Solving drop out problem in JMF Spectra 30-T

[zinc1024]

The amp works fine for about  5-7 minutes.  Then for a period of 5-15 seconds, volume drops (but not to zero) and gets crackly.  It recurs ever minute or two thereafter.  I've replaced all tubes; didn't solve it.  I cleaned the effects loop jacks; didn't solve it.  I surveyed the board and found one push connector that was a little bit unseated and reseated it.  Didn't solve it.  It appears to have the original filter capacitors.  My suspicion now is that with those about over 35 years old, they are the likely culprit and I should just bulk replace them.  My question: is the issue I've described likely to be failing filter caps?  Or something else?  Many thanks.

[vampwizzard]

Id say filter caps are a likely culprit. But honestly, could be a lot of component value drift in the power supply. check the resistors in there too. Does the problem exist in the non reverb channel too?

[pdf64]

I dunno, I don’t associate that kinda issue with failing ecaps. Seems more like a bad connection, eg cracked resistor, failing solder joint. May even be a failing solder joint on a HT cap, in which case, yeah, a recap would get the credit for fixing it.
It looks a neat amp, well worth fixing up right. It’s amazing how cheap they go for, considering their rarity and fundamental parts value. Does the ratty looking speaker do justice to the amp?
I suggest to monitor some key circuit voltages to see what changes during the fault mode.

[zinc1024]

It has a spring reverb.  The problem exists with that both enabled and disabled, as well as with and without the lead channel turned on.  I replaced the speaker with a Celestion V12-60 Silver long ago.  Otherwise, as far as I can see, the amp is all original.  Okay, I'll dive into some more serious board analysis.  The value of the amp just doesn't justify taking to a tech, but it does justify some home effort to get her ripping again, and it's a good time to learn some new skills.  I'll report back what I find.  Thanks folks.

[vampwizzard]

if this is the correct schematic it may not be the filter caps necessarily. Theres some solid state circuitry in there than can let you down as well. https://www.tdpri.com/threads/nad-jmf-spectra-30t.341067/

I thought there were two channels but I was wrong. If it doesnt change with channel switching then Id start my search at the output + power section and work backwards to the input. Good luck!

[PRR]

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