Need help troubleshooting SA-7500 II

Discussion in 'Pioneer Audio' started by Cuppy, Mar 18, 2017 at 1:59 PM.

  1. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    105
    Location:
    Maine
    Hoping to get some help diagnosing a problem with the protection circuit on this amp. This amp was given to me as non working. Initial power up on DBT passed indicating no major shorts. Symptoms on house power..., after initial power up, protection relay clicks after 5 secs as normal for muting function. After approx. 30 secs, relay will turn off. Then relay will continually cycle on/off with no real consistent time between cycles. Sometimes 5 or 10 secs. Sometimes longer. I have researched as much as I could find on this but not in total understanding of how this circuit works. Any help would be greatly appreciated. The one document I could find from the Pioneer Tuning Fork article on troubleshooting protection circuits, mentioned that if the relay turns on after the standard 5 secs after power up, it is likely a fault in the detection side rather than the relay driver side.

    The output transistors passed testing (in circuit) for shorts and they have the correct voltage on all collectors with power on (41vdc, -41vdc).

    Not sure what to include here for diagrams and the SM from hifiengine is of poor quality.
     

    Attached Files:

  2. bberkom

    bberkom AK Subscriber Subscriber

    Messages:
    390
    Location:
    Nashville, TN
    Have you measured DC mv at the speaker terminals when the relay is on? Excess DC going to the speakers could be shutting off the protection relay. I'd suspect one of the transistors, maybe Q4 or Q5, in the protection circuit if the DC looks normal.
     
  3. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    105
    Location:
    Maine
    OK. I was able to take a few readings while the protection relay was pulled in. DC mv at speaker terminals measured 40mv right and 15mv left (with speaker selector set to A).

    Regarding transistors Q4 and Q5, I believe these are actually Q20 and Q21 on the board. One thing I did notice is that the voltage fluctuates at the base of both even with the relay still pulled in. Base of Q21 (9 V) and base of Q20 (-1.0 V) fluctuate from 9.5vdc-1vdc and -1.5vdc-0vdc respectively. Not sure what that means or if it is helpful. I was hoping not to have to shotgun transistors or caps but will if it makes sense.
     
  4. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    105
    Location:
    Maine
    Still hoping for a little guidance as to what to check next. Should I replace all transistors, caps, diodes, etc. in the protection circuit? This one has me stumped.
     
  5. bberkom

    bberkom AK Subscriber Subscriber

    Messages:
    390
    Location:
    Nashville, TN
    Not being familiar with that amp, it's hard to say. I'd pull and test all of the protection circuit transistors. Has the amp been recapped or refurbished at all otherwise? I'd imagine a recap would do it good but I understand not wanting to shotgun it.
     
  6. MarZutra

    MarZutra Super Member

    Messages:
    3,129
    Location:
    New Scotland, Canada
    Boy I wish I could help. The only experience I had with my problem child SA9900 was getting screwed by techs and selling it for $200.00 parts....
     

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