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Pioneer SX-780 need help signal tracing. Powers on no sound. Darlington packs are good.

Discussion in 'Pioneer Audio' started by Kingbleu, Oct 2, 2018.

  1. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    I have a Pioneer SX-780 that powers up but produces no sound and no relay click. The STK's (Darlington power packs) are good. I would like some help tracing the signal. I have test equipment that I have picked up over the years. Mostly at garage sales or Craigslist.

    I have two SX-780's one that works and one that doesn't. I was thinking I could probe around and trace the signal, which would lead me closer to the issue. I thought it would be easy because I could use the working unit as a base line. However this is much harder than I expected because I don't know where to put the probe.

    I know for sure that the Darlington Power Packs are good because I swapped them out of a working SX-780.
    The unit passes the Dim bulb test.
    The unit powers up, the three lamps, source, and tape monitor lights come on but the relay doesn't click
    No voltage to the VU Meters
    FM needle is dead
    The rail voltages are +40.6v and -40.6v
    I tested the three transistors on the heatsinks Q25, Q19, Q20 and resoldered them. They all tested good.

    Q25
    E 13.7v
    C 27.3v
    B 14.4v

    Q19
    E 40.4v
    C 51.5v
    B 41v

    Q20
    E -40.4v
    C -53.3v
    B -41.1v

    R1 The big ceramic resistor mounted on the chasis tests 26.7v and 51.9v
    The emitter resistors all measure .4 ohms
    NO signal when probing at R233/R234 (the input to the power amps)

    Opamps TA7136 Q3 and Q4 are suspect not even close to specs.

    Q3
    pin7 -21.8v
    pin6 -24.6v
    pin5 -21.2v
    pin4 -24.8v
    pin3 -21.3v
    pin2 -20.6v
    pin1 -21.2v

    Q4
    pin7 -21.8v
    pin6 -23.9v
    pin5 -21.2v
    pin4 -24.8v
    pin3 -21.2v
    pin2 -20.6v
    pin1 -21.2v

    STK-0050

    Left STK (looking at front of unit)
    pin1 39.2v
    pin2 -41.3v
    pin3 39.2v
    pin8 39.2v
    pin9 41.3v
    pin0 40.3v

    Right STK (looking at front of unit)
    pin1 -1.1v
    pin2 -41.4v
    pin3 4-9mv
    pin8 18-25mv
    pin9 41.3v
    pin0 1.2v

    Side note when I first got the unit it powered up the relay clicked but no sound. Now after replacing the STK's the relay wont click.
     
    Last edited: Oct 6, 2018

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Test the regulated voltages at Q17/18. It would appear they may be O.K. due to your readings at the OP amps Pin 7, but that doesn't show the -23V supply.

    What signal source is being used?
     
    Last edited: Oct 2, 2018
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  3. Watthour

    Watthour Electron Rancher - JS3600

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  4. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    Q17 and Q18 are both showing negative values?

    Q17
    B -21.1v
    C -21.9v
    E -21.9v

    Q18
    B -25.4v
    C -36.6v
    E -25v

    The signal source being used is a Circuitmate Function Generator FG2
     
  5. rcs16

    rcs16 Super Member

    Messages:
    3,888
    Location:
    Baldwin, Ontario, Canada
    Because its supplies are wrong.

    q17 is the positive +24v regulator, which is toast (-21.9V) so you are going to have to figure what is blown, test the comps assoc with the reg, Q17
    q19 emitter is the supply, should be close to scat ~ +39.5VDC
     
    Last edited: Oct 3, 2018
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  6. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,493
    Location:
    Mass
    Left channel DC problem should be fixed. 39 Volts on pins 3 and 7... Relay won't click until this is fixed.

    As noted by others - fix the supplies before going after the ICs.
     
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  7. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    After testing the components associated with Q17 I found Resistor 305 was blown. It is a 430 ohm resistor. I replaced the resistor and wrote down the before and after values. Is it possible, that after the resistor blew, it toasted Q17, Q18? Should I go ahead and replace Q17 and Q18 at this time? Or should I look further for other faulty components?

    rcs16 can you please explain in more detail what you mean "q19 emitter is the supply, should be close to scat ~ +39.5VDC" thank you.

    Q17 blown resistor values
    B -21.1
    C -21.9
    E -21.9

    Q17 values after replacing resistor
    B 15.7
    C 15.1
    E 15.1

    Q17 actual values in service manual
    B 24.6
    C 34
    E 24


    Q18 blown resistor values
    B -25.4
    C -36.6
    E *-25

    Q18 values after replacing resistor
    B -10.9
    C -10.5
    E-10.3

    Q18 actual values in service manual
    B -24.6
    C -34
    E -24


    Q19 blown resistor values
    B 41
    C 51.5
    E 40.4

    Q19 values after replacing resistor
    B 41.1
    C 49.3
    E 40.5

    Q19 actual values in service manual
    B 40.1
    C 51.6
    E 39.5
     
    Last edited: Oct 4, 2018
  8. Watthour

    Watthour Electron Rancher - JS3600

    Q17 seems to be hurt. It is likely that is what caused the resistor to fail, rather than the other way around. Something may be pulling down the +24V at the emitter.
     
