Pioneer SX-990 Trimmers

Discussion in 'General Audio Discussion' started by mustang1102, Jul 12, 2018.

  1. mustang1102

    mustang1102 AK Subscriber Subscriber

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    Love this forum, really appreciate the wealth of information I have gained from being a member for just the short time I have been on........I'm turning to all you experienced folks for a problem I currently am having. I have a Pioneer SX-990 receiver with no left channel output, blows the 2 amp left channel fuse as soon as it is turned on. Actually, all the output transistors are checking good, emitter resistors, drivers as well. Only thing I can find wrong so far is that the 50 ohm trimmer pot for the left side is shorted, so I was wondering if you know of any suitable source and replacement for it, as well as the other trimmer on the same channel which is a 30k pot. Started looking at Digi-Key and Mouser, but have come up blank for replacement so far. Any suggestions would be greatly appreciated.
     

     

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  2. chazix

    chazix AK Subscriber Subscriber

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    Welcome to AK! I'm not in a position to help with what you actually asked about, but I have a couple of suggestions:

    • You'll probably get more experience-informed responses in the Pioneer forum (http://audiokarma.org/forums/index.php?forums/pioneer-audio.90/). You could ask a moderator to move this thread to that forum.
    • If you really mean the 50 ohm pot is shorted (zero ohms), that would not only be an unusual failure mode for a pot, it would also not explain the excessive current. I'm catching a whiff of red herring here.
    • That pot going open (infinite ohms), or the STV3 diode going open, would explain excessive current.
    • Look around AK for how to rig up a "dim bulb tester" (DBT) - it would be a really, really good idea to use one while you're debugging this.
    HTH,

    chazix
     
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  3. mustang1102

    mustang1102 AK Subscriber Subscriber

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    chazix
    Thank you for the advice, really appreciate your taking the time to respond. I will post it in the Pioneer forum as well. You are correct about the trimmer pot NOT being shorted. My mistake when I drafted the thread. The pot is OPEN between the two outer terminals, and I’m reading about 1.3 ohms from the center terminal to one of the outer legs. There is no change to the reading when adjusting the pot. You are right on with your analysis.
     
  4. OMGCat!

    OMGCat! AK Subscriber Subscriber

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  5. mustang1102

    mustang1102 AK Subscriber Subscriber

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    Thank you much. It looks as if the first one shown in the link may work, although the middle pin is offset from the other two, I can probably make it work. The trimmers in the 990 now have all three pins of directly in line next to each other.
     
  6. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    This is a nice multi-turn pot that has all of the legs inline.

    https://www.digikey.com/product-detail/en/bourns-inc/3296W-1-500LF/3296W-500LF-ND/1088056

    The search function on Digikey is weird. These didn't show up looking for 50ohm pots but I knew the series, searched for it and then specified 50ohm and there they are. Takes a bit of detective work to find stuff but eventually you get good at it or lucky.
     

     

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  7. mustang1102

    mustang1102 AK Subscriber Subscriber

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    That looks good ! I have dealt with Digi-Key and Mouser both in the past, for some reason I prefer Digi-Key better. My only other question would be I see these trimmers are available based on the number of "turns" you want for the pot. Is there any advantage of getting a multi-turn pot, over say a single turn pot ? Would a single turn pot maybe be too sensitive when adjusting, and harder to lock in on the desired setting, or doesn't it really make any difference ?
     
  8. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    It's much easier to dial in a multi-turn over a single. I prefer them for things like bias for just that reason and the price difference isn't enough to matter when you're only buying two.
    That said, new potted single turns will still be worlds better than the old open frame pots.
     
  9. mustang1102

    mustang1102 AK Subscriber Subscriber

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    In regards to the single turn, thats kind of what I was thinking. Multiturns it is !!!
     

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