 0V. But there is some intense ringing on the square wave and some distortion on the negative side when I try to drive the output negative. But I wonder if the ringing is measurement technique/scope... and maybe distortion due to the negative power supply problems happening at the output?I really hope it is one of the diodes involved in the negative half of the output stage. I'll be trying to step through the troubleshooting procedure in the manual. The documentation in these old manuals is SO GOOD!As always I'd love to hear anybody's input, cheers!I've attached only 3 images here because I can't be arsed to downsize them. There are a few more pics of waveforms to be seen here:https://imgur.com/a/hp-8116a-Tkf4w8F

author: MarkL
author_rank: Unknown
timestamp: October 23, 2024, 03:47:34 pm
content: 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

author: henmill
author_rank: Contributor
timestamp: October 24, 2024, 04:04:34 am
content: 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 Q501/502I 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!

author: MarkL
author_rank: Unknown
timestamp: October 24, 2024, 02:24:37 pm
content: 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.

author: stevopedia
author_rank: Contributor
timestamp: October 24, 2024, 03:22:21 pm
content: Quote from: henmill on October 23, 2024, 03:56:32 amYeah--the older HP manuals arestellar. 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!

author: henmill
author_rank: Contributor
timestamp: October 24, 2024, 05:24:29 pm
content: QuoteYes, 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.QuoteFair 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 hahaThanks again folks, and hope to update again tonight!

author: MarkL
author_rank: Unknown
timestamp: October 24, 2024, 08:16:21 pm
content: Quote from: henmill on October 24, 2024, 05:24:29 pmIf 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.

author: henmill
author_rank: Contributor
timestamp: October 25, 2024, 01:45:22 pm
content: 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!

author: MarkL
author_rank: Unknown
timestamp: October 25, 2024, 03:31:27 pm
content: 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?

author: henmill
author_rank: Contributor
timestamp: October 25, 2024, 03:59:48 pm
content: 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.

author: MarkL
author_rank: Unknown
timestamp: October 25, 2024, 06:17:24 pm
content: 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.

author: henmill
author_rank: Contributor
timestamp: October 28, 2024, 04:36:22 am
content: Hello, and sorry to keep you all waiting, no doubt anxiously for my update haha.I have not gotten as much time as I'd like the past few days to dig into this old beasty, due to silly things like spending time with familyBut, I have managed to think a little bit more logically about how to step through testing and thus developed a crude plan. Said plan involved probing a handful of points I deemed informative:1) U502.2 the inverting input of the offset control op amp driving the moving reference of the output (at least I think that's what's happening)p.s.) looking back, maybe I should have looked at the vernier control input at C5012) The node at CR501 cathode / CR502 anode - output post R509 of offset opamp U5023) Vbe of Q5014) Vbe of Q5025) The node where R528/529 meet which is the output reference potentialSo I checked these at a few different settings, first starting with no waveform, 0V offset, output disabled. Then with output enabled, everything was essentially the same, except output reference was closer to 0VSo the meat of what I've gathered is just looking at a 3Vpp sine wave, 1kHz and stepping through different offset voltages. The first attachment shows those values as well as a screen cap showing exactly where on the board I was probing.My initial takeaway from these measurements is that U502 appears to be just saturating to the negative rail as soon as offset is set to 0V and below. Whereas CR501/502 node stays about a diode drop below 0V when offset is positive. I reckon this is due to something not working (duh) and it immediately driving as hard as it can negative in attempt to get something but alas.Now, I decided to also poke around with the scope, to see if I could find where in the chain I lose the bottom (top?) half of the signal. So I have some scope captures looking at the signal coming from the shaper IC at R511 (ch2 blue), then the other side of R511 (ch3 fuscia), and then the final output (ch1 yellow). You will see the signal come in and out on ch3 on R511, which really is the inverting input at U502.2. I bothered to do proper scope caps but didn't bother to setup measurements on all relevant signals but oh well. Maybe next timeHopefully others can glean more from this exercise than me. But the big takeaway to me is that something is not working on the Q501 side probably, or something in the feedback network of the offset amp is buggered? Which isn't getting me much further but I think I am slowly absorbing the linear amp function and expect my eureka moment any day now...Until then, cheers!edit: note the "dots" on the drawing are the probe points. I also labeled the nodes base and emitter as well as relative polarity so it's a bit cluttered.

