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AMPEG SVT3 Pro Fan and 12AU7 not working

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  • AMPEG SVT3 Pro Fan and 12AU7 not working

    Hi,

    I have an SVT3 Pro where the fan and 12AU7 filaments are not working. Replacing the tube didn't change anything. Obviously if fan doesn't work the amp wouldn't start either. I didn't dig through the schematic in detail but I suspect amp's input is muted somewhere (there's slight hum at the speakers' output) to prevent overheating. You can see some voltage readings below. F1 fuse is good.
    I'm not transistor guy but from what I gather Q17 (TIP32C) is not working. Or 'maybe I'm missing something?
    Attached Files

  • #2
    Check F1 and ensure D21 is OK and look at the 65v supply. Is the fan OK. Make sure the 65v is there. Both valve heaters are in series and ignore the fan speed controller as that won't stop the heaters.
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #3
      Fuse is good (not open). 65V supply is there. Fan is working from external power supply.

      Comment


      • #4
        Voltage of pin 4 of V2?
        Voltage on fuse?
        Q17 dry joint?
        Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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        • #5
          Those voltages are on the pic above.

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          • #6
            So they are.
            I would suggest Q17 is collector emitter open circuit as there is enough voltage to make it conduct.
            Have you checked Q17 for continuity and hfe?
            Take it out and measure it.
            Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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            • #7
              Do you measure continuity from Q17 collector to fuse?
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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              • #8
                I checked Q17 with my DMM in diode mode with V1 and V2 out. Readings are as follows:
                1 /negative probe on B
                B/E and B/C ~570
                2/ positive probe on B
                B/E - OL (open), B/C starts from 920 and goes up to 1120 which doesn't look right.
                Also C/E (-/+) 540, C/E (+/-) OL (open).

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                • #9
                  Do you measure continuity from Q17 collector to fuse?
                  No continuity from Q17 C to fuse but it's there from V2 pin 4 to Q17 C and from pin 5 to fuse as per schematic.

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                  • #10
                    Always remove the component from the board to test it unless you know exactly what you are looking for.
                    As Q17s base goes more negative with respect to the emitter, the more the collector will increase in voltage. That is all there is.
                    Check with another transistor.
                    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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                    • #11
                      The way I see it with the tubes out al transistor's pins can be considered "free" which should not affect the measurements unless there's some conductivity PCB problem or something.
                      I was hoping to avoid removing the whole power amp board but it looks like it's inevitable.

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                      • #12
                        If you don't have continuity from pin4 to pin5 it won't work. And by continuity I don't mean a dead short, just the proper expected resistance. Probably a bad solder joint at the socket, or bad socket.
                        If you were going by a continuity 'beeper', check on resistance range. Compare to pin 4 to 5 resistance of V1, should be similar.
                        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                        • #13
                          You mentioned "tubes out". If there is no 12AU7 in the socket there can be no continuity between the fuse and the transistor Q17.
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #14
                            You mentioned "tubes out". If there is no 12AU7 in the socket there can be no continuity between the fuse and the transistor Q17.
                            That's what I said earlier. By tubes "tubes out" I meant tubes "out of their sockets".

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                            • #15
                              But were the tubes out when you tried to measure the continuity?

                              With a 12AU7 INSTALLED in V2, is there continuity from Q17 to the fuse?
                              Education is what you're left with after you have forgotten what you have learned.

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