Help needed please Tracing fault in preamp or tone board

Discussion in 'DIY' started by grabread, Aug 9, 2018.

  1. grabread

    grabread Active Member

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    Location:
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    Hello all, I need some advice on tracing a weak channel in an integrated amp. Fault is in the preamp section or tone board.
    I think I have tracked it down to the tone board (passive type) but I'm not sure. I have set up a signal generator with 1khz square wave on one channel and sine on the other so I can see which channel I'm following. I am probing with a very shaky old scope which reads only about 10 mv sometimes 5mv on the bad side. about 20 mv on the good channel. Any advice on how to go about tracing this fault down would be very much appreciated.
    There is no schematic available for this amp so it's down to good old fashioned detective work.

    Many thanks
     

     

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  2. triode17

    triode17 Super Member

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    1,169
    Assuming you are talking about a SS amp, I would check the signals on each side of any electrolytic signal coupling caps. If the amp is more than 20 years old, it may need re-capping. Also, inspect the parts closely with a strong light. Make sure there are no burned or singed parts, like resistors. That's a start.
     
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  3. dr*audio

    dr*audio Fish fingers and custard!

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    Crank that generator output up to 1V peak to peak. It will make it a lot easier to see on the scope.
     
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  4. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I'm not saying it's available but if you say what it is, perhaps someone has a schematic or experience and could simply the operation
    .also assuming your input is the same amplitude, the signal should appear the same (a stereo amp should have 2 of the same channel) so if you follow signal through where it becomes lower , is where you should be concentrating.
    I hope that makes sense.
     
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  5. grabread

    grabread Active Member

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    Yes it is a S S amp from 1976. Would I be expecting the signal to be the same either side of an electrolytic cap?
     
  6. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I wouldn't necessarily say that.
    What I would say is that cap would exist in both channels.
    So if you read hghi and lo in one channel, you should read high and low in the other channel.
    Is there any reason you can't share what it is?
     

     

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  7. grabread

    grabread Active Member

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    It's an Eagle A6400 quite a rare one made in Korea (same factory as sherwood)
     
  8. elnaldo

    elnaldo Addicted Member

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    I'd start checking the signal at the Input connectors (to check the signal is the same at the amp input), switches, loudness, tape-monitor, function selector. And then at the volume and balance controls (or do this first,check the signal arrives OK or not to the volume control)

    I'd use the same signal, not different. It doesn't matter which one is L or R as long as they match.

    Sometimes I use a 1khz sinewave in one channel and 1.25 KHz sinewave in the other, to see the difference but have the same response from the circuit
     
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  9. triode17

    triode17 Super Member

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    Yes. The caps. are signal couplers, there to block DC voltage.
     
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  10. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Nice looking piece! Has a Conrad Johnson look to it:idea:
     
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  11. triode17

    triode17 Super Member

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    Yes it should. The cap is there to block DC voltage from getting into the next stage.
     

     

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  12. grabread

    grabread Active Member

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    Thanks Triode 17, I have used your advice already but it does not appear to be a cap at fault (same both sides). Getting some strange readings around the A/B dubbing switch maybe this is the culprit?
    I will update later if I find the fault.

    Many thanks for all your help.
     
  13. triode17

    triode17 Super Member

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    Have you cleaned all the switches and controls with DeOxit or an equivalent yet? That can solve many issues.
     
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  14. grabread

    grabread Active Member

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    Yes cleaned all switches, I will keep plugging away at it and hopefully I will one day find the cause.
     
  15. maxhifi

    maxhifi AK Subscriber Subscriber

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    Tracing is the right approach, first make sure its at the inputs, then go through the stages, follow it through all the switches, controls transistors, until you find where it stops.
     
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  16. grabread

    grabread Active Member

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    I have found that the 2 treble pots behave differently. seems a lot of signal is lost on one of them. I have cleaned them with deoxit. Not sure if it's the cause or it's not receiving the full signal.
     

     

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  17. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Jump the wipers of the treble pots . See if signal is the same.
    You could try cleaning them again.
    You are working the control the full range 20-30 times after you shoot the deoxit in?
    Also you are spraying into both sections?
    Lastly, controls sometimes fail.
    If cleaning doesn't solve the issue , you may need to replace the control.
    Your observation points that way.
    Also look carefully at the,connections to the pots. Maybe iffy (cold) joint
     
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  18. maxhifi

    maxhifi AK Subscriber Subscriber

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    Also consider that the signal to the pot is probably coming from the emitter circuit of a transistor, coupled through a capacitor. Check that the signal is present at the emitter of the transistor, then at both sides of the coupling cap. If it's not at the emitter, check that the voltages are the same on the transistors in the left and right channels, and then see if the signal is at the base of the respective transistors.
     
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  19. grabread

    grabread Active Member

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    Location:
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    Thanks for your help. I will try these ideas tomorrow.
     
  20. grabread

    grabread Active Member

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    Some of your replies are now missing from this discussion. Due to the weekend updates I guess. Not sure if they will come back?
     

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