SA-7500II recap

Discussion in 'Pioneer Audio' started by loweran, Oct 6, 2017.

  1. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Well, seems to be working ok now. Maybe the pot just needed to dry out after all the cleaning. I will monitor it.
     
  2. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Well it acted up again. Definitely the volume pot. I turned it on cold and the right channel played, left did not. Worked all the pots and switches with no difference in right channel. Moved the volume and left channel kicked in. Going to go over it again. I will reflow the solder and clean it again.
     
  3. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    All, just wanted to provide an update. The unit has been working correctly on startup for days. I think the issue was the volume pot was wet from all the cleaning and needed to dry out a bit. Not sure if that makes sense, but it's a hypothesis.

    Should I be worried about the offset voltages at the speaker terminals? Seems high at 36.5mv and 44.5mv. Also getting bias at ~60mv one channel and ~16mv on the other.
     
    Last edited: Nov 7, 2017
  4. Shadowdog

    Shadowdog Super Member

    Messages:
    2,489
    Location:
    Flin Flon, Manitoba
    Doesn't seem right with channels being different. BTY I have one of those I bought new in 78 & haven't used it in several yrs. Bought a new A-717 in 88 & both are mint.

    Hopefully caps are still good on the older one! I don't think you have to replace transistors unless caps or overheating damages or ruins them.
     
  5. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,200
    Location:
    Victoria, Australia
    I would be more worried about the high bias in one channel, the dc offset would ideally be closer to zero also.
     
  6. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Question is, how do I sort out the high bias. I suppose I could try a different pair of transistors where the double transistor was. Also, not sure what effects the offset. I pulled and tested the emitters resistors and they are within tolerance. I did buy new ones to replace them just in case.
     
  7. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,200
    Location:
    Victoria, Australia
    Ahh l see, l assumed the 7500 would have an adjustable bias circuit like the 8500. I had a look at the service manual and it appears to have the same setup (a wire that is to be cut if under 10mV) as some of the smaller Pioneer's like the little SA-5300 l did a while back that l added bias trim pots to.

    You could fit a multi turn 1/2 watt 50ohm trim pot in place of resistors R45-R47 and R46-R48. Alternatively you could fit a 20-25ohm trim pot just in place of the R47 and R48 resistors, should work well either way. Here is a link for the 50ohm part https://au.mouser.com/Search/Refine.aspx?Keyword=3296W-1-500LF

    Your bias test points are across BOTH 0.5ohm emitter resistors so l would personally go for around 30mV bias which equates to 30mA.

    To lower your dc offset you will have to replace the amplifier inputs Q3-Q4 (2sa798) with two preferably matched (although not normally necessary if from the same batch) KSA992's with both emitters fitted through the same centre hole. I had good results recently with the same double diode replacement on a Technics. Here is a link on the subject http://audiokarma.org/forums/index....nsistor-2sa798-is-there-a-replacement.281614/
     
    Last edited: Nov 15, 2017
  8. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,200
    Location:
    Victoria, Australia
    Sorry, l just looked back through your thread and noticed you have already tried the KSA992's in place of the 2SA798's, maybe try again with closer matched pairs if possible. Alternatively just remove one of the new 992's (of the pair) and fit another from the same batch and recheck.
     
  9. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Interesting idea on the trim pot for the bias.

    I thought the idea of the double transistors was to reduce fluctuation of the voltage, and not necessarily the voltage itself. Maybe I miss understand the circuit. I well try to test some more of them and swap them out on the high channel.
     
  10. loweran

    loweran AK Subscriber Subscriber

    Messages:
    878
    Location:
    Austin, Texas
    Question about installing a trimmer in place of a resistor. The resistor has two legs where the trimmer has three. How would you mount the trimmer?
     
  11. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,200
    Location:
    Victoria, Australia
    I think because they are in one package they would be ideally matched, ie low offset outcome and also be well thermally coupled to reduce fluctuation.

    Just tie the centre leg to one of the outer legs, that way if the wiper ever fails the trim pot won't open and cause circuit damage. There are many units that have two of the legs tied together (solder side of board) with their factory trim pots. Just make sure if you do decide to fit trimmers to orient them in such a way that a clockwise adjustment increases bias, in your circuit this will mean more resistance so check your pot with a meter before fitting.
     

Share This Page