Event ASP-8 Problem / Diagnosis

My father-in-law has a set of Event ASP-8 monitors that I've fixed before. They get so hot that the board expansion caused some of the board-edge SMD caps to fracture. I soldered on new ones years ago and added a big heatsink to the back of it, and it seemed to work pretty well for several years. I'm not an expert, but I'm the family expert, if you know what I mean. I know these monitors are plagued with problems, but he likes them and I'd like to keep them working as long as possible.

However, one of the speakers is now acting up again. Behavior is as follows: Intermittently changes randomly between wayyy too loud, a more normal volume, and totally muted. Not sure if the levels are discrete or if it's an actual random volume. It happens on both the high and low end outputs. I've run a frequency sweep with input audio and I've also isolated the woofer and the tweeter and it definitely happens on both, so I doubt it's far down the signal path. I've checked the rails from the crappy power supply and they do seem pretty steadily at +-18VDC, even when the problem is occurring, so I doubt it's the problem.

Looking at the schematic, I'm guessing it's in the front-end/initial opamp/crossover area, since that's common to both low and high frequency. It does not seem to be the input connectors. I'm guessing it's not one of the ceramic SMD caps since, in my experience, they fail open, maybe closed, or somewhat predictably oscillate, and it's also not an issue that's sensitive to pushing or tapping on different parts of the board. Also, I checked the gain knob and it seems to be fine.

My guesses are that it's the front-end/crossover op-amp that got fried/out of spec, which I guess is common on these boards. Or, it's something in its feedback path, but that seemed unlikely to me based on what I know about SMD cap failures. The resistors all seem fine. My only other guess is that, despite the signal path being somewhat isolated from other parts, there could be something wonky still happening with ground, the power supply, or something else common to both channels. The fact that it is so intermittent and random in presentation makes me think it's something with silicon, though, but I don't know if that analysis is BS or maybe I'm missing something obvious. I haven't hooked up a scope to it yet, but maybe that's the next logical step.

I have ordered a drop-in replacement opamp for the original (which is a OPA2134) to figure out for sure if it's the culprit, but I wanted to just see if anyone else has any ideas.

Thank you.

Edit: Can find the service manual at https://dokumen.tips/documents/event-asp8-service-manualpdf.html or lots of other places
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Intermittent faults are the hardest to fix.
Those monitors were a bad implementation of the "suggested" application circuits from National Semiconductors.
There are several issues; but the main ones are the high voltage of the rails of the power amps (high dissipation of power ICs), three amps in parallel with just a small series resistors to mitigate the "inter IC currents" caused by voltage offset and poor heat management.
Based on the fault you describe, I suggest to start checking both power supplies (power amps and pre-amp), maybe the Zener's/resistors for the pre-amp circuit are out of value, then a thoroughly inspection of the solder joints with a glass magnifier.
Semiconductors do not have intermittent faults, they work or they do not.
You mentioned that you already got the schematics; but if you need them I may look for a good copy of them that I have stored some place.
Thank you for helping.

Both supplies seem very steady, at least on a multimeter. The "low"-voltage one is +-18V, the power amp rails are +-36V. The zeners (D6 and D8) seem intact but I don't think I can accurately test them in-circuit, although if they were bad wouldn't the rails seem wonky?

Preamp resistors are all in spec. Can't really test the caps without removing, which I might try next... I might just replace all the preamp SMD caps just in case, since they are what broke and/or exploded last time. Solder joints all looked good, except the pad under pin 7 of U5 was a bit messed up. I resoldered it and tested the continuity between the pin and something it had a trace going to, and it's fine. In fact, I resoldered w/some fresh solder/flux everything in the preamp/crossover area and the problem is the same.

To add to my original description of the problem, I also noticed that, when no signal is passed through the amp/speaker, it still makes little soft pop thump noises somewhat erratically but seems to be at least once a second. However, often the volume changes can be less frequent than that. Could just be coincidence, though.

Maybe I will break out a scope...
Once you checked the power supplies, I will suggest to do a "re-heating" of all suspicious solder joints (and even some that are not suspicious...).
Get a good solder iron and touch the solder joints adding a little bit of new solder to add some resin.
A fairly common issue with old equipment is the oxidation of the pins of the headers that causes the intermittent contact, hard to detect by the bare eye but with a glass magnifier you can do it.
I will strongly suggest an even easier solution: the "brute force" approach of re-heating all those solder joints prone to fail.
I will bet that you will fix it just doing it.
I found the culprit. I was too focused on the power supply rails and the pre-amp area. One other thing is also common among both channels, and that's the "mute" circuit on the LM3886 chips. It is basically not used in any meaningful way on this board, but it still has to pull to a certain voltage, so it has an RC circuit composed of a single 15.4k 0805 SMD resistor (R36) and a 100uF electrolytic cap. I think the cap is fine, but the resistor is not.

Like previous failures on this board, it's close to a heat source and the edge of the board, creating a lot of shearing/tension/whatever on the part. It isn't visibly damaged, even under a microscope. I tried resoldering it, and that seemed to improve the problem temporarily but I think that's only because it shifted the resistor a bit. When I mess with the resistor enough by poking/pushing on it, I can get the problem to worsen and improve. When I test the rail before the resistor (on the resistor contact) it's steady, but the voltage on the other side of the resistor changes with the onset of the problem.

I do not have any 15.4k 0805 1% resistors, so I will have to order some. I considered replacing the resistor with a through-hole one w/bent legs. I recognize that this would result in possible damage to the pad if I wasn't careful, but if I silastic the resistor body to the board it should be stable. Doing this would also remove the board thermal expansion problem in the future, at least for this specific part. I had to do this for a ceramic cap that exploded in these monitors before and took part of the trace with it. It'd also allow me to fix this thing and get it out of my hair ASAP, but for now I'll wait for the SMD resistor...

I gotta say that these monitors are so, so bad. They just constantly fail. Parts just break and explode all the time. Oh, and I can't forget, they actually installed the C21 cap for the wrong model. This is an ASP8 and they factory-installed the ASP6 cap, according to the service manual, so that's nice.