Fender Frontman 212r - Doesn't mute properly

[Teknous]

Fender Frontman 212r problem - The amp works fine when a cord is plugged into it. As soon as you unplug there is noise and random pops (loud pops). The mute circuit is obvioulsy not functioning properly. The mute (ground) from the input jack seems to be ok. The line goes to ground and is applied to the junction of r73 and c46, ( I even tried jumpering it manually to ground) the pops and noise persists. On a scope the output from the power amp (to the speakers) even sauturates into a square wave - plug in a jack and the noise stop (even without a guitar on the other end). Anyone seen such a problem?

[nosaj]

Clean your jacks with some deoxit, on some of the input jacks there is a switch built in that closes when you remove the instrument plug.
jason

[Enzo]

That is what he is calling a mute.

I agree, spray some cleaner into the jack and push a plug in and out a few times to spread it around inside.

And check the solder under the jack.

[g1]

This mute circuit is more than the standard input shorting jack. There is a mute circuit at the power amp input (to bottom of differential).
Do you have freeze spray? Q6 & Q7 would be my guess.

[Enzo]

Maybe, but how would it know if you had unplugged the guitar or not?

[g1]

He "told it" the guitar was unplugged by grounding the junction of R73 and c46. But it is still possible the mute circuit is non-functional and if so a dirty switching jack would be noisy.
(I typed this on an iPad and does that ever suck)

[Enzo]

I'm sorry. I mean if he grounds R73, and the amp makes noise with nothing plugged in but stops with anything plugged in, how does the power amp know something is plugged in if R73 is grounded?

There is a ground connection at the power amp in jack (was that cleaned?) which then goes through the two input jacks in series and ultimately to R73. Plugging into any one of those three jacks lifts the ground connection to R73. As far as I can tell that would be the only way the power amp could know to unmute. And if it is grounded manually...


I could easily be overlooking something obvious, I admit.