Pioneer SX-3800 Distortion and Low Idle Voltage

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

  1. Brivan

    Brivan Active Member

    Messages:
    113
    Location:
    Sioux Falls, SD
    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. I only see clipping when there is a load across the output (speakers connected) - with no load, the waveform looks normal.

    [​IMG]

    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. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,777
    Location:
    Las Vegas Nevada
    My guess is bad/poor solder connections on the traces. Somewhere in the NPN side of the amp.
     
  3. Brivan

    Brivan Active Member

    Messages:
    113
    Location:
    Sioux Falls, SD
    I've got the amplifier board removed, right now. Testing components, but nothing major jumping out at me. I'll have a look at the solder connections under a magnifying glass. FYI I previously replaced the two driver transistors for the left channel, since one of them was bad and was preventing the safety relay from engaging. I might just go ahead and replace the right channel drivers since I'm having issues - both meter-test okay, though. I'm also planning on replacing the sockets for the driver transistors while I'm in there. Drivers all tested good. Thanks!
     
  4. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,777
    Location:
    Las Vegas Nevada
    You bring up some good points.
    The original output sockets suck which might be part of the issue.
    And - The amp is a hot one when running. Heat related issues.
    If the transistors check good then they should be fine but I understand about when TS'ing (While your in there, change it with a known good component).
    A few tricks I use, one is looking at the base voltages of the drivers while adjusting the trimmers. I look for a good range of voltage from .4 to 3.5v of so . Compare them to the good working channel. This gets me close to the circuit that is suspect.
     
  5. Brivan

    Brivan Active Member

    Messages:
    113
    Location:
    Sioux Falls, SD
    Good advice. I'll keep that in mind. Thanks!
     

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