SA-7500II protection relay issue?

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

  1. JoshHendi

    JoshHendi vintage audio student Subscriber

    Messages:
    897
    Location:
    southeastern Washington State
    Thanks for the info!

    So update: Last night after it was on for 20+ minutes, it stayed on and played constantly with no more protection engages. Turned it off, let it cool down. Fired it up once it was cold, it came out of protection and played for a few minutes. It then went back into protection for a few merely seconds, came out, and has been playing fine for the last hour or so.

    It seems like it may be a resistor or cold solder joints? Once it warms up it seems to function totally fine.

    Also, did the tap test on the two 2SA798s (Q3 and Q4) and didn't make the ~50mv offset jump one way or the other.

    While waiting for more advise, I am going to reflow some solder joints and (just as a precaution) replace some of the 1/2w and 1w and the 2w resistors around the relay


    I am still waiting for a chance to measure the DC before the relay when it randomly engages the protection, but since replacing Q21, haven't had a good chance since it is rarely happening now, and when it does, only stays on for seconds. I also wouldn't be opposed to shotgunning a certain area if someone thinks a certain area is worth doing (I just hate finding the correct modern replacements for transistors)
     
  2. JoshHendi

    JoshHendi vintage audio student Subscriber

    Messages:
    897
    Location:
    southeastern Washington State
    After replacing about 12 resistors, the DC at the speakers is still in the 50mv range, but has seemed to stable out... From starting the unit cold after resistor replacements, have yet to run into any issues.

    May have the protection issue resolved (hopefully)

    What would be the best way to try to find the cause and/or reduce the DC offset now?
     
  3. petehall347

    petehall347 the brandy coffee man

    Messages:
    23,506
    Location:
    uk.. the middle bit
    if i remember right you need to match collector voltages of the diff pair:idea: .. please correct me if wrong .
     
  4. QSilver

    QSilver Super Member

    Messages:
    1,094
    If there is no DC adjustment... I think a lot of it depends on the matched hFE of the dual transistor. They're dual so that their gain is matched as they share the same die i think? But as they get old.... they seem to start drifting. You could try pulling one of them out and replacing it with two KSA992FBU's. You just feed both of their emitter's into the single emitter hole. I usually snip one short and solder it to the other... but you need to gain match them if you can otherwise it can be a pointless effort.

    I think that's as far as I can explain for now... but I know that if any of the resistances around those dual transistors drift, it can really throw off the offset too. Look for resistors around the power amp section that look discoloured or baked...

    Also, I hope you didn't uprate any resistors as some are marked specifically at that watt rating for safety reasons and to help stem the flow of damage should something nasty happen.
     
  5. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    141
    Location:
    Maine
    QSilver is correct in that the only way to reduce the DC is to replace the 2SA798's with gain matched KSA992FBU's. I can also tell you that this particular unit already has an extra hole in the board so no need to feed both emitters into the same hole.

    One final note: Service manual shows acceptable DC voltage from 10-70 mV. I recently did a full restore my 7500 II but left the original dual 798's and was running about 45 mV.
     
    Last edited: Aug 12, 2017
  6. JoshHendi

    JoshHendi vintage audio student Subscriber

    Messages:
    897
    Location:
    southeastern Washington State
    First of all, thanks to everyone for the help! It has been running great and stable :)

    I did up the wattage on some of the resistors, I will go back and replace with the same value.

    I think I will stick with the 50mv of offset. At least the offset is stable and I will not mess with the 2SA798s unless absolutely required. These will be paired with Speakerlab 4s which can take some abuse and 50mv of offset should be fine with them.

    Again, thanks to everyone for the help! This site is the best and I hope one day I can contribute to helping others like y'all have done for me :)

    Cheers
     
  7. Cuppy

    Cuppy AK Subscriber Subscriber

    Messages:
    141
    Location:
    Maine
    Congratulations on a successful repair! Presently my SA-7500 II is performing garage duty and sounds great paired with my EPI 150's. I would love to hear how it would sound paired with some Speakerlab 4's. My main system is an SA-9900 paired with some Speakerlab 7's.
     
  8. JoshHendi

    JoshHendi vintage audio student Subscriber

    Messages:
    897
    Location:
    southeastern Washington State
    Sa-9900 and 7s I bet is great :)

    I use my Sansui AU-9500 with my Super 7s, and as a bass lover, couldn't be happier :)
     

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