Pioneer A-9: Rebirth...more like a heart/lung transplant w/- kidney failure

Discussion in 'Pioneer Audio' started by Tangerine, Jan 30, 2017.

  1. Imanoldee

    Imanoldee AK Subscriber Subscriber

    Messages:
    468
    Location:
    Livermore CA
    Hang tough.
    That beast will come back to life.
    If worse comes to worse you can get a hold of Davy Jones in Sydney.

    Davy Jones of EEVblog fame.
    That is a joke by the way.
    Davy is known to us, even in the US. :beatnik:
     
  2. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    I'd be keen to review notes with Davy, especially since he's just up the road from me.
     
  3. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    Eureka!!

    After a couple of days tap dancing around the board with my Fluke, I've found thatp Q13 in the left channel has had the cct track pads lifted off the board. It's in the signal path for that channel. Without my LED lit magnifier I might have missed it.

    So once this is repaired, the protection cct is next.
     
  4. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    Further to my last, it wasn't broken pads but corrosion between the pad and solder. That's a new one!
    So, with Q13 back in cct...still no joy with the left channel :(
    Therefore, we went back to the firm favourite for the fault and we took out the OpAmp running the DC servo and bingo!! Both channels passed the input audio source! So it's just a voltage follower w/o the servo but it's working.
    A new one goes in tomorrow to prove the "theory"...hopefully. Going to mount the OpAmp into a socket to facilitate repair/fault finding.
    Right now the Amp is in a Frankenstein state, components are still out of cct to maintain a decoupling of the protection cct.
    New 100V 470uF caps go back into the power supply also tomorrow.
    Prediction is that it'll be 100% by ANZAC day!!
     
    Last edited: Apr 6, 2017
  5. slow_jazz

    slow_jazz Lunatic Member

    Messages:
    14,032
    Location:
    SE Michigan, Downriver....
    Good luck bringing her back to life.
     
  6. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    With +/- 0.5V at the outputs of both channels (should be 1/0th of that) we're hoping the new OpAmp, that's going in tonight, will bring stability to the board once again.
     
  7. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    Eureka.
    OpAmp is in, erroneous voltages down to zero as expected, and the board is able to pass audio through both channels, via the phono input.
    We're one step closer to putting it back together!!
    Next step is replace RL3 and confirm all functions of the protection cct work.
     
  8. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    Last night the amp ran for an hour with speaker load.
    Power supplies are still rock solid.
    However, it still goes into oscillation (heard as low level noise) after an hour of from cold.
    So, going back over the specs of the original driver transistors we saw a rather large discrepancy.
    The originals have a Vbe of 1.25 - 17V, whilst the ones we put in (that matched HFE, Ft) was approx. 0.95V.
    This seems to be a big oversight, and could be the cause of the oscillation, as the varistors D1 and D2 maybe causing biasing issues with the lower Vbe.
    Mind you the new trannys have copped a lot during this rebuild, but I may have to find something closer to the originals Vbe.
    Testing tonight by replacing one channel with the 2526/1076 pairs I originally sourced is the option to test the theory.

    There are a couple of design changes happening too.

    A 12V zener is going across RL3's coil to protect it from time zero turn on where the +60v rail is applied across a 12V coil.
    Also a pair of 1N4004 diodes are going across RL1 and RL2's coils to stop transients appearing on the +/- rails after switching the amp from the front panel.

    It's so close to being 100%

    If you're interested, I've posted a poll here on what I should play first once the A-9 is back in service.
     
  9. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    The RF issue has been solved. There was a dry joint that should have connected a couple of 220uF filter caps to ground, put any leads near it and you have a tank cct.
    The zeners have removed rail spikes when the protection relays switch on.
    Vbe for the driver transistors was lower than the originals, it wasn't a spec we focussed on, have had to alter the varister to match the new trannys.
    The last annoyance is a low right channel when using the phono input, but not when in Mono!
    I've attached pics of all the dead parts replaced in the A-9. IMG_4934.JPG IMG_4935.JPG IMG_4936.JPG IMG_4937.JPG IMG_4938.JPG
     
  10. Tangerine

    Tangerine Thanks for all the fish!

    Messages:
    97
    Location:
    Torrens, ACT, AUSTRALIA
    image.jpg
    My tech, who is also a guitarist suggested Kashmir, as a Zep tragic I couldn't say no!!
     

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