SX-626. The trials and tribulations of troubleshooting when you hardly know what your doing.

Discussion in 'Pioneer Audio' started by kaplang, Dec 31, 2018.

  1. kaplang

    kaplang Works for me ! Subscriber

    Let me start out by saying that my troubleshooting and schematic reading skills are marginal at best. That being said here is what i think I need to know to try and get the Pioneer SX-626 working again. Or at least a starting point

    The main amp unit AWH - 002 has several pins connected to ground that include E1,E2, E3 and E4 if I am understanding the schematic correctly. I currently have everything disconnected from the board except for +B of 56 volts DC to pin B1, B2 and pin E5 to ground. With only these 3 connections the 4 pins E1,E2,E3 and E4 are all showing a reading of 56 volts DC and that makes no sense to me since those are suppose to be ground connected. I have tested every component on the amp board out of circuit and have found nothing that explains why 56 DC volt readings are showing on pins to be grounded.

    Anybody want to jump in here give me some help. It will be greatly appreciated and who knows, I’ll probably learn a few things

    Cheers and Happy New Year
    George

    Main amp unit AWH-002
    View attachment 1372161 View attachment 1372162 View attachment 1372163
     

     

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  2. ConradH

    ConradH Addicted Member

    Messages:
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    Location:
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    It sounds like you don't have all the grounds tied to ground, so the whole circuit will probably float up to the power supply voltage. Why do you have things disconnected? Most troubleshooting is done in-circuit, with the power off. What was the original problem?
     
  3. kaplang

    kaplang Works for me ! Subscriber

    At the beginning I did have everything to the amp board connected. I was bringing the receiver up slowly with my variac and the amp meter jumped up to a point that it was pulling more amps then expected and I backed off the variac before any fuses blew. If I was using a dim bulb tester it would have gone bright.
     
  4. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    Location:
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    So it's not causing an issue (Shorted) in the current condition?
    What was the initial problem?
    It would be better to use the DBT. 75 watt bulb.
     
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  5. ConradH

    ConradH Addicted Member

    Messages:
    8,034
    Location:
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    Have you tested the output transistors and drivers with a DMM on diode check?
     
  6. kaplang

    kaplang Works for me ! Subscriber

    Finally got my 626 working today out of both channels after replacing one on the output transistors, an NPN transistor on the driver board and 1/4 watt resistor that was fried. I next want to set the bias but have not been able to find what value It should set it too. I know where to take the reading from but not the value and the service manual is no help.

    Anybody know what it should be?

    Thanks
     

     

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Make sure your manual is the correct one for your 626. THERE ARE 2 DIFFERENT MANUALS as PIONEER re-Designed it midway thru production. Dowload the 1973 manual and it'll give you the serial #'s that are the demarkation point for each version (KCT,etc)
     
  8. kaplang

    kaplang Works for me ! Subscriber

    So the bias setting is different for early and late version? Which manual has the biasing procedures?
     
  9. tjg79

    tjg79 New Member

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    Location:
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  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Location:
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    Not necessarily. They redesigned the whole thing. There are enough differences between the two that using the wrong manual will have most techs pulling what little hair they have, out trying to figure it all out. Neither manual has instructions. PIONEER at this point in the game was still learning how to put a decent manual together. With the next series (x3x) they pretty much got it right, although some of the BIAS calculations are off due to "Math errors by the engineers").

    Basically you'll measure the voltage at the emitter resistors (0.5ohm 3w). One will show close to the B+1 or B+2 voltages and the other should be approx 1/2 that. Make sure you have a DUMMY LOAD connected to the spkr output's. Adjust the lower voltage so that it's 1/2 of the B+1 or B+2 voltages. (IIRC this is VR3 & VR4). Offset is VR1 & VR2 IIRC.
     
  11. kaplang

    kaplang Works for me ! Subscriber


    My 626 doesn't have the IC lable above the power cord so it uses the 1971 service manual. It is the FW with the 5 position power selector
    Thanks Larry for pointing out the different manuals. I didn't know.
    Excellent source of information for the Pioneer line. Thanks for sharing. Quality service at an affordably price!
     

     

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  12. tjg79

    tjg79 New Member

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    The 1973 manual indicates the serial number that starts the IC model. I wouldn't rely on the absence of a label that may have been removed.
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Location:
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    +1 with tjg79. Verify by Type, then SERIAL # as laid out in the 1973 manual. Both versions have FW 5 position Voltage Selectors. Compare the serial #, then verify each board # with the manual. Use the manual that matches your's with regard to the boards and serial #.
     

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