Pioneer SX-636 Intermittent DC Offset How to Diagnose?

Discussion in 'Pioneer Audio' started by Johnny_Law, Mar 19, 2017.

  1. Johnny_Law

    Johnny_Law AK Member Subscriber

    Messages:
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    Location:
    North Bay, NorCal
    Have an SX-636 on the bench. It was acting like an output transistor had shorted, giving me ~20VDC offset across the right channel at one point. Got it to the bench (a couple days later) and checked all the pins on the Power Amplifier board (AWH-034) and the Power Supply board (AWR-060). The only voltage that looked strange was 13.9V at pin 14 of the power supply - but the resistor right there tested 270ohms on the nose. All other pin voltages were within 10% of spec.

    Confused by the correct pin voltages, I wanted to double check my DC offset. I measured again and it was much better. Around 8mV on the left channel and 30mV on the right. Hooked it back up and it started singing sweetly again. Even pin 14 on the power supply is back at 6V where it's supposed to be.

    Question is - how do I diagnose this if it's not acting up any more? Should I just keep it on until it happens again? Is this weird or does this happen normally?

    Thanks,
    Ryan
     
    Last edited: Mar 20, 2017
  2. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    Have you gotten rid of the (weird and problematic) 2sc1451 transistors in the power amp? No less the 2sa725/2sa726 transistors.

    There are unconditional preventative replacements, the 2sc1451 can blow out the power amp circuits when it finally fails instead of operating intermittently.

    I know, I know the schematic says 2sa763, but the 2sa726 part number is sitting right there after the "OR" that follows the 2sa763 number....
     
    Johnny_Law likes this.
  3. Johnny_Law

    Johnny_Law AK Member Subscriber

    Messages:
    1,156
    Location:
    North Bay, NorCal
    Hi, Mark - thanks for stopping by. I read some of your posts in the past concerning those transistors (e.g. http://audiokarma.org/forums/index.php?threads/sx-636-hiss-crackle-pop.373254/, also, a thread I found with a similar DC offset issue http://audiokarma.org/forums/index.php?threads/pioneer-sx-636-30v-on-left-output.726515/) and was thinking it'd be a good idea to replace those regardless. I'll likely do so and see if the DC offset ever spikes again.

    You had listed the following as replacements:

    And larry had this list:

     
    Last edited: Mar 21, 2017

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