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Fisher amp help please

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  • Fisher amp help please

    A Fisher CA2310 came in for a cleaning, lots of switches cutting out, etc. Got it all cleaned up and when I went to button it up, it made loud popping and crackling noise and the output relay wouldn't stay on. A little investigation and a few power cycles later, it came back to life. Thinking it may be the relay, I cleaned the contacts. Next day, I listen to it all afternoon, when all of a sudden, there is that loud noise again with the switching relay on and off. Volume turned off made no difference.

    I probed around and found the noise on the left power amp. Rails were clean, so I traced backwards. I can see the noise all the way back to the collector of Q06, but not the base. I sprayed it with freeze, and it did get somewhat worse, but not sure if it was just a coincidence, it wasn't a dramatic change. So I pulled it, and the noise is gone, but now the protect relay doesn't energize. I don't see what relationship there is with Q06 and the relay board, probably because I don't understand what Q06's job is. The D06 on Q06 collector is a STV3H thermister mounted on the heat sink, I can see how this would interrupt the signal, but not via the relay.

    Could a failing D06 have caused the noise? Power amp is on the last page.

    It's weird, because it WAS working fine.....

  • #2
    OK, right to left, Q10,11 are outputs in push pull. Q8,9 are their drivers. Q12,13 are limiters we will ignore for now. Finally we get to VR01 and D6, which connect the driver bases together and also provide their bias. This is like just about any solid state power amp. All that is one big current amp, takes the voltage signal and increases its current for the speakers. Whatever voltage is on D6 and VR01 is what voltage will be on the output, more or less.

    Now we want the output to sit at zero until the signal says otherwise.

    So we have current coming through Q7, down through D6 and on through Q6. What does Q6 do? It is your voltage amplifier stage, but also it controls the current through that string I just described. If you pull Q6, then there is nothing to draw current through those other parts, you have opened the circuit. And that means the voltage at D6 and friends will climb up to V+ or close to it, and that in turn means the output will have V+ on it. Q6 has to conduct enough to pull the voltage at D6 down to around zero so the amp will sit where it needs to be. Then during operation, the signal controls Q6, which in turn directs the outputs.

    Is that connected to the relay? No, but look at the relay circuit. Follow the orange highlighted line. From the amp output at L1/R24, we go over thrrough the relay contacts and on up to the speaker terminals. But wait, that output also branches through that 68k resistor R1, is rectified by the bridge of four diodes and on to Q1. Q2,3 are a darlington pair that turn on the relay. Look upper left on the protect board. 34vAC comes in and makes a 13vDC supply with D7,8. That supply is fed through the two posistors on the heat sinks, and ultimately to the bases of Q2,3, which turns them on, enabling the speakers. If the posistors get hot, their resistance rises, dropping the voltage at the bases of Q2,3, turning them off. That is how the relay turns off if the amp overheats.

    But back to Q1. If we send a large enough voltage to that small bridge and long enough to charge up C3, then Q1 turns on, and grounds the bases of Q2,3. That turns them off and the relay drops out, disconnecting the speaks.

    Now back to Q6 on the amplifier board. You removed it, that allowed the voltage on D6 and all to go to V+. That ran the amp output to V+. V+ is what, +47v? SO by pulling that transistor, you made the output 40 some volts, which turns the relay off. You cannot run the amp with Q6 missing, this is not like a row of 12AX7s in a Fender. If you think Q6 is noisy, then you need to replace it, not just remove it..

    Our friend Teemu K. has written a great book about solid state amps, and has posted it for download over at in the sticky section. i recommend you get it and read it.

    Q6 seems to be one of those oversized TO92 types 80v 700ma. You have any parts like that? MPSA43 in my drawer might be close. leg order different though. Oooh, I have 2SC2235 120v 800ma, I think I have those for Fender FM212R amps. But the real part I think is still available. if you want to be sure, swap it with the one on the other channel. That will tell you.
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      Enzo, brilliant as always! Thanks very much for that very concise explanation. I don't have anything on hand that would sub, but I did find an original on eBay for $4 shipped. Yes, I realize it is Chinese, but hey. And yes, swapping sides, that got by me! I am mostly self taught, so there are holes in my theory, mostly in the SS realm, so I ALWAYS appreciate assistance and guidance from the big dogs.
      It's weird, because it WAS working fine.....