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I'm having difficulty identifying the cause of parasitic oscillation that manifests at the output of this amplifier circuit, based on now-obsolete AN7062N. For context, this is the first amp level from an old Technics audio amp. Upstream there's the volume potentiometer and it is followed by other amplifier levels. There's a negative feedback loop that closes at the inverting input of the IC, setting the overall gain (to approx. 30).

voltage amp circuit based on obsolete AN7062N

Observations on the parasitic oscillation

  • manifests only for the left channel (Ch-1 on scope);
  • has a frequency way higher than the signal's;
  • is triggered towards the lower peak of the wave;
  • manifests only for output amplitudes higher than 15Vpp;
  • sometimes after playing around with various input signal shapes and amplitudes, the problem disappears; turning the unit off and back on after a while reactivates the oscillation; this made me assume that an yet-to-be-determined part is acting-up below a certain temperature.

capture of parasitic oscillation at L-ch output of AN7062-based voltage amp

What I did so far

  • eliminated the volume potentiometer and the downstream amp stages as possible causes;
  • tested all junctions in-circuit and they seem fine;
  • tested all resistors in-circuit and also seem fine (including the R411);
  • replaced the AN7062 but the problem persists;
  • added a 100k resistor in parallel with R407 to alter the gain (to approx. 16); this led me to conclude that only the output signal's amplitude is triggering the oscillation;
  • carefully air-blower heated to approx. 100C various parts of the left-channel amp with no effect.

My current assumption is that some capacitors used for parasitic oscillation countermeasures around AN7062N may have aged out of specs (assuming they're ceramic, the service manual doesn't specify). I read that heating them reverts this phenomenon but nothing good came out of this attempt. I could exhaustively test/replace the remaining culprits but I don't have all the parts yet, the PCB traces tend to exfoliate way too easy and, above all, I'd like to understand what's happening.

Does anyone have other ideas about the causes and how to identify them?

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    \$\begingroup\$ Is the AN7062N coming from a reliable source? (My apologies. But this is just an i-dotting and t-crossing before considering investing thought.) \$\endgroup\$ Mar 10, 2023 at 23:48
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    \$\begingroup\$ It came from China. I really cannot vouch for it. However, both ICs (the new and the one on the amp) exhibit the same behavior leading me to believe external causes. I do have another AN7062N but I hesitate replacing the chip again before I eliminate other causes. \$\endgroup\$ Mar 10, 2023 at 23:56
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    \$\begingroup\$ Okay. I see your point of view. It would still make me want to source exactly one more of them from a certain source (and pay more) and then to, because of your valid worry about the circuit itself, sit back and work out incremental steps. Starting with Ohm meter measurements between pins on the new certain part and making those same on the other two, graduating to simple proto-designs to verify sections of the IC and testing all three in these safer circumstances, and slowly work myself back to this schematic. (I'm assuming you knew what you were doing in the design when I say that.) \$\endgroup\$ Mar 11, 2023 at 0:01
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    \$\begingroup\$ That would be a sound and considerate approach. Now that I think about it, I could also replace the new/unreliable chip with an IC DIP socket not only to limit further wear on the PCB or to test chips one after another but to be able to breadboard-test a channel swap (I know this is a big no in terms of stray capacitance). This could help eliminate the chip as a cause. \$\endgroup\$ Mar 11, 2023 at 0:13

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I was able to eliminate the AN7062 as the cause and instead found that Q401 (2SA1123) was acting weird. I replaced it with the nearest PNP equivalent at hand and I was able to get the amp working.

With doubts about the AN7062 and hesitating to play further with a frail PCB, I decided to put an IC DIP socket instead of the chip. With this new setup, a breadboard and 14 wires I managed to swap the channels only to observe that the oscillation was manifesting on the same channel still. It was clear that the AN7062 was OK and I started looking for parts around its output, based on my previous observations. I replaced R411 (even though it was testing well) but nothing changed, then I pulled-out Q401 which was also testing well in-circuit and outside (using my DMM gain tester). To my surprise, replacing it with a BD244C (the one with the highest Vce I had) got it working and it is still working after 4 hours.

Some conclusions:

  • In a complex loop with multiple cascaded amp levels (and negative feedback), it is best to reduce the complexity of the problem at each testing step (sort of a divide-and-conquer). I was lucky in this particular case to have the amp chain on 2 different boards interconnected with a ribbon cable. This gave me the opportunity to swap channels and observe if the issue changed channel also.
  • Assuming that a transistor is good based on its low current junction drop can be misleading. It can reveal the most common failure modes but that's pretty much it. In this case the transistor was beginning to fail at higher (but within specs) currents and voltages - the only explanation given the oscillation manifestation.

I will eventually replace the transistor with another of the same type but for the moment the BD244C solved the problem. I haven't decided yet though if I should keep the DIP socket or put back the IC how it was.

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