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Pioneer SX-737 protection circuit problems

Discussion in 'Pioneer Audio' started by M&M, Nov 3, 2015.

  1. M&M

    M&M New Member

    Messages:
    6
    Hi folks,
    as you can see I am pretty new here. I found this great forum by looking for help with a broken stereo.
    My friends Pioneer SX-737 has problems that match exactly on DinD's description in the thread "Pioneer x737 bad relay or s.th.":
    After switching off the stereo and waiting for a couple of hours it works fine again - for a while :(

    So far, i followed the measure-instructions mark gave (Thanks) and got following signals on the AWM-025:
    Pin 1: 0.3V
    Pin 2: -13.0V
    Pin 3: 0.3V
    Pin 4: GND
    Pin 9: 42 V when sound is cut out, and 9.7V when it all works fine
    Pin 10 and 11: 42 V
    Pin 12: 6.6V AC

    Intuitively i'd renew the capacitors but i didn't understand the circuit. Can someone help me with the error analysis, or does somebody know what devices need to be exchanged?
    Thanks ahead
    M
     
  2. pauls1

    pauls1 Active Member

    Messages:
    129
    Location:
    Essex, UK
    dlucy likes this.
  3. zebulon1

    zebulon1 Into Phase Linear's lately Subscriber

    Messages:
    3,719
    Location:
    Las Vegas Nevada
    Perfect Link!
    I would check the idles per Larry's thread above first. They might be drifting after it warms up.
    If they need adjustment report back first. Adjusting an old trimmer has it's perils.
    But the DC Balance is spot on at 0.3mv's. (2SA726's - Differential Pair)
    Totally agree with "pauls1" on the transistor replacement.
    Check the Power supply voltages for GP.
     
  4. JURB

    JURB Super Member

    Messages:
    3,336
    Location:
    NE Ohio
    THIS IS GOING TO BLOW YOUR MIND

    There are transistors on the phono preamp board with the part number 2SA725.

    Though the unit has a regulated power supply for this, when these transistors go intermittent like shorted or open they change the load on the power supply. Those power supplies are only designed to handle line regulation, and for the legs of it to come up and go down in an orderly fashion. Like you can't have a 12 volt source throwing voltage into something that only takes 5 volts. And then there is the positive/negative thing.

    But anyway, those power supplies were not meant to deal with load variances. Every stage in the preamp is class A, and the current drain never changes.

    Until a transistor acts up. that causes fluctuations in the current drain and that causes the voltage to vary to certain parts of the power amp which cause a DC offset shift. I have seen it cause the protection to kick in.

    Anyway, it could be any other stage as well. the thing that got me in my 737 was that even though the phono pre was causing the problem, it affected the power amp. That is because of how it was designed. Not that that's bad, it's just how it is.
     
    dlucy likes this.
  5. M&M

    M&M New Member

    Messages:
    6
    Hi. Thanks for your help :)

    I followed pauls instructions. This is my result after adjusting the trimmers:
    Pin 3 -> 0.2V (0V is not possible i turned it to the minimum end position)
    Pin 17 -> 0.2V (0V is not possible i turned it to the minimum end position)

    Pin1-2 -> 12mV (10mV is not possible i turned it to the minimum end position)
    Pin 18-19 10mV

    can i improve the adjustments?
    @zebulon:
    i hope i got you right about the dc balance: i measured 0.78V on R7 and 0.76V on R8
    what power supply voltages i need to check?
    on Pin6 i've got 40V
    on Pin 10 i've got -13V

    @JURB: sorry, i couldn't really follow. its a while ago that i worked as an electronics technician. and all in english here ...makes me a little insecure.
     
  6. zebulon1

    zebulon1 Into Phase Linear's lately Subscriber

    Messages:
    3,719
    Location:
    Las Vegas Nevada
    I don't know what I was looking at but the Amp adjustments are off as in your first post.
    Are the readings you where getting stable?
    If the Balance is that off (Idles are not right as well), you got some work ahead.
    Take voltage readings from the Power supply. Both amps are behaving the same, you need to look at the supplies first.
     
  7. M&M

    M&M New Member

    Messages:
    6
    I measured following signals with the actual trimmer positions:
    AWM-025
    Pin 1: 0.2V stable
    Pin 2: -13.04V stable
    Pin 3: 0.18V stable
    Pin 9: 41.9 V (stable) when sound is cut out, and 10.3V (stable) when it all works fine
    Pin 10 and 11: 41.8V stable
    Pin 12: 6.28-6.30V AC

    Power supply circuit
    Pin 12 -12.9 stable
    Pin 14 39.4-39.7
    Pin 15 34.6 stable
    Pin 17 25.6 stable
    Pin 19 22.0 stable (too high?)

