fender fm212r possible muting problem

[georage]

thanks for watching. my 2003 fm212r has a strange problem where it will go silent after about 12 seconds from turning on. this problem is intermittent, which makes it really hard to track down. wernersaurus made a really good post on how the mute circuit works but its over my head. if anyone is interested, here is what im working with....sometimes when i turn the amp on it will play for about 12 seconds and then go quiet. when this happens the preamp will play through a seperate power amp via the fx loop. observing the voltage on the two big resistors that convert the 40v rails to 16v for the preamp, the voltage will be full upon start-up and then slowly drift down to zero in about 12 seconds. my question is...is this the effect of the mute circuit? i took these measurement 2 years ago and now i turned the amp on again and it works perfectly again...so i cant double check the measurements untill it fails again, but it does this intermittently while im playing it..it just cuts out and wont stop failing until it sits for a long time and then it works like nothing ever happened. any ideas? thanks.

[The Dude]

......sometimes when i turn the amp on it will play for about 12 seconds and then go quiet. when this happens the preamp will play through a seperate power amp via the fx loop. observing the voltage on the two big resistors that convert the 40v rails to 16v for the preamp, the voltage will be full upon start-up and then slowly drift down to zero in about 12 seconds ......

That makes no sense. You say the preamp out still works when the amp fails. Then, you say the 16V rails go to 0V when the amp fails. The preamp (op amps) runs off the 16V rails, so both statements cannot be true.

[Jon Snell]

I would suspect the effects loop sockets for dirty contacts.

[georage]

i know it makes no sense The Dude! i must be mistaken somehow and i cant get the amp to fail now. i've had it on the bench for 2 days and it wont cut out..so sorry for wasting time. i will get back to it when the amp fails again.

[The Dude]

Don't sweat it. Electronic repair is often a class in wasting time.

[georage]

the amp hasnt failed yet and so i went ahead and replaced q12 and q13 since that is recomended. i have a thermal camera and i think it is interesting to view the heat of components. before the swap q12 and 13 ran at 130°c...they are rated at 150°c max, but the new transistors run at 60°c

[georage]

question: what is an acceptable amount of dc on the speaker output? without a cord plugged in, this amp is pushing 60mv dc on the speaker and it appears to increase the longer it is on....i have had it on for about 10 minutes and it is hopefully stopping at 60mv dc. when i plug a speaker in and a cord in the front, i cant read any dc. does this sound normal? thanks for any insight

[g1]

Do you also hear a hum? When nothing is plugged in, the mute is engaged. I think you may be running into the issue that came up in the wernersaurus post.
You can do the mod, or just turn it off when you have nothing plugged into the input.
https://music-electronics-forum.com/...305#post588305