author: henmill
author_rank: Contributor
timestamp: October 29, 2024, 03:41:13 pm
content: Very quick update:I checked the Offset Vernier Control signal coming from the control board, and it lined up with what is shown in the manual. I don't have those numbers in front of me, but the important thing I observed is that the analog signal coming in is indeed dual polarity, ranging from (about) -2.8V to +2.8V for offsets +7V to -7V.So, I'm still betting it is some failed component on one side of the amp. Just having a little trouble figuring out how to determine that with everything in circuit.Thanks!

author: MarkL
author_rank: Unknown
timestamp: October 29, 2024, 03:51:29 pm
content: A couple of observations...U502.2 should always be 0V since the non-inverting (U502.3) input is ground. Note that when U502.2 is 0V, everything is ok. When U502.2 goes positive, it causes the U502's output to saturate negative, which explains the extreme negative readings you're getting on Q501's base (and causing Q501's collector to go positive).The collector of Q501 is clearly signalling for more negative output by going positive, but is being ineffective.Your yellow output trace is always at +250mV when things aren't working, and is probably what's causing the positive input to U502.2. The output should be 0V in static conditions, so something is dragging this high and not allowing it to get below +250mV, or the negative pull down is not working.I think what I would do at this point is start looking backwards from the output and check bias and compare voltage levels on the output drivers Q510 thru Q513, then Q508/Q509, and so on.

author: henmill
author_rank: Contributor
timestamp: November 05, 2024, 05:25:45 am
content: Hello again and thanks as always for the feedback. (speaking of which, in a moment...)MarkL, you are absolutely right about your explanation of the offset control amp. I am slowly but surely starting to understand the components of the amplifier. Emphasis on slowly hahaBut to that point, I decided to take a crack at simulating the circuit for funzies. I started with just the input stages and was happy to see it more or less function as a buffer. Then just tonight I've built out pretty much the rest of the gain stage and final power output. I am not using opamps yet for the offset voltage and current control. And I'm happy to report it works!....but I had to add in my own feedback network to set the gain (i.e., amplitude) instead of it kinda just maxing out.For some reason I am not understanding where the feedback network is and how it's controlled, so I'm asking for your help!Does it have something to do with Q590 and Q591 and their resistor networks? (this seems like the most logical). But feeling like a duntz I don't understand the schematic Number/Lettering scheme? Do adjacent letters connect across sheets? (i.e., A to B, E to F, etc.)So, first pic is those transistors in question, do the nodes labeled 1E both get tied to 1F? Surely not right?Next pic is the schematic of my simulation in LTSpice, followed by an example output waveform.One really big observation I've gotten already is that tweaking the resistor network for Q507 (labeled "Bias Adjust") has a huge effect on the offset of the output waveform.AND, I poked at those nodes the other night and found them all to be at the same potential, about 1.25V.... which I found curious. I measured across the resistors with the power off and it checks out about 1k across Vce and R536 measured about right too. So it's not a dead short through Q507.To me, this leads me to think maybe any one of Q503-Q506 could be screwy and not turning on to allow current through the Bias Adjust network.Is it worthwhile to measure resistance across the different nodes of each transistor with the power off to potentially assess a failure? Or check for diode Vf across base to emitter?Thanks in advance for any ideas! And let me know if you have any questions or suggestions on the simulation, or want to see different settings run!

author: MarkL
author_rank: Unknown
timestamp: November 05, 2024, 05:37:25 pm
content: Quote from: henmill on November 05, 2024, 05:25:45 amCool! Can you share your spice file?QuoteThe feedback path from output to input starts at R541/R542 and continues through R528/R529.Q590 is associated with "ERROR FEEDBACK", and Q591 with "LEVEL CHECK" according to where they are connected. They are both TTL level signals, so the unit is probably using these signals for checking the output during self-test.QuoteNo, not right. The "1E" refers to the schematic sheet number shown in the lower right corner of all the schematics. Sheet 1E is in the VCO and Width generator section (sec. 10.3). The number in the circle refers to the node number on the sheet.Quote"Those nodes"? All of them? Can you be a little more specific?QuoteI would be careful measuring resistance in-circuit. Depending on the voltage being used by your meter, it may be turning on junctions and you might not get an accurate reading, or there may be other passives interfering. Measuring Vf (Vbe) under power I think is a better test since you don't have to wonder about interfering components, but it still does not say if a transistor is 100% ok.QuoteI would really encourage comparing quiescent voltages with the schematic sheet I posted of the working unit. The problem area will likely become very obvious. If it helps at all, below is a version of the schematic with blanks in all the measurement boxes if you want to write them down as you go. The source document is in LibreOffice Draw - if you want that let me know.Note that Q510/Q512 and Q511/Q513 are just drivers in parallel, and they are being driven as emitter followers by Q508/Q509, which are emitter followers too. So the bases of Q508 and Q509 should be about 2 diode drops above/below the output. From your readings, it sounds like there is an issue around Q507. Perhaps Q506 is not pulling down for some reason. Look at the voltage levels at Q506 and Q504 with respect to ground.

