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  • Fender Frontman 212R distorting

    On the bench is a Fender Frontman 212R that is distorting on both inputs. Inject a signal in the PowerAmp in and still distorted. I traced it down to this...

    on U6 i get a good signal on pins 1,2,3,5 and 6 but distorted on 7. I swapped out the U6 but no go. My voltage on TP29 is off too. I'm getting almost nothing there. I'm getting correct voltages on either side of D38/39. And I get distorted signal on both inputs of Q9/10.

    Any suggestions on why this thing is distorting only on pin7 of U6?

    thanks in advance!

  • #2
    Hi and welcome to the place.

    We have just had, and i guess are still having, a couple of threads on the FM212R power amp, one in the repair section and one in the theory part. May I suggest reading through those both to add some insight into the operation of this circuit, before we get into the questions.

    Be aware that solid state power amps are massively fed back, so any problem causing distortion in the later stages will cause a compensating distortion in the differential circuits trying to correct it.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      thanks Enzo! That is some terrific information I have been looking for for a long time. To continue with the "learning" aspect from a troubleshooting point of view.... Where would be a logical place to start troubleshooting? I would assume to check to see if you have the 2V differential on the drivers to check for bias? Am I correct there?

      thanks again for the patience and understanding. We're all trying to learn something everyday....

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      • #4
        First thing I'd do is scope the output to see what the nature of the distortion is. If I see half the waveform missing, that suggests a totally different approach from something like clipping top and bottom. Or I might discover that the amp has gone into oscillation at RF freqs. You can;t hear that, but it makes what audio still comes through sound crappy.


        If your amp basically amplifies, then the bias is probably OK. If the bias circuits short out, you get crossover distortion, but my guts say that isn;t why you are here. But maybe. And if the bias were open, then you'd be blowing fuses or speakers.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Lets assume you don't have a scope.... then where would you start first.

          Since I can listen to a distorted signal right on pin 7 would that not rule out things such as crossover distortion since at that point the signal isn't being split out to the + and - sides?
          Last edited by 1WoodHippie; 11-05-2010, 02:57 AM.

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          • #6
            AH, but it is. The output is being sampled back into pin 6, so the IC will be trying to correct any distortion by pre-distorting the signal to compensate. That is why we would prefer to see the nature of the distortion. At this point in my career, I usually recognise the sound of clipping versus half waveform or some such, but I have no way to describe such a thing. And before we go there, I don;t have sound on my computer, so don't send me a recording of it.

            You already swapped the IC, so it isn;t likely the problem anyway.

            Um have you tried a different speaker, by the way?


            NO scope... OK, first, with no speaker connected, is there zero DC offset at the output? Assuming so, here is something one could try. Just in my head, but ought to work. We will make a little sensor. Get a diode and connect it between the output and your meter probe. And just to complate a circuit, also connect a 10k resistor between your probes. SO now there is a 10k resistor across your meter with a diode sticking out from the red probe and connected to the output. Almost forget, also connect the black probe to ground. APply a nice steady signal to the amp. What we are doing is rectifying the output to be read as a DC voltage on the meter. Now, reverse that diode. In other words if it was positive before, we now measure the negative. SO you see, since we will be doing it both ways, it doesn;t matter which way was first. Now the bottom line - are the positive and negative voltages that result more or less the same? Or is one substantially larger than the other. That is my very crude half-waveform detector. In a way it is a sort of not quite RF detector probe. Oh, and the 10k is not critical at all, 6.8k, 22k, 47k, whatever you have.

            If you actually do that, I am dying to hear if it works.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              I GOT IT! Got hardly any voltage on the + side. Checked the bias on the driver and was at zero. Turned out to be the solder joints on R102 and the driver. Once reflowed it jumped right up!

              Thanks a ton enzo for your help. I've learned a TON on this project.

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              • #8
                My diode idea actually worked?
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  Absolutely! I had say -17V on the bottom rail and .6 on the positive. So I knew that something on the + rail was wrong. Checked the bias on that rail and that let me to the fix. EXCELLENT idea for someone without a scope....

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                  • #10
                    Far out, man.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #11
                      Albert Enzo-tein

                      When two British Scientists (Astronomers) published an experiment that actually *proved* Einstein's Theory of Relativity, he scratched his head and said, in disbelief: "ach !! is that so???"
                      Juan Manuel Fahey

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                      • #12
                        This isn't just far out, it's positively groovy.
                        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                        • #13
                          Hello, I'm new here, and I have the FENDER FRONTMAN 212R amplifier and I have the same problem, someone knows how to improve the design of the electronic part, so it does not heat much ????, it's a problem of BIAS, some have a solution, thanks

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                          • #14
                            I would recommend starting a new thread for your amp issue. Measure DC offset on the output with the speakers disconnected. If you have any substantial DC voltage on the output it can damage the speakers. It is important to figure out if the distortion is coming from the preamp or the power amp, also need to rule out damaged speakers. There is an effects loop on that amp where you can isolate preamp or power amp. To test the preamp inject signal into main instrument input jack. Then find preamp out (effects send) and connect a cable from that jack to the input of another known to be good amp, if no distortion then it's not in the preamp. You can plug the guitar directly into the power amp in jack (effects return) to test to see if the distortion is in the power amp. Start with this test and report back. Figure out what section of the amp has the fault, preamp or power amp.

                            Note that the problem in this old thread was narrowed down to a cold solder joint. A transistor was not getting voltage to bias the driver transistor. It was not a general amp bias setting issue but rather that a specific transistor was not biased or even receiving bias voltage. So not a bias being too hot issue but rather bias too cold on the positive rail.
                            Last edited by DrGonz78; 12-05-2018, 07:43 AM.
                            When the going gets weird... The weird turn pro!

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                            • #15
                              Fully agree: start a new thread and describe exactly what happens to *your* amplifier.
                              Adding yourself to an 8 year old dead thread makes anybody lose interest.
                              At least from reading old questions which to boot do NOT apply toyour problem.
                              And please update your profile to state where are you from.

                              And: welcome to the Forum
                              Juan Manuel Fahey

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