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Ampeg SVT 350H issue

Discussion in 'Amps and Cabs [BG]' started by Oren Hudson, Mar 21, 2013.

  1. Oren Hudson

    Oren Hudson

    Dec 25, 2007
    Gastonia, NC
    I've got a '95 Ampeg SVT 350H solid state that's served me well for the last 15 years. My first problem with it. I noticed a warm smell from it and found that the fan was not working. I removed the fan and tested it, it's OK. What's next to check for? Everything else is working fine. :meh:
  2. lokikallas


    Aug 15, 2010
    los angeles
    How'd you test the fan?
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    next is for a technician to check the fan circuit. Maybe a bad connection, maybe C17, the capacitor across the fan is shorted, maybe something else.

    The fan should run all the time slowly, and speed up as the amp gets hot. The "base level" of speed is set just by resistors from the positive power supply "rail", so if it doesn't run, and doesn't start when the amp is hot, then either there is a bad fan, a bad connection, fan power shorted, no power to the circuit, or the circuit is not working.

    You checked the fan, apparently, and the amp works, so we assume that power is available to the fan. It is unlikely for both sections of the fan circuit to be bad together. So probably a bad connection, or a shorted C17.
  4. Oren Hudson

    Oren Hudson

    Dec 25, 2007
    Gastonia, NC
    Put snap connectors with black and red leads on a 9 volt battery. Then touch the red and black wires from the 9 volt to the red and black wires of the fan. :)
  5. Oren Hudson

    Oren Hudson

    Dec 25, 2007
    Gastonia, NC
    Thanks! :)
  6. t-bird freak

    t-bird freak

    Apr 11, 2016
    I have a mint condition SVT-350h or b2r-e.....sell either for$180 each
    BasturdBlaster likes this.
  7. popcat

    popcat Supporting Member

    Jan 7, 2016
    Q18 and IC1 are immaterial, because whether they are working or not, or are open/shorted, the fan should still operate.
    But if IC1 is working properly and the unit overheats, the output relay should open and remove the load.

    I suspect that the engineer chose the proper value of C17 at 35V for normal operating conditions.
    However, if the fan becomes disconnected or open, then the 35V capacitor can experience 60V and will probably fail at some point.

    Presuming the fan is okay per your testing:
    1) Either the +60V is not there or too low.
    2) C17 is shorted.
    3) R54A/B are open or high resistance (possibly burned).
    4) Bad fan connection/connector (also check for poor solder at components or open traces/wires).

    All of the above can easily be checked with a low-cost multi-meter.
    BasturdBlaster and Oren Hudson like this.
  8. bassfears


    Mar 26, 2008
    Raleigh , NC
    Fan has temp control switch on the control board

    Famous for going bad
    I bypassed mine so the fan runs all the time
  9. mapleglo

    mapleglo Gold Supporting Member

    Sep 7, 2013
    phoenix, az
    Funny that you should post this. The fan in my SVT-350 just stopped working like last week. I noticed that the amp was running real quiet, then I realized that the fan wasn't running. One of these days I should take a closer look at it, though it seems to run fine for bedroom practice without the fan.

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