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Old 19th November 2011, 07:42 AM   #1
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Default New Problem with Fender Frontman 212R

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.
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Old 19th November 2011, 10:11 AM   #2
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The power amplifer is duff, it 'may' just require the output transistors replacing, but often something else caused them to blow. If you don't replace everything that's faulty, they will simply blow again.

Essentially you need to understand exactly who amplifiers work, and find all the faulty parts.
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Old 19th November 2011, 05:16 PM   #3
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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?
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Old 19th November 2011, 08:44 PM   #4
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Quote:
Originally Posted by diynow89 View Post
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.
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Old 20th November 2011, 07:52 AM   #5
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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?
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Old 20th November 2011, 08:41 AM   #6
sbrads is online now sbrads  United Kingdom
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Schematic at http://support.fender.com/schematics..._schematic.pdf

There's AC coupling everywhere up to the power amp section so it's difficult to see how the cone moves when a guitar is plugged in. Perhaps there were multiple leaky caps first as well as now transistor failure.
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Old 20th November 2011, 12:37 PM   #7
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Quote:
Originally Posted by diynow89 View Post
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.
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Old 20th November 2011, 03:01 PM   #8
sbrads is online now sbrads  United Kingdom
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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.
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Old 20th November 2011, 09:10 PM   #9
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Quote:
Originally Posted by sbrads View Post
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.
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Old 20th November 2011, 10:53 PM   #10
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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.
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