Sx-424 left channel faint and static

Discussion in 'Pioneer Audio' started by october13, Jan 2, 2019.

  1. october13

    october13 New Member

    Messages:
    13
    Hi,

    Looking for some help.

    I have a minty 424, and it was working perfectly fine. Had a scratchy pot or two, so I went in and cleaned all pots and switches with deoxit. Also made a caps list while I was in there.
    Turned it on and now I have a low volume, scratchy left channel. Swapped speakers all around, and it's the same on all inputs.
    My fear is that I pinched or disrupted something.
    Is it cooked? Fixable?
    Where to start?

    Right side is working perfectly.

    Thanks....
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Welcome Aboard!

    Everything is fixable. Well, almost. In this case it most probably is. It sounds as if something in the left channel audio path is asking for further cleaning. The usual suspects are Loudness, Tape Mon, and Mono/Stereo switches, although the rotary selector switches are also in the group of suspects, including the speaker switch since it has some strange-ish functions in the 424.
     
  3. october13

    october13 New Member

    Messages:
    13
    Update:

    Now right channel is toast....

    Left channel is faint and distorted.
    Right channel is basically non existent except for some clicking/distortion.
     
  4. october13

    october13 New Member

    Messages:
    13
    IMG_7920.JPG IMG_7921.JPG What is this component?

    Varistor?
     
    Last edited: Jan 6, 2019
  5. Watthour

    Watthour Electron Rancher - JS3600

    Why the photos of the STV-3? Does it check bad?
     
  6. october13

    october13 New Member

    Messages:
    13
    I discovered one of these was bent over and pinned under the heatsink/mounting bracket during reassembly. Pretty sure this smoked on powerup. What chain of damage may this have caused?

    I want to repair this receiver, but at this point it is over my head
     

     

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  7. Watthour

    Watthour Electron Rancher - JS3600

    These are usually mounted to the heat sink to sense temperature, and alter the bias of the circuit to prevent thermal runaway as the components gain temperature. Most often they are screwed to the heat sink(s), but can be clipped as well.
     
    americannigh likes this.
  8. october13

    october13 New Member

    Messages:
    13
    These are soldered to the board and appear as they shouldn't make contact with the aluminum piece
     
  9. Watthour

    Watthour Electron Rancher - JS3600

    Usually, they actually need to physically connect to the heat sink to monitor temperature of the output stages. The 424 may be different, but I have doubts that they would just be allowed to reside in the vicinity of the heat sink since that would slow their response drastically.

    Judging by this image from LD, they should be clipped to the heat sink:

    [​IMG]
     
  10. october13

    october13 New Member

    Messages:
    13
    Oh yes... I see that now.

    In my top pic you can clearly see the clip that they should be clipped into...

    So what damage may have occurred if one was pinned to the board at reassembly? And also is this component resilient or easily fried? What is the likeyhood that this component is still ok?

    Thanks for your help
     
  11. Watthour

    Watthour Electron Rancher - JS3600

    It is unlikely the STV suffered damage being clamped under the heat sink. Unless it is cracked or crushed, it should be physically O.K. It can be tested electrically with a meter on a diode test function. The most fragile aspect of these devices is usually the leads. They tend to break if flexed extensively.
     

     

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  12. october13

    october13 New Member

    Messages:
    13
    Could it have cooked other components?
    I did see a wisp of smoke
     
  13. Watthour

    Watthour Electron Rancher - JS3600

    It is nearly impossible for this device to damage other components. However, if it is damaged, other components can suffer as well.

    If something was hot enough to emit smoke, the circuit needs further analysis before ever applying power.

    If you haven't already done so, download a copy of the service manual and schematic for your unit. We will need to point you along a path to check items, and having the manual will make it a lot easier for you to locate devices and follow along. All the testing can be done with a typical DMM.
     
  14. october13

    october13 New Member

    Messages:
    13
    Ok, I have the service manual downloaded
     
  15. october13

    october13 New Member

    Messages:
    13
    IMG_7924.JPG Will this work?
     

    Attached Files:

  16. Watthour

    Watthour Electron Rancher - JS3600

    It will work acceptably. Some meters have a diode test function which actually displays the voltage drop at a junction, and that can provice a bit better information, but a regular ohmmeter can work. You'll have to use it on the 2KΩ scale, and will only get an reading of junction resistance.
     

     

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  17. americannigh

    americannigh Active Member

    Messages:
    123
    Location:
    Bellevue, WA
    Did you actually see or hear a component smoke? If not, I would take another run at those pots and switches. Both channels worked fine before cleaning the pots and switches, correct? Work them all back and forth quite a bit and see if you get any signs of life on both channels. Look at the wires soldered to each switch and pot to make sure one didn't pop off when you were in there poking around with the spray nozzle. Also test all the fuses with your meter. Put it on continuity check (looks like the far left setting on your dial in the picture) and make sure each fuse has continuity across it. Sometimes fuses can look good when in fact they are blown. I know these are simple things you probably have done to some extent but really the only thing you did is clean the pots and switches so my money is still on something with the pots and switches.
     
  18. october13

    october13 New Member

    Messages:
    13
    Yes I saw smoke
     
  19. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    How does one hear smoke?:rolleyes:
     
  20. Watthour

    Watthour Electron Rancher - JS3600

    Well, that's a special skill, reserved for naval personnel only.
     

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