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Yamaha P4500 / P3200 Power Amp | Distortion

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  • #31
    Did you figure the amp out?

    I admire your guts to tackle such a complex amp with not test instruments to speak of. But I am afraid you introduced more problems then resolved with your ill advised technique. By pulling all the transistors to check out of circuit you have introduced dozens of possible new problems that are harder to trace. Don't pull anything out until you determine a reason. The voltages on a transistor in a live circuit will tell you all you need to determine if it functioning, pulling it test little except junction drop with a constant current source across it. If you value the amp, put everything back, even the original caps in there original positions. The odds of putting all those signal transistors back in correctly unharmed is actually poor. Take the amp to a tech who has the test gear needed to evaluate the problem. It will likely save the amp.
    The distortion sound was like breakthrough distortion, with only peaks turning on the current devices in the output. You are also working with a no-no condition of using a load on a defective solid state amp. Your heart is in the right place but you are doing everything to ensure an unhappy ending. If you had a scope you could see, without a load whether the waveform got better with no load. I suspect it does to a degree, indicating a problem fairly late in the amp signal flow. A level shifter or constant current source would be suspect but with your test method you are not going to find such a dynamic problem, that depends on the circuit to be live to duplicate the problem.
    Good luck

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    • #32
      Progress on this has been very slow, but I have since replaced every transistor, including the outputs with no improvement. I've got a scope on order, hopefully it will be here soon and I can post my findings. Thanks a ton for all the help so far!

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      • #33
        Here's a picture of my scope when on the output of the amp on the B channel. The Signal was a 1khz tone from a cd player. Power amp volume was halfway up.
        I also noticed that the 'Signal' light only lights up on this channel when the volume is full, while on the A Channel it lights up with the volume barely on, which is normal for most power amps I've used.

        Another quick question as this is my first scope; what should I NOT touch with the scope probe?

        Thanks again for all the help, great forum!
        Attached Files

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        • #34
          I do not see a "signal" circuit on the schematic.
          I would assume it is part of the preamp, not the power amp.
          If that is so, then you have a problem in the preamp.
          It may be that there is nothing wrong with the power amp.
          One of the First Rules: Divide to Conquer.

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          • #35
            The distortion only follows the B power amp 'module' after swapping them with each other.

            When the B module is hooked up, that's when I can see the distortion in the preamp section. Here is a 'map' of where I'm seeing that nasty wave before it hits the 'B' amplifier.
            Attached Files

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            • #36
              We are used to thinking of the negative terminal of power amp outputs as "ground." But in bridge mode, both channels work together. Don;t assume either is grounded, so don;t cojnect your scope probe GROUND clip to an output terminal COnnect it to actual ground in th amp and monitor what each channel is doing.
              Education is what you're left with after you have forgotten what you have learned.

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              • #37
                I've been testing it in stereo mode, but I understand what you mean.

                Yesterday, I went ahead and mapped out where I can 'see' the distortion in the B channel preamp and power amp, but I'm not sure where I go from there...

                I thought this was gonna be a quick output swap after the owner described one channel being out, but once i heard that distortion i knew i was in for a fun time. haha.

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                • #38
                  Can you post more of the schematic?
                  You have a signal led that comes on late.
                  Also the proect & temp leds do not light.
                  I am curious why.

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                  • #39
                    The entire schematic should be in the first post.

                    The protect and temp lights don't come on I'm guessing because the amp actually 'works', it just doesn't work correctly. If I unplug that power amp module and run the amp, then they do come on, so i know they work.

                    The signal light is a mystery to me so far. I would have thought it was a preamp problem, but i don't see any crazy distortion until I connect the power amp.

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                    • #40
                      Here are images showing where i can scope the distortion There is a bit of overlap on the power amp images since i can't fit it all in my viewing area, so keep that in mind.

                      Pre:
                      Click image for larger version

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                      Power:
                      Click image for larger version

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ID:	820057Click image for larger version

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                      Three of the vertical lines that go past the edge of the image are going to IDL TP and one is to SP OUT

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