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SX-737 Not working after recap

Discussion in 'Pioneer Audio' started by MusicRobb, Apr 15, 2018.

  1. MusicRobb

    MusicRobb New Member

    So I recently got a non-working SX-737 from a friend. When he had it it was working okay but, when warmed up, it would occasionally pop and the audio would and cut out for a bit.

    I guess he figured a recap was in order so he went ahead and did some work on AWH-033, AWR-057, and AWF-011. Somewhere in the mix of things it became a very nice looking paper weight. Now I have it and I'd like to resolve whatever's going on with it.

    I know from his notes that he replaced the following:

    • 35v 47uf cap x 2
    • 6.3v 220uf cap x 2
    • 10v 22uf cap x 2
    • A726 S56G1 transistor x 4 (replaced with KSA992FBU)
    • 50v 220uf cap
    • 50v 100uf cap
    • 50v 330uf cap x 3
    • 16v 220uf cap x 2
    • 35v 100uf cap
    • 16v 100uf cap
    • 25v 22uf cap

    • 6.3v 330uf cap x 2
    • 25v 100uf cap
    • 16v 220uf cap
    • C1313S54G transistor x 4 (replaced with KSC1845FTA)
    • A725S56F transistor x 2 (replaced with KSA992FBU)
    I also know he referenced this post (http://www.audiokarma.org/forums/index.php?threads/deciding-what-caps-to-use.494440/) for finding the caps and transistors.

    So the current state of the receiver is this: it powers on but there is no relay click and no sound from anything. Not even the AM/FM or headphones. I am not super knowledgeable about diagnosing these things but I suspect that it's the transistors that have been installed incorrectly. I only say that because a) none of the caps were reversed or installed incorrectly and b) I know from talking with my friend that he was a little unsure on the orientation of the transistors. (I should also note that all fuses are okay.)

    That all being said I would really appreciate some guidance in figuring out how to narrow down where the issue lies. I have a multimeter (that's a little hard to read but it's better than nothing) and the ability remove/replace whatever component is needed.

    Any advice would very much appreciated!


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  2. rcs16

    rcs16 Super Member

    Baldwin, Ontario, Canada
    Double check those bjt orientations. if there is no relay click then you have to
    1) learn to use a DMM
    2) check/post the power supply voltages compare/match against the schematic.
    3) check the power amp DC offset to see if that is the fault.
    4) check the protection circuit
  3. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Calgary, Alberta
    Think you will need to check the dc offset before the relay if its not clicking open...
  4. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

    Milwaukee, WI
    I've put a lot of transistors in backwards, in my time. I would probably just shotgun the transistors on all those boards, seeing as it'll cost around $5-10 and shouldn't take that long. You can do the whole checking voltages thing, but if you're fairly good at doing ECB orientations correctly, I'd either double-check all of his or just redo them all.
  5. MusicRobb

    MusicRobb New Member

    So I just went ahead and checked and I got nothing in A or B. I also checked to see if there was anything coming from any of the pins in the protection circuit and there was also nothing. Another thing to note was a slight buzzing hum coming from the power transformer.

    Any ideas where to go from here?
  6. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Calgary, Alberta
    you got zero dcv ? Maybe there is a bad/backwards cap before the relay causing not enough volts for relay to engage? Think you just need to start checking the power supply voltages and check them against the schematic. If the power supply is good then i would double check or shotgun the transistors as mentioned earlier. It was working before, so an error in part replacement seems likely...
    Pioneered likes this.


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  7. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

    Las Vegas Nevada
    A basket case from a failed repair is the worst type of job a tech can be saddled with.
    Lots of mental effort is needed. A natural failure is predictable but components when installed incorrectly pose a whole new dimension.
    Everything has to be checked. Piece meal approach will waste time and components.
    After checking components for correct installation. Check for a signal at the output of the preamp. If it’s there you’re halfway.
    Remove the power to the amps prestage and outputs.
    Begin solidifying the power supply. Next to the preamp and then to the amps.
    You can check for the signal through the process of eliminating the boards.
    I love these repairs when I have time. It’s a challenge and there is a lesson in it.
    Last edited: Apr 16, 2018
    merlynski likes this.
  8. petehall347

    petehall347 the brandy coffee man Subscriber

    uk.. the middle bit
    check these first for correct pin outs .
    • A726 S56G1 transistor x 4 (replaced with KSA992FBU)
    q1 q2 q3 q4 ,

    but firstly check volts at pins 3 and 17 amp board awh-033
  9. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Build a dim-bulb tester before you go any further.
  10. two.dogs

    two.dogs Active Member

    Denver, CO
    You're lucky that you've already got some great advice from some really smart guys. This is also a great receiver to start out with. The blue face Pioneers are all awesome, from the 434 to the 1010. Your 737 is relatively easy to work on, so the stars are all aligned for a good repair.

    You will absolutely need to make a dim bulb tester, so do some searching on that. If there is a short somewhere, then the receiver will try to pull infinite amps and things will burn. The dim bulb tester puts a light bulb in series with the receiver. If there are no shorts, the bulb will burn bright for about a second while the power caps are charging, then it will dim. If you have a short, then the bulb will stay bright and limit current until you shut it down (quickly). Meanwhile, take some pictures of the power supply board from different angles, top and bottom, and show us what you have.

    Last edited: Apr 17, 2018

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