fender fm65-r solid state.plays quits then cycles on and off repeatably

[EDWARDEFFECT1]

any ideas! i have a fender fm65-r amp that plays for about an hour then it turns off for about 10-15 seconds.then it starts up playing normally for a bit then cycle off and back on again.like a computor loop.when it starts up after quitting it starts low in volume and gets louder as it comes back to normal playing volume.just like an amp that is initially turned on.i replace 2 transistors as they were getting hot to the touch. i thought they might be leaking,but guess that wasn't it.do you think it might be an output chip?anyone had this problem with this model?how do i test for a bad chip.i have a signal tracer ,scope ect.i replaced a 1013 and c 2383 which are larger in size than the other chips on the board.thanks...ed!!!

[phatt]

Hello edwardefx,
Sounds like the thermal OLoad is working.
If you can verify it is that? Then Consider adding a much larger heat sink.

I once fixed a Fender BXR Bassamp that had melted the back panel plastic.
The poor chap even had a big fan on it.

I cut some 4mm thick alloy panels for the whole back panel and it never came back. Same amp was seen 5 years later still working with no issues.

Fender make some good gear but some are Just plain SAD design.
Phil.

[Enzo]

If the amp is so hot it is thermalling, then the chassis will be very hot to the touch.  They all get warm, but this will be noticably hot.


In my humble opinion, enlarging the heat sink in one of these to cure overheating, is like fixing a broken arm by taking pain killers.  The problem that needs to be addressed is why the amp is overheating, not how to dump the excess heat it should not be making.

If amp bias is adjstable, start there.  But a lot of these are not, so we need to explore the bias string in the amp, or if it is a chip amp, we need to make sure the zobel network is keeping oscillation at bay.  I find sometimes I need to shunt across one of the series diodes in some power amps when they run too hot.


I don't spot any thermal protection in the FM65r, other than possibly a thermal fuse in the power transformer.   And in my experience, most things that thermal off don;t cool down enough to come back on in only 10 seconds.

There is a pair of jacks - preamp out and power amp in.  Take a spare guitar cord and plug it from one to the other.  Does that help?  If so, the power amp jack needs service.


When this happens next, plug the guitar into the powr amp jack.  Does it still fall down and get back up, or is that fairly steady - yes, all the preamp controls will have no eeffect then.  You can also run a cord from preamp out to the input of some other amp.  So when your amp falls quiet, is that preamp signal coming out the other amp also falling off, or does it remain striong?

[EDWARDEFFECT1]

got the amp working. it had a cold solder somewhere on the board.i measured the temperature with a infared temperature gun and got 170 degrees on q11 transistor.when i touched it with my hand there is no way it was that hot. played it 4 1/2 hours with no quit, so i put it back together and it is ready to rock....thanks for your help again.....just thought i'd let everone know what was up...latter....thanks....ed!!