Fender FM 212r low level hum

[agent86oz]

With no inputs plugged in get a low level hum.

Found if I plug a jack in either J1 or J2 hum disappears but get a bit of very low background noise normal to amplifier.

On the schematic the switching on the Jacks goes to a Mute circuit. I have two units here and all voltages seem to be near identical but the second one behaves correctly.

Even though power electrolytic caps check OK replaced them anyway.

Can anyone explain the operation of the Mute circuitry so I might be able to troubleshoot further as it has me pulling my hair out.

http://www.fender.com/support/amp_sc..._Schematic.pdf

Thanks

[Steve L]

FM 212R Hum

Unless there is a cold solder joint on the printed circuit board, you probably have a bad input jack. It should be making connections when there is no jack inserted to stop the noise.

I would replace both of the jacks (J1 and J2) since you will have the amp apart.

Steve

[Teknous]

I have a similar problem - 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).

[Jazz P Bass]

The muting circuit requires a plug inserted in the jack.