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Old 18th November 2011, 04:26 AM   #1
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Default Problem with Fender Frontman 212R amp

I bought FM 212R in non-working condition for a good price on spec that I could fix it.

The symptom: When you turn it on it w/o guitar plugged in) it puts out a strange loud hum. Strange in that it is not the type of normal hum you get when you have a guitar pluggged in. A friend who is knowledgeable thought it might be the power filter caps so we turned it off immediately because if it is the caps it could damage other components.

I removed the chassis. And located the two large 4700 uf electrolytic caps. They looked okay upon visual inspection. But I decided to replace them just to see. While I was removing the board the wire from the AC power input connector (the right pin looking at the chassis from the back) to the lower left pin of the on/off switch pulled out of the female connector (which was still on the male spade on the switch. It came out very easily without my pulling on it as I was disconnecting the wires to remove the board.

The power transistors look good from the top, i.e., no pitting, or bubbling. The solder connections for the transistors have some brownish discoloration around the edges of the solder on the board and salty looking residue.

Could the wire that pulled out of the female connector have damaged something?

Does anyone have any ideas as to what could be the problem? Thanks in advance for any help.
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Old 18th November 2011, 06:15 AM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
They will probably move thos to the instrument amp section.

When you have a loud hum from an amp like this, 99% of the time it is one of two things.

One is the output has failed and gone to DC. You would find a large DC voltage across the speaker, and if you looked at the speaker, when power went on, the speaker cone would move one direction and stay there. Bad for the speaker so don;t leave it on. That can be caused by failed otput transistors or many other things.

The other thing is the filter caps. Usually what has failed is their solder rather than the caps themselves. I run a Fender aithorized repair shop, and I don't think I have ever had to replace one of those caps. Fender uses the same basic layout on a lot of models - the pair of cans sticking up from the center of the circuit board. They shake around and crack their solder. This results in hum, but not DC on the output. If the cap is thus disconnected or open, it won;t hurt the amp further, it just hums loud.

Bad parts usually look no different from good parts.

The brown stiff around the solder joints is just solder flux, and the salty looking stiff is just scale from the water they washed the board with at the factory. Neither is a problem.

Disconnecting the mains power wire is the same as turning off the power switch, it won;t hurt anything unless is touches some other circuit part. If you put it back in the right place, you hurt nothing.


The amp can be powered up with no speaker connected, and in fact we'd prefer you work on it without a speaker load until we are sure the amp is stable and not putting out DC to the speaker.

The tests here are to first check for RC on the speaker wires - I don;t know, 30-40 volts maybe. And then check the main power supplies for ripple. A convenient place to find that is the metal tabs of the output transistors. The ones on one end will be positive and the other ones negative. Put your meter on DC and see that both supplies are about the same other than polarity. Of one is much lower in voltage, flip the meter to AC and remeasure. Now you are only measuring ripple. There should be less than a volt, if you get 20 volts or more of AC that way, you have lost a filter caps, either from solder or failure.

Unfortunately, even though this is an entry level moidel amp, the power amp circuitry is way more complex than it needs to be, and won;t be trivial to fix unless you get lucky. I won;t get into why other than they wanted to make it as loud as they could with the limited power available.
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Old 18th November 2011, 07:41 AM   #3
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Enzo:
Thank you for for the quick detailed reply.

Are the 2SC3263, the 2SA1294, the 2SC3298, and the 2SA1306 the output transistors? Or just the 3263 and the 1294? The filter caps and the 3263 and 1294 are only about $15.00 at Digi Key. I think I might gamble the $15.00 and see if, as you say, I get lucky.

You say that other than the output transistors it could be "many other things". If you mean components on the board that could be easily replaced. Can you suggest several other components I could change out to see if that does it? The components are cheap enough. The 3263 and 1294 were only $3.08 each. Thanks again.
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