fm212r souns like garbage and sometimes clears up while being turned off

[georage]

ok. well thanks for the effort. i thought about soldering a resistor in series with ground, but it looks like the gain channel volume doesn't ground. it looks like it provides negative feedback instead. im not extremely knowledgeable. it looks out of my league to attempt a hack.

[Mars Amp Repair]

Try AmpRepairParts.com They have tons of pots of all kinds.

[georage]

thanks i will look that up. i buttoned the amp back up and gave it back, but then i started on fm212r number two. i read the voltages on r 101 and r105 and i get .e volts on both. now im worried. FM number1 read 5v on r101 and .3 volts on r105. even though the amp sounded great, i replaced the zener and standard diodes associated with those resistors. i should have read the voltages before reassmbly.

[georage]

when i replaced resistors r101 and r105 the amp started playing great, but the amp came back today with the exact same problem. i must have missread the multimeter as well which is too bad because im getting different readings now. im starting over from scratch. there must be some clue to the fact that this amp has reduced volume and distorted output, but when the power is turned off the volume and clarity returns to normal just before the amp goes quiet.

[John G]

Hi, I don't have an answer for you, and I gave up repairing these amps years ago.
I did find that when this problem occurred, I could sometimes pulse the on/off mains switch, and it would right itself.
Other times, I would apply my Digital voltmeter to any of the collector diodes of the power amp differential pair, and it would also right itself. I could never get any consistent fix, so I gave up.
I did wonder if the pulse on the diff pair collector was enough to kickstart the pair and have since thought that maybe the constant current transistor beneath the pair was not working correctly or the startup circuit feeding the base of the current transistor was also not working correctly.
Just some thoughts.
John

[Mars Amp Repair]

One caveat on these Frontman amps for those who perhaps have never worked on one is that of course it's nearly impossible to work on these when they're not in the frame but if you try to run them for very long w/o the power xistors screwd to the heatsink (basically a bar of aluminum screwed to the aluminum frame) the output transistors (esp in the FET versions) will overheat & blow.
Don't know what the hell Fender was thinking of when they designed this triangular mess! glen

[georage]

i was just trying to check tp18 on the schematic. its not great that i cant exactly follow the test conditions as my signal generator only goes down to 10mv instead of 3mv as recommended. my test was 10mv into the clean channel at 3volume. i made the following measurements with my scope on u6 a and b...
pin5..56mv
pin6...46mv
pin7..310mv
pin3..46mv
pin2...46mv
pin1...46mv

on power off, pin 7 jumps up to 510mv and the wave changes shape several times.

[g1]

You said earlier that the preamp sounds good through another power amp.
So if that is still the case, adjust the volume control so you get the 91mV at TP15 and proceed from there.

[georage]

well i went back to tp15 and set it all up and that was great. i tracked the weird wave form on shutoff all thebway back to tp10, but while doing that i touched the mid control button a few times and the problem completely corrected its self. the sound was perfect and clear. everynow and then after that (with a guitar plugged in and a dummy jack plugged in to the pwr amp-in) there would be a periodic crackle and sizzle noise. the noise wouldncome and go randomly or sometimes when i measures the -42 rail around resistor 105. then while measuring r105, r105 apparently died! i cant read any voltage acrossed it and the whole sound of the amp got really really distorted.