SA-7500II protection relay issue?

Discussion in 'Pioneer Audio' started by JoshHendi, Aug 10, 2017.

  1. JoshHendi

    JoshHendi vintage audio student Subscriber

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    So have a SA-7500II I acquried. Previous owner said it was in the 'needed work' pile. After a cleaning and initial checks, I fired it up and it did come out of protection, had clean sine waves on both channels. But then after a couple of minutes, the protection relay would click on and off, randomly doing so.

    I checked for DC on both channels when the protection relay was off and each channel had less than 50mv... At this point, would it be safe to assume that the relay is just bad and needs to be replaced? Or could something else be amiss? I already ordered a replacement relay, but was wondering if there is anything I should check before/after/during relay replacement
     
  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    most like relay driver q21 . monitor its base whilst its playing up .
     
  3. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Thanks for the quick reply! Think it'd be worth checking before replacing the relay?
     
  4. petehall347

    petehall347 the brandy coffee man Subscriber

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    yes .. the relay itself is very unlikely to fail intermittently
    the circuit is in need of a small mod too . it really needs a flyback diode across the relay coil .
     
  5. KingBubba

    KingBubba "Too Much Stuff" Subscriber

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    If you are going to dig that far in, you might as well replace the relay. They are not very expensive. The driver transistor is not an unusual issue in the Pioneer line. Search AK for the modification suggested by PH. You will need a solder sucker and a soldering iron capable of fairly high heat to deal with the relay.
     
  6. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Would it be wise to first replace q21 before adding the relay coil diode?

    Edit: better question. Would it be wise to fix my issue first, THEN add the relay coil diode?
     
  7. petehall347

    petehall347 the brandy coffee man Subscriber

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    yes fix it first then make it better . the diode is to stop the transistor failing . saying that its worked all these years .
     
  8. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Okay one last question then Ill quite buggin ya (for now ;) )

    Suitable replacement for 2SC1166? The cross reference site I use is showing 2N4410

    But I am not very good at picking correct replacement transistors. Too many specs and just not confident in my selection
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

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    KSC2690AYS
     
  10. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Even tho the KSC2690AYS is a TO-126 and the original is a TO-92?

    I have some new KSC2690AYS on hand already, which would be a plus :)
     
  11. petehall347

    petehall347 the brandy coffee man Subscriber

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    will far outlast the old one . its a switch .
     
  12. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Okay just wanted to check.. Ive read you want to replace transistors with the same casing type, but Im sure that is just a general guideline and not a hardfast rule. Besides, I'd think if you found a replacement with the same specs and installed the legs correctly, it should still perform as intended...
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

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    well yes of course same spec should behave same way .
     
  14. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Okay so a quick update...

    Swapped in the KSC2690AYS and new relay.

    Now when powered up, the amp comes out of protection and plays music for a few minutes (this wasnt the case before two new parts installed), then will get a pop out of the left test speaker, then goes back into protection.

    When I first turn on the amp, I measured DC at the terminals, both were in the 50mv range, but one was very stable and the other channel was a little more up and down. I am guessing this channel has something else going on that is eventually engaging the protection.

    Anything I should check next?

    While waiting for a response, I will connect meter to the left channel (the suspect channel) and monitor DC while music is playing
     
    Last edited: Aug 10, 2017
  15. QSilver

    QSilver Super Member

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    The main reason for the larger TO-126 style transistor is that the smaller style cannot handle the heat as well. I believe it was MTF who originally suggested this (at least that's where I first read about it)

    IIRC the transistor die inside is subject to rapid heating a cooling which wears it out far faster than would usually happen.

    The popping could be a dry solder joint... do you have the schematic?

    You could try just gently tapping Q3 and Q4 gently with a wooden stick and see if this aggravates the problem?

    Other than that, 50mV is a little on the high side, you might want to check the resistance of R34 and R33. If anything is wrong around Q3 and Q4, it can cause problems with DC balance...
     
  16. JoshHendi

    JoshHendi vintage audio student Subscriber

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    So the problem is coming from the left channel.

    Amp will play for a few minutes then the protection kicks on, but then after a few minutes the protection again turns back off. Also if I restart the amp, it will come out of protection right away every time
     
  17. QSilver

    QSilver Super Member

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    When it goes into proteciton, leave it and measure the DC offset on both channels BEFORE the protection relay and post the results.
     
  18. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Yes I have the full digital manual and schematic... Tapping Q3 and Q4 didn't cause anything on a few quick attempts.

    I will check the R34 and R34 resistors... I was originally considering replacing the Q3 and Q4 transistors to see if that could bring the offset down, but have never messed with the 5 legged transistors
     
  19. JoshHendi

    JoshHendi vintage audio student Subscriber

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    Copy that... Have had it on and playing for nearly 10 minutes now and it has yet to kick the protection back on... but I will report my findings asap


    Update: After playing for 10+ minutes, it went into protection, and before I even had a chance to take DC measurements at the spots before the signal enters into the relay, it kicked out again, and now has been playing fine ever since. Still waiting for a chance to measure DC while in protection before the relay

    2nd update: After being on for 20+ minutes, it has been playing fine and hasn't kicked back into protection... Seems that the longer the unit is on, the less chance of the protection re-engaging. I just shut it down to let it cool off and will check R33 and R34.

    I am almost tempted to replace Q3 and Q4. Even if they aren't the exact parts causing the protection issue, would it help possibly lower the DC offset? The five legs on each one are dark and probably wouldn't hurt to replace them?
     
    Last edited: Aug 10, 2017
  20. QSilver

    QSilver Super Member

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    If you have no exact reason to replace them, just leave them. They could lower the DC offset but then again, they might not. Those resistors and surrounding circuitry are equally as important for DC balance. If tapping Q3 or Q4 with a wooden stick doesn't make the DC offset jump all over the place, they're not the main culprit....

    The protection issue could be a capacitor in the protection circuit reforming as you use the amplifier more... or a faulty transistor in the protection circuit. What you're describing tends to indicate the protection circuit itself is acting up but its best to be sure so when it does get into protection again, just get a mV reading on its output if you can.
     

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