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  9. rcs16

    rcs16 Super Member

    Messages:
    3,888
    Location:
    Baldwin, Ontario, Canada
    Agree with above.
     
  10. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    In the above statement about q19 what is scat. Is that a typo. What do you mean it should be close to scat
     
  11. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    Are you suggesting that I keep hunting for the thing that may be pulling down the +24v at the emitter of q17? If so any advice on where to look. Thank you
     

     

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  12. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,812
    Location:
    Las Vegas Nevada
    A first step:
    Check for continuity between pins 4, 6 and 7 of the TA7136 IC?
    Check for continuity between pins 1, 4 and 8 of the HA1457 IC?
    It could be the EQ or the Pre amp IC's. Or something else.
     
    Last edited: Oct 8, 2018
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  13. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    Do you mean HA1457.
    No continuity between pins
     
  14. rcs16

    rcs16 Super Member

    Messages:
    3,888
    Location:
    Baldwin, Ontario, Canada
    scat = schematic
    HA1457, you need to check all the places that the +24VDC regulated voltage goes too.
    Q1,2,3,4
    First thing to do is remove Q17 and measure the resistance from the Q17-emitter to chassis ground.
    check resistance of R301,302,305,309,310
    check &/or change C301,303,305
    Q17, use 512-KSC2383YTA (Vce=160V,Ic=1A,Pd=900mW,Ft=100M,hfe=60-320,TO-92L,ECB)
     
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  15. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    Thank you very much. I am grateful for this knowledge that you share. Now I got some work to do on this receiver.
    You say to remove Q17 and measure resistance from the Q17 emitter to chassis ground. I am very confused on how to do this. If I remove Q17 from the circuit board how do I measure resistance from emitter to chassis ground?
     
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  16. Watthour

    Watthour Electron Rancher - JS3600

    Test the emitter connection (circuit board trace) resistance to chassis ground.
     
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  17. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,493
    Location:
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    Very unscientific, but check for hot components. If one of the chips is loading down the supply, it should get hot...
     
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  18. rcs16

    rcs16 Super Member

    Messages:
    3,888
    Location:
    Baldwin, Ontario, Canada
    There are series resistors & ecaps to ground, running to each load (supplies for Q1-4), these act as supply noise filters to remove hum/noise
    If you remove these R's you can isolate the loads from the supply.
    get the supply up and running at the correct voltage, then you can figure out if/which load is drawing too much current, (try one set of series R's +/-24V at a time) to figure out if/why the regulator has failed or is out of spec V.
     
  19. Kingbleu

    Kingbleu Active Member

    Messages:
    180
    Location:
    Southern California
    :banana: This is so fun. Its like a drug. I love it.

    It is the HA1457 that is drawing all the current. It took me awhile to figure it out but here is the process.
    Removed Q17 and measured resistance from chassis to ground 46.5k

    Checked the mentioned resistors. All check ok except for R309
    R301 391
    R302 219
    R305 425
    R309 13k Should be 22k
    R310 9.3k

    Checked the mentioned Capacitors as well as C213, C214 they all tested very bad and I replaced them all.
    C301 57
    C303 62.5
    C305 35
    C213 16
    C214 15

    Replacing the caps did nothing for Q17, Q18 so I started to pull the IC's. I started with the Q3, Q4 TA7136P I compared them to each other and tested resistance from pin to pin. They both had the same values so I figured either both are good or both are blown. Next I tested Q17, Q18 while the TA7136P's were pulled and no love there. So I moved on to Q1, Q2 HA1457 I pulled them out and compared them to each other, testing resistance from pin to pin I saw that they were not matching. So I pulled a HA1457 from my working SX-780 to use a base line and found out one of the HA1457's was blown.

    I replaced the blown HA1457 and powered her up on a dim bulb and Voila Q17, Q16 are closer to spec but still no relay click. I have made some forward progress and it feels good. Now what should I do to get the relay working? Below are the readings while on dim bulb.
    Q17
    B 24.6v
    C 32.8v
    E 24.2v

    Q16
    B -24.6v
    C -29.2v
    E -24.2v
     
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  20. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,493
    Location:
    Mass
    Looks like you need another fix...,
    Until you solve the reason for pins 3 and 7 being 39.2 volts, you won't get a relay click.


    Edit: Looks like pins 7,8,9 should be 8,9,0 (thnx merlynski)
     
    Last edited: Oct 5, 2018
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