Pioneer SX-3800 Distortion & Low Idle Voltage

Discussion in 'General Audio Discussion' started by Brivan, Jul 13, 2018.

  1. Brivan

    Brivan Active Member

    Messages:
    127
    Location:
    Sioux Falls, SD
    Hi, folks

    I have a Pioneer SX-3800 with distorted sound in the right channel. Injecting a 100Hz tone gives the best-looking waveform results, which shows some clipping on the negative-half of the waveform.

    20180712_140426.jpg

    I started the procedure for setting the 0 volts and idle current on the amplifier board according to the service manual. The left channel set up fine, but the right channel gave me some issues (no doubt related to my bad waveform). I was able to set VR2 for 0v, but was unable to set VR4 to 120mV - I could only get about 5mV out of it. I didn't even try VR6.

    Is there a common failure component to look for here? I'm trying to avoid having to pull components to hunt this down.

    Thanks!
     
    Last edited: Jul 13, 2018

     

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  2. DaveVoorhis

    DaveVoorhis Super Member

    Messages:
    3,624
    Location:
    Behind you.
    100mhz? You're supplying an RF signal?

    Or do you mean 100hz?

    If the latter, it looks like a failure of an output or driver transistor in one side of a push-pull amplifier.

    First, swap the pre-out/amp-in jumpers to verify that the problem isn't in the preamp.

    Assuming it's not a problem in the preamp section, then, verify that power supply rail voltages are correct.

    Assuming power supply voltages are correct, then make sure idle current & bias pots are not corroded or dirty. Give them a bit of a cleaning with contact cleaner (e.g., Deoxit) and rotate slightly and back into original position. Then test again.

    Assuming the pots are fine, then test the amplifier transistors, working backward from the final outputs back toward the preamp. You'll likely find one of them is defective.
     
  3. Brivan

    Brivan Active Member

    Messages:
    127
    Location:
    Sioux Falls, SD
    Dave - thanks for the reply. Yes, it's 100Hz (I corrected my original post - oops!). I didn't clean the pots, but I did run them back and forth a few times before setting the amplifier board. I'll hit them with some DeOxit and see if that helps. My assumption was that using the the "aux in" to inject the signal would bypass the preamp, but I didn't verify this, yet. Voltages were checked previously, but it wouldn't hurt to check them again. Something I didn't mention in my previous post - I only get the clipped waveform if have a load across the outputs (speakers connected), so I'm hoping this won't make a bad transistor difficult to find.

    Thanks!
     
  4. quaddriver

    quaddriver 120 What's per channel Subscriber

    post over in pioneer and the smart cookies will help ya easier and it makes the fix easier to find for posterity
     
  5. Brivan

    Brivan Active Member

    Messages:
    127
    Location:
    Sioux Falls, SD
    Okay, will do. Thanks!
     

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