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Thread: Fender Frontman 212r sound breakup and crackles diagnosis

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  1. #34
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    May 2019
    Suffolk, UK
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    Thanks, freeze spray on the way.. none of the transistors are overly hot after use however, though I'll still go through with the spray when it arrives.

    I know the lack of advice is probably down to an amp like this being exceedingly hard to help repair via a few internet instructions! I can see it's a big winding mess of signals and the whole things being fed back on itself makes it so hard to pinpoint a problem. But I have nothing really to go on so I'm gonna just post my findings back here in the hope someone spots something obvious I can't see!

    The signal with the negative cut off, posted above, appears all over the later stages of the power amp, interestingly it's ALWAYS with the negative cut off, the same signal on transistors on both the negative and positive side of the amp. Does that sound weird?

    The OpAmp U6-B gives out an odd, lumpy sin. At the q12 collector the sin is normal except for the extremely clipped negative side.

    Both the power rails are at 44.1v, + and - respectively, rather than 42v, could that mess things up?

    I'm finding it hard to trace the signal as at most points the positive or negative rail DC voltage means I have to increase the V/Div up to 40v ish. At this zoom I can't see if the signal looks any different, I just see a slightly wavy DC. Should I be able to make the oscilloscope only show the AC and ignore the DC? I've tried doing this on AC trigger mode but it just gives the same +40dc reading...
    EDIT - OK, I've found the AC switch, I thought it was the same as the trigger switch but now I can see the ac component without the DC, very odd bendy sin though. And I can hear the sin wave coming from the PCB, sort of coming in and out like a slow tremolo effect. The sin wave at these points, eg. q9 collector slopes down from high to low on the oscilloscope screen, while maintaining a sin, very roughly...

    The q9 emitter gives a pretty good output of the sin wave at the EMITTER, I can't see how that could be right, as that just goes down to q8 and some kind of biasing circuit as far as I can see. Can we expect to see the signal duplicated at the emitter or is that a fault? Is it just a feature of transistors, or this circuit and the way the transistors are used?

    I think I can see where the signal should be going now, though I still see a load of weird behaviours I don't understand!

    Would it be advisable to reflow all the solder in the power amp?

    I've checked all the transistors for shorts and got none, I think next I might just buy a load of the smaller transistors and just shotgun replace them, I know it's not elegant, or efficient, but I'm nearly ready to just take the nuclear option!

    0 Not allowed! Not allowed!
    Last edited by OwenM; 11-11-2019 at 01:55 AM.

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