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Thread: Fender Frontman 212r sound breakup and crackles diagnosis

  1. #36
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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!

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    Last edited by OwenM; 11-11-2019 at 01:55 AM.

  2. #37
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    Quote Originally Posted by OwenM View Post
    Hi all

    So after fixing one 212r I find myself with a second with a more curious symptom. The amp works but breaks up in an almost bitcrushed, digital way on the fadeout of chords, and sounds generally very unwell.

    It's in the power amp section, the preamp going to another amp sounds fine, and the problem remains the same sending a preamplified signal from another 212r to the power amp input.

    The board looks clean, I've cleaned all jacks and pots, the power resistors that are often faulty on these amps are securely soldered (as far as I can tell) and in spec. The board looks generally good and connections check out wherever there's any question over them. The big caps are in spec from a quick test in circuit with an ESR meter.

    The 4 big transistors have only been checked in circuit, but they show 0.6v voltage drop in most directions, not open in some as expected, but I've ONLY tested these in circuit and haven't checked the schematic properly to see if this is normal yet (in truth, I'm still learning to see that part of signal flow on a schematic, haven't worked on many transistor amps) so I don't know if I'm reading that voltage drop because of other transistors in the circuit. I can take them out and test but wanted to get some advice first as they are connected in a fairly robust manner that I didn't want to mess with without input from people here!

    So something is out, but this type of symptom is new to me, I'm probably more comfortable dealing with 'no sound', 'loud hum', 'intermittent sound' etc.

    I can start taking out things in the power circuit and testing but I really would be shotgun-approaching it at this stage, so wanted to see if the sound it makes (recording attached) points to any rough area I should start with!

    All the best, and thanks to anyone who would like to help!
    ive a similar problem . And the problem was in R88 & R90. Are you test the R88,R90,Q12,Q13??

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  3. #38
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    Quote Originally Posted by binito View Post
    ive a similar problem . And the problem was in R88 & R90. Are you test the R88,R90,Q12,Q13??
    Thanks, I just read the enormous thread over at DIY audio and those parts came up there, too. I think after reading 300 ish posts over there I have a bit of a better understanding of what's going on in the circuit, roughly at least, so I'm going to start pulling some of the same parts and testing, I've ordered a few bits already, too!

    I had a thought though, after peering at the circuit for hours, and I'm probably wrong.... but I'll ask anyway, does the side of the power amp connected to the positive rail actually amplify the negative half wave of the signal, and vice versa? I just ask because of the diode orientation, D28 and D29 in particular. It looks like they deal with the negative half wave at d28 and positive half wave at d29, then go into the bias circuit and on into the power amp proper. I just ask as my amp has a clear, extreme, clip on the negative half of the sin when scoped, and I want to make sure I'm focusing on roughly the right 'side' of the power amp. I know loads more stuff may be at play, and I'll be testing a lot more than just one side, I feel, but just wanted to clear up that!

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  4. #39
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    The power transistors connected to the positive rail are responsible for the top half of the signal.
    As the upper transistors turn fully on, the output is basically shorted to the positive rail, and vice versa for the bottom side.

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    Quote Originally Posted by g1 View Post
    The power transistors connected to the positive rail are responsible for the top half of the signal.
    As the upper transistors turn fully on, the output is basically shorted to the positive rail, and vice versa for the bottom side.
    Thanks, yeah I sort of knew that but was allowing myself to get confused with d28 and d29, now I see they aren't forward biased by the signal, on the input side, but probably from the rails on their other side. I can't hazard a guess as to why they are there but I can see the signal flow much more clearly now anyway, cheers!

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  6. #41
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    Ah... as if by magic I've found that r87 is fused! Everything else checks out on the board as far as I can test in circuit. There are a few tolerances on the edge but I think they are basically other circuit interactions. I'm gonna replace that resistor and see what I get. Do these sometimes just go spontaneously due to manufacturer error etc, or should I be looking more for a cause cause of it blowing?

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    If Q11 is ok, then R87 may be the only problem. I suppose the crackling could have been R87 on it's way out.
    But it could also be that Q11 is going bad when powered, yet tests ok unpowered.
    If the noise is still there with R87 replaced, hit Q11 with the freeze spray.

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    Ok, sounds like a plan... it's not just erroneous crackles unfortunately, the negative side of a scoped sin is chopped off too, and the whole sound has a sort of fuzzy bit-crushed sound as it fades out. But the shape of the scoped sin has altered quite a bit over the period of time I've been looking at it.

    So yeah, I think I've seen R87 slowly dying. Q11 showed high resistances (not open) on B>E and E>B, but tested fine out of circuit with both resistance and diode test. But yeah, I'll keep my eye on it once the resistor arrives

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    The replacement resistors finally arrived, replaced R87 and I now get the perfect sin I'm feeding into the power amp on the speaker output, major success! I still need to test it properly and then play a guitar through it, paying attention to Q11, but very happy that the sin is making its way through the circuit properly for now. This amp has been a nightmare sitting on my bench for weeks, I hope I'm not prematurely excited that I can soon put it back together and be done with it

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    Last edited by OwenM; 11-28-2019 at 01:18 AM.

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