author: henmill
author_rank: Contributor
timestamp: November 15, 2024, 04:47:35 am
content: Hello again,I had hoped to make more progress by now and/or update sooner, but alas.Anyhow, see attached spice file. Since last posting I've added the offset control amp and honestly the whole thing kinda works. If you have time maybe you (and the greater "y'all") can dig up models for the original components rather than the ones I used included with ltspice. Could have a digital twin, so to speak.Thanks for attempting to explain the feedback path. That makes a lot more sense although I don't quite get how to understand the gain network with the dual polarity feedback(?). I have a lot to learn about how the different stages of this amp work, which boils down to my understanding of bjts..... which I haven't thought about in ages.Thanks also for showing me how schematics work *facepalm*On Q507, by all nodes I meant all the transistor pins: emitter, base, collector.To your point of comparing the voltages, I once again had to photoshop myself a better guide because it is so tedious trying to probe each pin from underneath and know what you're poking. See attached pic of underside of amp with Q503-513 pins labeled. I also made an excel sheet to record my measurements, see screencap of said sheet. I would totally take your source document so I can type in the numbers directly! I don't have a printer at home so don't have an easy way to record values except by the ol' pen and paper.Now, on to another thought that's been creeping into my mind... if it's difficult or expensive to find replacement transistors, what if we bypassed the output power stage with a modern high speed op amp? Call me crazy but there is probably a part or two out there that could do what this unit does, maybe slightly less amplitude into 50 ohms. I also barely know what I'm talking about, if at allThanks for any more feedback on this resurrection project! From my measurements, the only thing that's really jumping out at me is Q507 being equipotential at all pins... love to know your thoughts.-H

author: MarkL
author_rank: Unknown
timestamp: November 16, 2024, 08:58:18 pm
content: Here's what I see from your latest readings:Recall that the output is idling at about +250mV and not reaching 0V, where it should be.Q506 is not pulling down, which in turn explains why emitter followers Q509 and Q511/Q513 are not pulling down on the output.Q506 appears to not be on by any significant degree, given the Vbe of only 0.5V.Q506's base is being driven by Q504. Q504 appears to be in saturation with a Vbe of 0.8, and a Vce of 0.5V.Q504's base is sitting at the negative rail of -24.1V, which would say Q502 is not pulling up on R527 (100R) at all. So why isn't Q502 on?From some of your earlier measurements (reply #20), the junction of CR501/CR502 is very negative at -8.7V when the offset is set to cause a failure. (Unfortunately, the table doesn't have 0V offset filled in, so I'm going to assume the unit is in non-operational territory with voltages similar to setting a negative offset.)R517 is pulling down on the other side of CR502 (the cathode), so cathode of CR502 should be one diode drop less, or approx. -8.7V - 0.7V = -9.4V. This is also Q502's base.But Q502's base is not at -9.4V, it's at +1.05V.Ok, so is Q502's E-B diode pulling the base up to +1.05V? No, because the emitter is at -9.4V.I didn't notice this discrepancy when you posted it. I don't know where a positive voltage on Q502's base is coming from when it's surrounded by negative voltages. Maybe the wrong points were probed? I think those numbers need verification. You could add Q502 to your spreadsheet.Maybe R517 is open and Q502's base is floating. But I would expect the readings to be more random. Your earlier reading in reply #16 would imply R517 might is ok. But then again, that diagram shows (after correction) that Vbe is 1.0V, which is not right.I've never looked around for a replacement power opamp. I suppose there could be one out there somewhere. But I wouldn't give up on a repair yet. This amp worked once upon a time, and it can be made to work again. It already works half way.Attached is the source document for the annotated schematic.

author: MarkL
author_rank: Unknown
timestamp: November 22, 2024, 02:14:32 am
content: Also... That's a nice photo overlay you made there!On Q507, you have E and C reversed. A couple of clues: Variable resistor RV535 is between the base and collector and, the tab on metal-cased transistors (such as TO-5, TO-18, etc) is next to the emitter which in this case is pointing to the back.