New Problem with Fender Frontman 212R

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Originally I posted this under the "solid state category. But I repost here because this is the more appropriate category and because I made a
repair. Now the symptom is different so it is basically a new problem.

Originally:

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 replaced the two 4700 uf electrolytic caps/ power filters.

Now when you turn it on, speakers connected, no cable plugs in the inputs the amp is dead quiet, the LED is on. Not even the slightest hum but I didn't leave it on for for very long.

When you plug in a guitar, then turn it on you get a mild hum and the left speaker cone pops out and stays out. Turn off the power and the cone releases. It does this in both inputs.

In the other thread Enzo stated as follows:

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.

This is an exact description of what happens now but only with the guitar plugged in and only the left speaker.

An output that has failed would include which components?

Also in regard to the statement "many other things" ; Can anyone suggest some of the most likely things that could be causing the problem. I am a novice here (only my second thread) and in regard to fixing amps. I am interested in using this amp as a way to learn about state guitar amps. Thanks in advance for your help and suggestions.
 
Nigel:

The black wire coming from the power input connector to the bottom of the on/off switch was so loose that it came out of the female connector to which it was crimped. It came out with out my pulling on it when I first removed the board. I have already been told it is not that but I am wishfully thinking that the problem could be solved that easily.

Could operating the amp with that power supply wire loose in the connector and perhaps amking intermittent contact while operating the amp cause the initial problem that made the power caps and transistors blow?
 
Nigel:

The black wire coming from the power input connector to the bottom of the on/off switch was so loose that it came out of the female connector to which it was crimped. It came out with out my pulling on it when I first removed the board. I have already been told it is not that but I am wishfully thinking that the problem could be solved that easily.

Could operating the amp with that power supply wire loose in the connector and perhaps amking intermittent contact while operating the amp cause the initial problem that made the power caps and transistors blow?

If the switch is in the mains, then it's pretty unlikely.
 
Can you suggest several components most likely to be the culprit? (other than the power transistors)

A Question:

Theoretically, if one started to test and/or replace every component, i.e., every resistor, capacitor, diode, transistor, etc., on the board one would eventually find every component that had failed and fix the amp. Is that correct?

Are amps like this typically junked because they are not worth the time to find the problem? Someone I know who is knowledgeable was sure the problem was the power filter caps. When it wasn't that he was stumped. He had never seen this problem and couldn't understand why or figure out what faulty component(s) caused the amp to hum and the speaker cone to pop out ONLY when there was a plug in one of the inputs. A DMM was used to test the power transistors for shorts or opens. None found.

What would be the first several things you'd check?
 
Can you suggest several components most likely to be the culprit? (other than the power transistors)

A Question:

Theoretically, if one started to test and/or replace every component, i.e., every resistor, capacitor, diode, transistor, etc., on the board one would eventually find every component that had failed and fix the amp. Is that correct?

Are amps like this typically junked because they are not worth the time to find the problem? Someone I know who is knowledgeable was sure the problem was the power filter caps. When it wasn't that he was stumped. He had never seen this problem and couldn't understand why or figure out what faulty component(s) caused the amp to hum and the speaker cone to pop out ONLY when there was a plug in one of the inputs. A DMM was used to test the power transistors for shorts or opens. None found.

What would be the first several things you'd check?

It's quite a strange circuit, as a minimum I would try changing the output and driver transistors, and see what happens then - more than that you probably need a professional repair.
 
I just thought of a logical answer if there's no transistors blown. You had to replace the main power supply electrolytics so I'm wondering if they've leaked electrolyte everywhere. This can be quite conductive, certainly enough to turn transistors hard on with +-42v supplies around.
 
I just thought of a logical answer if there's no transistors blown. You had to replace the main power supply electrolytics so I'm wondering if they've leaked electrolyte everywhere. This can be quite conductive, certainly enough to turn transistors hard on with +-42v supplies around.

You're assuming the capacitors were faulty?.

I was assuming (as is more likely) that there was nothing wrong with the capacitors, but that the output transistors were S/C, which causes it to hum.
 
I really do not think anyone that does not understand how SS power amps work will be able to fix the amp by guessing and swapping out parts. One bad part tends to effect other circuits in the amp. You have to work your way through the amp troubleshooting each stage. You could have one faulty part or multiple and if you only fix the one the other may blow the other. SS amps are not as forgiving as a tube if something is wrong. I doubt you will be able to solve the problem asking questions here. Sometimes you need the proper knowledge and tools to get the job done.
 
Well, I am reading Teemu Kyttala's book, Solid State Guitar Amplifiers, or at least, I have it, I was just studying it to learn what exactly is meant by "power amplifier"; It seems to have two references: 1. to the entire amplifier unit; and to a specific group of components within the entire amplifier unit.

Anyway, I am not taking the repair of this amp too seriously. If I can get some leads as to the most likely sources of the problem, I'll try it. Luckily this stuff is still fun for me. If I can't fix it. I have already tried out the "specially designed for Fender" speakers as a 4 ohm cab. My drum machine with a Crate 160 watt head sounds good through it. Much better than the Crate Custom Design 12" speaker, anyway.

And as to the value of "guessing and swapping", I disagree. Most of what I do is by "guessing and swapping." I am just using as many resources as possible to improve my chances by making my guessing and swapping as informed as possible.
 
But the problem with direct coupled transistor amps is if you swap some components and nothing changed is that you might not have been wrong in the parts that you swapped but that if you miss a bad one the ones you swapped in will be blown. Then you will be scratching your head thinking if those were not the parts then which ones would they be. At a minimum you have to know how to test transistors to see if they are shorted, open or good. If you get to the point where your skills are not up to fixing the amp you could always put it aside for a later date when you understand more of how the amp works. It is not rocket science but you do need a certain level to tackle it. And with a few good resources you can learn enough to give it a go. But just being able to swap parts probably will not do it.
 
And as to the value of "guessing and swapping", I disagree. Most of what I do is by "guessing and swapping." I am just using as many resources as possible to improve my chances by making my guessing and swapping as informed as possible.

As Printer2 has already explained, you can't "guess and swap" parts in DC coupled power amps, all you're going to do is destroy more and more parts - including the new ones you put in.

Like I said before, try changing the outputs and drivers, and see if that cures it.
 
Nigel is probably right about the caps. They were not leaking and probably okay. I was going to try replacing the output transistors, but if something else is damaging them ... I am surprised o one has seen this problem on a Fender amp before.

As we've explained to you endlessly, pretty well anything duff in the power amp can blow the output transistors - so it's not a question of one standard component that blows it.
 
There is a problem thinking that problems are generic and that if someone else had this symptom then your amp will have the exact same cause. Not all that likely. My shop is an authorized Fender repair station, along with many other brands, and i have seen any number of Fender solid state amps of this model and others of similar circuitry, and a goodly percentage that have failed output stages. But there is no one part that will cause the symptom.

As others have said, solid state amps like this are direct coupled, and massively fed back, so it is one big loop. Not trivial to troubleshoot, and shoot and hope is not the way to go.

Output transistors can fail, and those take out driver transistors, and any of those will take out resistors. And this thing is not a simple conventional amp, it has little current mirrors and stuff over on the left half.
 
Thanks tubemax for the quick reply, I already read that forum, but my problem relates more with this one that deals with a loud hum. I checked the voltage supply of 16 and -16 that is going to all the opamps and they are getting 5 and -5 volts, so I checked the zenner diodes that regulates that voltage and the are fine, so i dont know where the problem could be that is getting those voltages down.
 
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