JMF Spectra 30-T Sound Drop Out

The drop out issue starts after about 5-7 minutes of the amp being on and played. The sound doesn’t die out, it just fades and crackles, then comes back on after a few seconds. This repeats every minute or two. QUESTION: is this most likely due to old (35 years or so, I believe they are original) filter capacitors, and replacement of those is a likely fix? I’ve fully retubed, that didn’t work, I’ve cleaned the send/receive jacks, that didn’t work, I’ve look over the board and found one little plug that was a bit unseated and fully reseated it, that didn’t work. So now I’m suspecting it needs a recap. Does the sound issue sound consistent with failing caps? Many thanks.

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for capacitor, normally testing is done by removing and checking using a esr meter to ensure it is within its range, so as the farads, for old sets, check for any "dry solders" you may need to retouch it.

share some pics with us if possible.



Hi @zinc1024 ,

Here's a link to the schematics for the amp that may also help if replacing the capacitors doesn't solve it. Scroll down to post #19.

It appears to be hand drawn but definitely better than not having it.


All the best for 2021. Stay safe and healthy



@jayeff nice find,

Happy New Year!

Stay safe and happy holidays!


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