Fender FM212R Problems

Hi Guys

The amp has an effects loop so it is a simple matter to split the amp in two and hear where the noise is coming from. Take a line from the 'send' to another amp, your stereo, or anything that you can use to monitor the sound with. Do you hear the crackling through the monitor?

Now plug your guitar into the 'return'. This breaks the signal feed from the preamp to the PA. Set the guitar volume to zero. Do you hear crackling through the speaker?

If you hear crackling through the monitor system, then the problem lies in the preamp.

If you hear crackling through the amp's speaker then the problem lies in the PA or supply.

If you hear crackling in both then the problem is in the supply but actually still comes down to poor solder connections.

Soldering a few suspect looking joints is not good enough - EVERY joint must be resoldered and done properly to take this aspect out of the picture.

Have fun
 
Thanks, I think someone mentioned that before and that's a great idea. My problem now is that it is intermittent and presently the amp is working fine, but it will show up again and I am waiting to take some pics. I know if I put it back together it will happen again so for now I'll leave it on the bench maybe throw some heat to it to see if it will do it again. I guess I will resolder the whole board and start over just to eliminate that problem. This mofo driving me nuts! lol
 
I'm sorry my question wasn't specific enough: I meant to say, it only hums loudly when nothing is plugged in. When I plug in a guitar or just a 1/4" plug adapter, the amp goes quiet. I assumed a problem with the muting circuitry, but can find nothing wrong there. I replaced the 10V zener, 10uF and .047uF in that circuit, anyway, for good measure. The schematic shows a PTC thermistor in this circuit, but I can't find it on the board. Then I looked at the input jack circuit and tested R1, the 10 ohm resistor to "chassis" ground. That's OK, I also replaced C1, the .1uF mylar that goes from the switching contact to R1. The mystery in this input, is that I don't see how R1 gets to chassis ground?
Hey Audiocarp, that thermistor you can't find is sitting on the left side of the heat sink nestled snugly in that little square slot. I'm assuming so if it gets to hot it will shut it down. I know, it took me forever to find it.
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Are we still talking about the ticking sound?

Please clip your scope to the output , show us a clean signal and then the noisy one.

Be certain that anything strong enough to be heard will be important enough to show some visible abnormality on the waveform.

Guaranteed.

This will sound sweet:
SineWave.gif


none of these will:

this is an ugly overdrive tube Marshall
An externally hosted image should be here but it was not working when we last tested it.


this is an unstable Acoustic 360 where the bottom of the waveform plain clips while the top briefly oscillates:
An externally hosted image should be here but it was not working when we last tested it.


I show this not because of the waveform but because of the terrible spikes, which would be absolutely unbearable in an amplifier:
mmv158.jpg


here's the same, mounted on a clean sinewave and making it raspy, annoying:
Xh0M5di.jpg


this is an output stage missing the bottom half drivers:
testwaveformoutputbeforediode.png


in a nutshell: when you hear something weird, you must be seeing something weird.
That may (should) help troubleshooting.
Have your camera ready :)

EDIT: there is one way to hear ugly/weird stuff and see nothing: when you have a *mechanical* problem at the speaker: junk/loose wire/bubble/torn/unglued voice coil - frayed tinsel wires - cracked/split cone - loose dustcap - loose speaker - etc.

To check that, try amp with another, known good speaker.
How do you post pics in this forum?
 
Well guys I'm back, after re-soldering the components around U7A Op amp, my amp is now working with no noise present. I had visually inspected the board and especially around that OP Amp, which was my suspected fault, but I could not visually see anything wrong, even under a magnifying glass. So lesson learned here. Thanks for all the help especially comments regarding re-sweating the solder joints, that was my problem. BTW the scope was showing noise spikes all along the sine wave similar to the photos above, so take note future troubleshooters. :)
 
Sorry about that.... I gave wrong schematic. The FM212R does suffer with solder joints. But intermittent distortion problems in recent repairs where finally tracked down to Q12 and Q13. Could be breakdown or internal transistor structure problems.
Replace with indicated substitutes...and this time with the correct schematic!
 

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Hi there,
I'm trying to fix one of these amps, that has a very loud hum and very weak distorted sound.
Unfortunately I'm not into solid state, only learning about tube amps for a few months. I'm a tech, not in electronics, but I learned some electrical things 30+ years ago. I can check voltages, resistors, etc... with a meter, and I'm aware of safety problems when dealing with amps.
First, just by looking at the board, I found R84 (470 ohms) completely fried. I replaced it but it made no difference. After reading this thread, I tried some measurements, and found 20+ DC volts on the speakers. Following Enzo's advice, I checked the output transistors (Q18 to 21) and found them shorted.
Any advice before I change the transistors ?
 
Same problem here. The power amp transistors measured "open" in-circuit because of oxidation. I then switched test probes for stainless steel pinpoint-type, then all transistors measured ok.
Turned out the pre-amp auxiliary +/- 15V supply was noisy/unstable because of cracked solder joints around the zener diode circuit D57/D58 and R144/145. Caused by very high temperature of these components, which operate close to their limits. There was a minor discoloration around these zeners. The solder joints were stressed, aged and broke as a result.