    Any suggestions? Should I start with changing the transistors 2sa726 and 2sc1451 on the AWH-033?
     
  8. rikardo

    rikardo New Member

    Messages:
    4
    Hi,
    When I got mine it was clicking again and again, think it was a noisy transistor on the protection board.
     
  9. M&M

    M&M New Member

    Messages:
    6
    Hi,

    i changed the transistors 2sa726 and 2sc1451 on the AWH-033. Now the amp is doing nothing anymore. Well, i can switch it on, but no sound. No relay clicking when switching it on.
    Can anyone help me with the next step?
     
  10. tsd71

    tsd71 Gittin Some Head!

    Messages:
    2,583
    Location:
    New Jersey
    ksa992.jpeg 737.jpeg View attachment 668564 Did you install the transistors correctly? New transistors Fairchild are E_C_B, some of he older transistors are different leg pinouts.
    How about a picture of the placement.
     
    Last edited: Jan 3, 2016
  11. Watthour

    Watthour Electron Rancher - JS3600 Subscriber

    M&M,

    What replacement devices were used for the 2SA726s, and were they gain-matched?

    What replacement devices were used for the 2SC1451s?

    Typical replacements for those would be 2SA726 ==> KSA992s and 2SC1451 ==> KSC2316Y (sufficient) or KSC2690A (overkill).

    Important to note is that the pinouts are as follows, just in case there was lack of information when installing:

    2SA726 - E-C-B
    KSA992A - E-C-B

    2SC1451 - E-B-C
    KSC2316Y - E-C-B
    KSC2690A - E-C-B
     
  12. Watthour

    Watthour Electron Rancher - JS3600 Subscriber

    Well, that figures. While I was checking it out and verifying the data, TSD comes in with the info right off the top of his head...
     
  13. tsd71

    tsd71 Gittin Some Head!

    Messages:
    2,583
    Location:
    New Jersey
    Two heads are better than one.
     
  14. Watthour

    Watthour Electron Rancher - JS3600 Subscriber

    Tell that to the Medusa... ;)
     
  15. tsd71

    tsd71 Gittin Some Head!

    Messages:
    2,583
    Location:
    New Jersey
    I would love to tell her anything she wanted to hear!

    Medusa.jpg
     
  16. M&M

    M&M New Member

    Messages:
    6
    I am quite sure that I installed them correctly. i attached a picture. I used following transistors:
    2sc1451 -> NPN 30 KSC3503DSTU TO-126 FSC
    2sa726 -> PNP 120 KSA992FTA TO-92-3 FSC
     

    Attached Files:

  17. tsd71

    tsd71 Gittin Some Head!

    Messages:
    2,583
    Location:
    New Jersey
    What about the other side of the amp, did you replace those too? Are the 992's gain matched?
     
  18. Watthour

    Watthour Electron Rancher - JS3600 Subscriber

    It appears the KSC3503 is also E-C-B. As a result, it appears that the devices are installed correctly according to the board artwork. As long as both channels are the same it shouldn't be a problem.

    We only asked about orientation is because it can happen to any of us, and it seems to happen regularly.

    Since the unit went "dead" after changing those trannies, it might be worthwhile to inspect the solder joints and traces just in case something is cold, cracked, or bridged. That includes the driver transistors (2SB527/2SD357) which may have been heat stressed in the past and the movement of the board may have cracked what was already a marginal joint. That applies to the other larger components (emitter resistors, etc.) and the wire wrap connections just in case one of them didn't like being moved or flexed.
     
    monte77 likes this.
  19. rikardo

    rikardo New Member

    Messages:
    4
    did you try turning it on with a DBT? I can take a picture of the board recapped if that helps, as i also changed caps on that board in addition to the transistors (AWM-025 protection board), i have made no change to the power board.
     
  20. M&M

    M&M New Member

    Messages:
    6
    Hi, thanks for dobblechecking the transistors.
    I exchanged the transistors on both channels the same way. What does it mean "gain matched"?
    I checked the solder joints. Nothing seems cold or cracked. The solder appears in a pretty good condition.

    @ rikardo: What means turning on with a dbt? How do i do that? did you have similar symptoms and you solved them by recapping?
     

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