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Pioneer SX-880 goes into protection when turned up - has anyone ever solved this issue?

Discussion in 'Pioneer Audio' started by EastPoint, Aug 9, 2018.

  1. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Hi folks. I have a Pioneer SX-880 that goes into protection when turned up to 10 o'clock or so. Sometimes it does it sooner, sometimes it does it later. It plays fine until you turn it up a bit, and then it starts cutting out. If you leave it at that volume, it'll play again for a split-second, then cut out for a few seconds, then come back in for a split-second, etc.

    It won't do it with no speakers playing--if I turn off the speaker pushbutton it will be fine. It isn't hooked to a dim-bulb. It does it with balance set to either side or in the middle. It's not an issue with my speakers--they are AR11s and I've had no issues driving them with other gear. DC offset and what-not are fine--this isn't an issue with that.

    This is a recurring issue with the 880, and there have been a half-dozen threads about it over the years. Here are a few:

    http://audiokarma.org/forums/index.php?threads/sx-880-protection-at-high-volume.620033/
    http://audiokarma.org/forums/index....880-enters-protection-when-overdriven.470111/
    http://audiokarma.org/forums/index.php?threads/pioneer-sx-880-cutting-out.81118/
    http://audiokarma.org/forums/index.php?threads/sx-880-shut-down-for-a-second.152177/
    http://audiokarma.org/forums/index.php?threads/sx-880-protection-keeps-tripping.41614/
    http://audiokarma.org/forums/index.php?threads/infamous-pioneer-sx-880-protection-on.415600/

    So clearly this is a common issue with the 880. Unfortunately none of these threads appear to actually resolve the issue.

    A few years ago, an AK member @jeffsplat88 claimed to have solved the issue, here:
    http://audiokarma.org/forums/index.php?threads/pioneer-sx-880-protection-circuit-fixes.679172/

    However, I tried upping the resistor to 440 ohms as he suggested, and my 880 refused to come out of protection. I tried bumping the 47 uf 16v capacitor up to 220 uf 25v and it didn't really seem to change things. I am pretty sure the polarity was correct as installed (and as marked on the board), but I tried reversing polarity and it didn't seem to change things.

    I have recapped the majority of the SX-880. It had this issue before the recap and it has the issue now.

    Any ideas?
     

     

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  2. john stumpf

    john stumpf AK Subscriber Subscriber

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    i have owned and rebuilt an 880 and an 890. i am by NO means an electronics expert. have you checked the bias and offset? i did do the jeffsplat88 mods to both sets and that resistor mod sure did cool down the power supply regulators. and both sets still performed well. as i recall its not JUST the capacitor value that is changed, its polarity is as well. i cant for the life of me remember exactly,but it seems the board silk screen was ass about from the schematic.i gave both sets away one is still here in an un aircon house and it plays great the other is in california. both owners seem quite pleased. at the time i was hoping that "mark the fixer" would chime in as to the validity of the mods.sadly he didnt, but as he wont let anyone steer the forum wrong i figured it was a mod that couldnt possibly lead to ruination and was quite easy to reverse if need be. my curiosity was piqued as i wondered if my sx 1080 might suffer with its super hot regulators in the same way. based on my experience. i recommend the changes. if it's "BUM DOPE" im sure someone will comment. i highly recommend bornes multi turns be used to replace the bias and offset pots.
     
  3. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    I will check bias and offset tonight and then try the mods again. I checked the schematic and I do think my cap is in backwards like he said.
     
  4. sregor

    sregor AK Subscriber Subscriber

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    I would try changing c418 (47uF16 V). The function is to filter out audio so only DC will trigger protection circuit. I would also change it to an NP-Non Polar (or BP - Bipolar same only different name) cap. The schematic and parts list show regular polar type cap, but IMHO it should be a non polar type. Good luck with it.
     
    dr*audio likes this.
  5. john stumpf

    john stumpf AK Subscriber Subscriber

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    do you mean opposite of the silk screen?
     
  6. sregor

    sregor AK Subscriber Subscriber

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    No - use a non polar or bipolar cap - silkscreen won't matter. (NP or BP caps don't have + or - designations.)
     

     

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  7. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Well, this is interesting.

    I checked the DC offset and it was very close to zero. Bias was pretty low--it's supposed to be around 30ma and it was around 5-10ma on one side and 15ma on the other. I adjusted the bias to 30ma and then tried installing a 220 uf 16v cap backwards in place of the 47 uf cap, and tried the extra 220-ohm resistor again. The receiver is now working fine and has no protection issue, so apparently we're all good.
     
  8. Eastham

    Eastham More Class-A than ever!

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    I'm no electrical engineer but from what I can tell is in the event of a failure that leads to DC offset on the outputs, some of that voltage is tapped to pin 4 of the protection IC which stops pin 3 of the IC sending voltage to the base of Q37 (or Q27 my schematic is blurry) which inturn stops any current flow thru the protection relay coil thus disengaging the relay contacts. (Q37 is basically a switch once it turns on it allows current to pass thru the relay coil to ground which then engages the relay contacts.) I think that 47µF cap you mentioned is part of the startup delay so you don't get a lovely THUMP thru your speakers at startup, have you noticed that it may take the relay longer to engage at startup? Might be worth upgrading Q37 to a KSC2690AYS as I have a sneeking feeling that was the original issue.
     
    Last edited: Aug 10, 2018
  9. Eastham

    Eastham More Class-A than ever!

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    Oh and if you do have this issue in the future again, it could also be something to do with the overcurrent protection circuit. Q33 and Q34. If they detect an over current condition or one of them shorts due to age they'll send negative rail voltage to the protection IC causing it to disenguge the relay and save the amp from damage or be a nuisence depending on why they engaged Q36.
     
    petehall347 likes this.
  10. john stumpf

    john stumpf AK Subscriber Subscriber

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    did you notice the reduced heat load on the power supply transistors? normally they will burn your finger!
     
  11. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    I actually upgraded a few of the transistors already, but I'm not sure if I did that one yet. I'll double-check it.
     
    Eastham likes this.

     

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  12. kevzep

    kevzep Its all about the Music

    Did you check all the emitter resistors on that channel? I have seen this with Pioneer's a few times (SX 50 and 80 series) where perhaps in the past the amp has had a catastrophic output failure, the output transistors are replaced, but the Emitter resistors were never checked, and on or two might have drifted high, so then when you bias it up, it all good running at low power, but when it starts drawing any significant current, the resistor is heating up and going even more hiR than before, so more voltage drop, and sends more voltage to the current sensing transistor and trips the protection.
    Had this on the SX1250 I just rebuilt...
     
  13. WE6C

    WE6C Well-Known Member

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    Kevzep, Once I even check my emitter resistors (SX-1250) on a power supply running some current through them to make sure their values didn't change etc. Ran them until they got quite warm to the touch, not burning hot but fairly warm. In my case the resistance value did not change and the current draw during the test also did not change as the temperature increased. Some thought I was wasting my time but since they play such an important role I wanted to be sure. Figured it was better than a plain ole resistance check.

    Bob
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    q35 voltages might help .
     

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