I've been repairing an old (~1982) Fender solid-state guitar amp. While I had it apart I took the opportunity to replace some of the RC4558 op amps with NE5532's in DIP sockets I installed.

It was working great (much quieter). I left it on the bench for a couple of weeks and today when I came back and switched it on, I got a loud low-frequency "motorboat" from the speaker.

So far it looks like the buffer amp at the output of the tone control stack is oscillating. It seems to come and go, with varying amplitude—sometimes it seems fine, sometimes it's a fairly low-level sine wave, and other times a clipped almost-square wave. Update: this is the op amp towards the upper right corner of the schematic, labeled "1/2 IC2". I notice that there don't seem to be any bypass capacitors on the supply leads, so adding some might help.

The only possible variable I can think of is temperature: my bench is in an unheated room and the weather has turned colder. Hardly seems likely, though.

Any ideas what could be causing this? When I read about op amps oscillating, it's normally at a high frequency, right? The amp's other, high-gain "lead" channel seems OK.

London Reverb Preamp Schematic

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    \$\begingroup\$ It's usually due to some feedback issue. See Wiki on electronics and motorboating for some useful discussion. We'd need a schematic, I suspect, to offer anything directly useful. Even then, not sure. I definitely think you should follow up on the temperature aspect -- it may point you in different directions, if it correlates to the problem. You should be able to easily test that. But I suspect you'll eliminate it, rather than confirm it -- just guessing for now. \$\endgroup\$ – jonk Oct 25 '19 at 6:19
  • \$\begingroup\$ "sometimes it's a fairly low-level sine wave, and other times a clipped almost-square wave." - what frequency? \$\endgroup\$ – Bruce Abbott Oct 25 '19 at 6:50
  • \$\begingroup\$ Didn't check but it's a low-frequency "motorboat". Pretty sure it's not 60 Hz, though. I've added schematic (which I notice doesn't have power supply bypass capacitors on the op amp (labeled "1/2 IC2")—should probably add those) \$\endgroup\$ – Rob Lewis Oct 25 '19 at 7:08

With the mid (possibly and Bass) controls fully CCW you have a hell of a lot of capacitance to ground hung directly on the opamp output.... Never a good look.

Also, where is the decoupling on IC2? 5532s need local decoupling caps and are known to exhibit stability issues if they are absent.

  • \$\begingroup\$ so 5532 opamps have POOR low frequency PSRR? \$\endgroup\$ – analogsystemsrf Oct 25 '19 at 7:53
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    \$\begingroup\$ Not particularly, they usually go off internally at high frequency, and it shows up as increased distortion, but with the best part of a microfarad of cap load on the output would you care to bet on what will happen? \$\endgroup\$ – Dan Mills Oct 25 '19 at 7:59
  • \$\begingroup\$ Looks like R8 (21.5k) is in series with the capacitive load if the pots are turned full CCW so the opamp doesn't have capacitive load directly at the output... \$\endgroup\$ – peufeu Oct 25 '19 at 12:25
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    \$\begingroup\$ @peufeu, How the F did I mess that? I blame lack of coffee! \$\endgroup\$ – Dan Mills Oct 25 '19 at 14:55

Low frequency oscillations require a rather high time constant to be inserted in a feedback path somewhere... the power supply is one of the places where this can occur.

So, I'd suggest checking that first, probe the +/- 15V rails at various places, wiggle the connectors and caps to check for bad contacts or broken colder joints, etc.

Suspiciously high ripple could mean some caps have dried.

  • \$\begingroup\$ Hmm. Since the other half of IC2 is the previous stage driving the tone stack, and there doesn't seem to be any supply bypassing, seems possible it could be feeding back through the power supply. \$\endgroup\$ – Rob Lewis Oct 25 '19 at 20:07
  • \$\begingroup\$ Another possible clue (which I'm now trying to confirm): the motorboating seems to only happen when the amp is first turned on after some time left off. That presumably is why I never noticed it when I was working on the amp earlier. After a warmup period of some minutes, it seems to have stopped. I'm leaving it on overnight (with power amp disconnected) to see if it stays off. But does this maybe hint at failing electrolytic caps in the power supply? Even though they haven't been used much, they are after all 37 years old! \$\endgroup\$ – Rob Lewis Oct 26 '19 at 0:58
  • \$\begingroup\$ Capacitor ESR depends on temperature, so maybe... Could also be a broken solder joint or bad contact which expands enough when warm... how's the supply voltage ripple? \$\endgroup\$ – peufeu Oct 26 '19 at 7:47
  • \$\begingroup\$ No visible supply ripple on the scope (and also, no motorboating). Will try again after leaving the amp shut off overnight. It's odd that according to the schematic, this suspect IC is the only one that doesn't have bypass caps on the supply terminals. \$\endgroup\$ – Rob Lewis Oct 26 '19 at 23:24

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