jukebox amplifier repair

Discussion in 'General Electronics Chat' started by mbdmtrasdl, Sep 26, 2014.

  1. mbdmtrasdl

    Thread Starter New Member

    Oct 15, 2012
    2
    0
    Greetings
    I am in need of help repairing a jukebox amplifier--it is stereo Seeburg TSA-5 and the right channel has very low sound. I measured DC voltage on all five transistors, both channels, and the only difference between working and nonworking sides --voltage is low on Q5103&Q5104 collectors being -12.7 instead of the noted 16 v. the two transistors form a darlington amplifier circuit and both checked ok after being pulled(replaced anyway) I checked resistorsR5113,R5115,R5116 pulled out and all were fine. I must have taken an extra stupid pill today because I just don't understand what is going on. schematic included .
    thanks for helping an old geezer.
    mike img007.jpg
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,772
    931
    First suggestion would be to put Treble at max and scratch compensator at min to rule out any problems with those caps.

    do you have access to an O'scope?
     
  3. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    It's really hard to track the negative voltage and current without the power supply section displayed.
    Q5105 looks like it is contributing 193 ua to Q5103 & Q5104 which puts point Y at 16.52 VDC compared to the collector of Q5105 but there is 1380 ua leaving through R5105.
    Where is that extra current coming from? R5111?
     
  4. mbdmtrasdl

    Thread Starter New Member

    Oct 15, 2012
    2
    0
    thanks for the reply's--I moved all three switches, no changes noted...I am uploading a complete schematic of the amp with power supply. I do have an analog and digital scope. Voltages for Q5101.02.03.04,05 are within 10 % of their counterparts Q5111,12,13,14,15 except collector voltage for Q5103, 5104 at 12.75 volts instead of left side pair 15.75 v. little vol on right side.
    thanks so much
    mike img010.jpg
     
  5. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    This could be as simple as a dirty connector on the pickup cartridge. o_O I think it's time to use the 'scope.
    Remember, this amplifier responds to millivolt level signals. You are going to have to...
    Set up a test signal and measure the AC component on the base of Q5013 and Q5113 to see if they are equal.
    and...
    Measure both sides of R5113 to find the DC current flow in the base circuit of Q5103.
    Same with R5146

    Remember, I might be barking at the wrong tree entirely, but you have to start somewhere.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,357
    Time to stop pulling components and get out the oscilloscope.

    Fortunately, the stages are AC coupled and hence you should be able to find the problem quickly.

    Begin by supplying an input signal and jumper the Left and Right Channel inputs so that they both get the same input signal.
    Then trace the signal along the bad channel and compare it against the good channel.
     
    Last edited: Sep 29, 2014
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