SX-950 Relay Bypassed and Burnt Resistors

Discussion in 'Pioneer Audio' started by Pageplayer, Jul 12, 2018.

  1. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    are the input differential pairs q1 q3 q2 q4 ?
     

     

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    yes the 992 you put in there
     
  3. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    Yes.. I went through and found matching pairs by recording hFE with component checker. then retested again.
     
  4. petehall347

    petehall347 the brandy coffee man Subscriber

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    am wondering if offsets will come down some when at operating temps. and on full mains power .
     
  5. petehall347

    petehall347 the brandy coffee man Subscriber

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    are the trim pots all one way to get it to the lowest you can ? they should reside somewhere near centre positions when set to 0v . turning one way offsets go positive and other way negative .
     
  6. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    I understand what you're saying.
    To be clear, I installed Bourns 652-3296W-1-103LF 10K ohm 25 turn pots in VR1 and VR2 positions.
    turning all the way counter clockwise gets DC offset down to +.07v.
    turning about 25+ turns clockwise raises the reading +.12v
     

     

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  7. petehall347

    petehall347 the brandy coffee man Subscriber

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    both channels the same ? i think you replied already on that but cant remember right now .
     
  8. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    yes.. both are the same
    I'm looking thru the Power Amp "again" and making sure everything was installed correctly
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

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    something common to both then .
     
  10. petehall347

    petehall347 the brandy coffee man Subscriber

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    looking at the schematic could you get all marked voltages ? pin 7 at the top ..amp board awh-050
     
  11. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    Across the top of Amp board..
    Pin 31 -50.0v
    Pin 36 +50.1v
    Pin 41 -50.2v
    Pin 46 +50.3
     

     

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  12. petehall347

    petehall347 the brandy coffee man Subscriber

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    14 and 19 amp board ?
     
  13. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    Pin 14= 50.4v
    Pin 19= 34.5v

    I also was checking transistor legs that have call outs on schematic and they're very close.
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    meant 4 not 14 but never mind ..
     
  15. petehall347

    petehall347 the brandy coffee man Subscriber

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    pin 2 -51.5v ..then d1 cathode 4v
     
  16. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    Pin 2 -54.6v
    D1 cathode -4.45v

    Pin 4 34.4v
     

     

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  17. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    Since I wasn't seeing any way out of whack voltages other than the DC offset being weird, I went ahead and added the remaining output transistors to the power amp.
    No issues with DBT power up as I cycled thru 60w, 75w and 100w bulbs, then full mains.
    I let the unit warm up for 20 minutes, monitored and set bias to about 18mV..
    Transistors mounted to heat sinks on power supply do get pretty warm, but everything else stays cool.

    And DC offset still won't zero or go below .05v
     
    Last edited: Aug 11, 2018
  18. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    The saga continues..
    It's been awhile since I got back to this project, as it took me awhile to figure out why the DC offset would not zero out. And sure enough it was an error on my part. Then I had to wait for parts and time to install them.
    Long story short, I ordered the wrong Bourns trimmers. Ended up with 2K trimmers rather than the 10K versions for VR1&2.

    I sent Pete an apology for all the work he put into trying to figure this out, but unfortunately it doesn't end there...

    Today I thought I was home free when I began to install the new trimmers.
    I hooked up my 2 DMM's and powered up the unit on the 100w DBT. Adjusted DC offset and they came right in to 0v
    I then powered down, hooked up the probes to check bias and dialed that in to 15mv.

    Everything seemed dandy as my DMM readings where good and the relay was working as it should, so I went ahead and connected speakers and powered back on..
    Sounded great! I was dancing around like Fred Astaire. Wife thought I lost my mind.

    I then decided to disconnect everything and go full mains to do a final bias and DC offset adjustment.
    I let the unit sit for about 45 minutes with my DMM's connected, Unit sitting flat on the bench. I had to back down the bias a couple times to keep it from going over 20mv (spec).

    I started to gather the faceplate and knobs to begin reassembly.
    While I was doing so, I heard the relay click off then back on a couple times. o_O :(

    Decided to go back to step one and get some voltages....
    Screenshot 2018-08-25 14.54.06.png

    Hoping you guys could help me out again?

    Chuck
     
    Last edited: Aug 25, 2018
  19. petehall347

    petehall347 the brandy coffee man Subscriber

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    fuse 6 1.5 amps
    also powers the lamps . so i guess lights are out too ..maybe knocked the wires .
     
  20. Pageplayer

    Pageplayer AK Subscriber Subscriber

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    all fuses good.. lights working
     

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