Fender FM212R Squeals at 4ohms, but not 8?

[parentheticalfact]

Hello there. I'll just admit I don't know much about solid state amplifiers (as anyone who has read my posts about music man amps can probably assess ).
I have an FM212R amplifier that squeals above a certain volume level when used with it's internal 2x12 4 ohm load, but when hooked to my bench load of 8 ohms. Components are reading true and voltages seem right according to the schematic. Could this be a problem with the output transistors? I'm assuming the resistors are also holding their value with current running through them. Where I get lost in this circuit seems to be where a feedback circuit is introduced. Could it be related to this section?
Any ideas or thoughts would be appreciated.
Thanks all!

[Enzo]

Is your 8 ohm load a speaker or a resistor? Bang on the chassis while connected to a speaker. Is the chassis microphonic? Or for that matter, while the chassis is out, connect it back to the speakers in the cab with some wires. SO it plays tjhrough those 4 ohm speakers but the chassis is not in the cab with them to be vibrated.

[peter jeurissen]

Is your 8 ohm load a speaker or a resistor? Bang on the chassis while connected to a speaker. Is the chassis microphonic? Or for that matter, while the chassis is out, connect it back to the speakers in the cab with some wires. SO it plays tjhrough those 4 ohm speakers but the chassis is not in the cab with them to be vibrated.

i had that same issue on a 212......
it appeared to be a bad solder joint on one of the 2 big load resistors which are in the amp
once i got it well soldered the amp was dead quiet

hope this helps

peter

[52 Bill]

Oscillations on those amps will sometimes be caused by broken snubber caps in different parts of the circuit. They are often in the feedback loop of an opamp. One lead breaks off and the circuit becomes a bit unstable.

Just do a quick look and feel of all of the small caps on the board to see if any are broken off.