Rowe R-91 Jukebox Amplifier has to much gain/output distortion

Discussion in 'General Electronics Chat' started by awdman, Sep 3, 2009.

  1. awdman

    Thread Starter New Member

    Apr 5, 2009
    I am working on an old rowe juke box and the amp has major distortion or gain on the output and I believe it may be the capacitors.
    1.I isolated the noise to the main amps by taking of the preamp and inputing a signal directly to the main board.
    2. the Left channel also sounds better that the right channel but not good.
    3. I have changed some of the transistors that were blown out. q3 of collector one it seems like the output is already hi.
    5. It says I am suppose to get 16vac at R32 but am not getting close to that.
    I can provide whatever info is needed but I do not have an oscilloscope, I am using a homebuilt signal tracer.
    Thanks for you Help
  2. mik3

    Senior Member

    Feb 4, 2008
    You need an oscilloscope in this case otherwise you just guess. Take it to someone who has or borrow one.
  3. kkazem

    Active Member

    Jul 23, 2009
    Hi Awdman,
    In this case, it's almost a lot cause to try to fix this without a scope. First of all, you may be the cause of the distortion by possible overdriving the input. Try turning your input signal down low with an attenuator and see if it helps.

    Shotgunning transistors is usually a bad idea and expensive. You can generally tell if they are bad or good by using an ohmmeter with a diode checker. Most often, bad transistors have shorted or nearly shorted junctions (check them out of circuit). You can easily check for bad caps with an ohmmeter also (again, out-of-circuit) if they are electrolytic and over 10uF. You should see the ohmmeter climb-up and then eventually level-off. THat's a good cap. If you have a 1,000 uF cap and the ohmmeter immediately goes to infinity or a high value and stays there, it's bad (open).

    Good luck.
    Kamran Kazem,
  4. Audioguru

    New Member

    Dec 20, 2007
    The output at barely clipping is supposed to be 16.12VAC across a 4 ohm load which is a power of 65W.
    Your multimeter does not measure audio frequencies accurately since it is made to measure only the mains frequency (50Hz or 60Hz).