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May 23, 2024, 02:00:06 AM

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FM212r burnt resistor/distortion on clean channel

Started by sd22, March 27, 2024, 09:17:23 AM

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sd22

I have a Fender 212r that isnt worth much, but I am trying to repair as a hobby. I am looking to get into projects after taking some previous circuits classes but I'm stumped. The amp works, but through the clean channel I have some background distortion. The distortion channels have separate tone and are fine/unnoticeable. The speakers seem to be fine too. This amp is usually pretty clean but doesn't have that same sound anymore.

I took it apart and found that R84 was on its way out, 470ohm resistor burning and increased in resistance to upper 500ohms. The increased resistance could be forcing more power into other portions causing that breakup but that might be completely wrong. I tried checking some of the transistors and diodes around to see if there was an issue surrounding that, but I didn't find anything conclusive. I'm hoping to get some advise on some places to double check for a source for the excess power flow since a bad resistor likely is due to something else going wrong. I plan to replace the resistor obviously along with any shorted transistors or whatever else I find.

Line diagram linked
https://pub.deadnet.se/Manuals/ciasteczkowypotwor/%23pro_audio/Inne/frontman_212r_sch.pdf

g1

It is very common for the FM212 (and 65) to develop problems with the differential stage of the power amp (Q9 and Q10), which needs to be well balanced.  That could very well be the cause of your distortion issue, but even if not, it still needs to be dealt with.
Check R83,84,85,86 and replace any that are out of spec.
Replace Q9 and Q10.  This may be tricky as they are no longer made and there will be a lot of fakes available.  Try to get them from a reputable electronic parts supplier, not from Amazon or China.  * edit:  looks like Audiolabs of Georgia has them and they are a legit supplier.
It may be better to find a modern production sub for the 2SC2362K.

sd22

Thanks g1. I took a look at the resistors and transistors and R84, 85 and 86 seem to be out of spec. The transistors seem to read alright. I'm going to take a look at that site.