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

[georage]

hi. is anyone familiar with the fender fm 212r? i have one with a problem. it sounds like garbage...what i mean is that the output is very weak and distorted. sometimes, just after turning the switch off, the amp will sound perfect for a split second before shutting off. i have used a seperate amps effects loop to determine that the power amp section is faulty. in anycase the preamp out sounds good into the other amp and the other amp's preamp sounds bad through the fm212r's power amp in.

[georage]

any help would be appreciated. i have a schematic and will be looking into a couple of other posts with similar symptoms. thank you

[The Dude]

See this thread:

https://music-electronics-forum.com/...-repair/47329-


I'm not saying for sure that your amp has the identical problem, but the most common problem I find with this amp is Q12 & Q13. Almost every time I've had one in the shop, this is the cause of the problem. The transistors fail when they get warm. They will often test good with a transistor test or diode test on a DVM, but in circuit voltage measurements will prove or disprove that they are the problem.

[georage]

i did view the board with a thermal camera and no transistors got hot. the distorted weak sound is present from power-on. i will locate those transistors though.

[The Dude]

Measure Q12 & Q13 voltages while the unit is in failure mode. B-E voltages will be either much more or much less than the expected .7ish volts if the transistors are the issue. It's fairly simple to diagnose. Also, it could be just one transistor and not the other, but if that's the issue, I'd either replace them with something larger or install heatsinks. As I said, it's a relatively common problem. They tend to run hot even in normal operation. Then again, this may not be your issue at all.

[Mick Bailey]

Weak distorted output is often down to one side not conducting. Apart from the transistor failures mentioned, check the solder side of the board carefully - in particular the solder joints on the output transistors and emitter resistors. Look for a circular crack around the component leg. I've had a couple of these in recent years with broken solder joints on those resistors.

Check to see if one half of the output is missing. See post #6 here if you don;t have a scope;
https://music-electronics-forum.com/...12r-distorting

[DrGonz78]

Weak distorted output is often down to one side not conducting. Apart from the transistor failures mentioned, check the solder side of the board carefully - in particular the solder joints on the output transistors and emitter resistors. Look for a circular crack around the component leg. I've had a couple of these in recent years with broken solder joints on those resistors.

Check to see if one half of the output is missing. See post #6 here if you don;t have a scope;
https://music-electronics-forum.com/...12r-distorting

This sounds identical to an issue I am having on an old SS amp. The top half of the waveform is missing and sounds distorted. Just like the OP mentioned when he turns off the unit it starts to sound good for a quick moment as the voltage in the amp depletes. I think in my case it is due to old leaky NPN transistors as that is about the only thing that makes sense after checking out everything else. I will probably open a thread on this amp soon since it is giving me a bunch of trouble.

[georage]

thanks for the responses. i found an issue around resistor s r101 nd r105. i dont know what they do actually but here is what i did. maybe someone can help me understand whats going on....i checked the voltage on the 42 volt rails for the power amp section. i am getting 39 and -40 ( i hope im remembering the right polarity) when i got to resistors 101 and 105, r101 read 39 and 5vdc(on either side of the resistor)....r105 read -39 and ,3mv. i replaced r105 and the amp started sounding really good and normal, but the voltage drop was still similar. so i replaced r101 and the amp sounded even better, but the voltage drop still remained the same. so now the amp is sounding really good and normal. i cant understand why these 2 identical 15k resistors have totally different voltages and i am wondering if the problem is still there or not.

[georage]

ps i should have been reading the voltage drop across the resistors but instead i was reading voltage to ground from either side of the resistors.

[The Dude]

Did you check the original resistors? If so, what value were they?

[georage]

i did. they both read 14k. r105 seemed suspect as it appeared to fail when power was aplied to it.

[georage]

thanks for the help everyone. the frontman 212r is back together and playing. does anyone know where i can get replacement volume pots for this amp? i have a damaged volume pot on the gain channel. the pot hits a bad spot around 0-1 volume and the amp pops and crackles at insanely loud volume.

[The Dude]

Have you tried cleaning it?

[georage]

yes i did., with conact cleaner. maybe i should take it apart and rub the resistive strip with an eraser. however the pot is damaged and the bushing that holds the wiper-shaft is loose and i can wiggle the shaft and cause it to crackle.

[The Dude]

My recollection is that the original part is no longer available. The schematic shows a 100K linear taper pot. You might just have to measure up parameters and find one that will fit. If you can post a picture, we may be able to help.