SX-737 Project

Discussion in 'Pioneer Audio' started by Motnick, Jul 4, 2017.

  1. Motnick

    Motnick AK Subscriber Subscriber

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    Wyandotte Michigan
    I picked up a Pioneer SX-737 the other day that has some issues. The seller described the issue as "plays good for a few minutes and then its like the antenna stops picking up". From reading other threads in this forum I asssumed it was having some protection circuit issues, so I took the chance. When I got it home I cleaned all of the switches and pots with deoxit, and then powered it up to see what happened. Sure enough the relay would open up after a minute or so and it would sometimes toggle back and forth a few times before ultimately staying open. I checked the DC offset and it was very high especially in the right channel.

    I found the corrected bias and DC offset procedure here on the forum and gave that a shot. I was able to get the offset down in both channels, but the adjustment pots did not provide enough adjustment to get things dialed down to 0 in both channels. I was able to get the protection circuit to stop cutting things out however.

    At this point the left channel is down around 0, but it does swing around a bit. Every once in awhile it will just run away to up around a half a volt. It usually will come back down after a few seconds, then it runs away again.

    The right channel seems to be stuck around -.2 volts. It also seems to swing around as well. Fine adjustment on this channel does not seem possible.

    I have found the recap parts list for this receiver, and I am planning to go ahead and order all of the parts to have on hand. I would like to go ahead and replace all of the parts on the list, but I am hoping that I can get it working correctly first rather than just taking the shotgun approach and replacing everything.

    Any advice on next steps would be greatly appreciated. I have been reading the service manual, and I do have the voltage readings from several of the boards that I can provide. I am new to this hobby, but would really like to expand my knowledge. If I can also end up with a nice SX-737 in the end then that would also be a bonus.
     
    Last edited: Jul 6, 2017
  2. moodydan

    moodydan I love the punk rock. Subscriber

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    While it's still 'shot gunning' to a degree, I would start recapping board by board and see how things settle. I'd start with power supply, then protection, the power amps, etc., testing between each board being completed and checking voltages against the SM. I'd also replace the transistors that are known for trouble in that unit, if they aren't on the list that you found, because they can cause protection issues (much like what you're already fighting!).
     
    Last edited: Jul 4, 2017
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  3. Motnick

    Motnick AK Subscriber Subscriber

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    Here is what I can see...

    AWR-057

    Pin 14. 34.7v
    Pin 15 16. 29.5v
    Pin 17. 18.3v
    Pin 19 20. 12.9v
    Pin 21. 12.9v
    Pin 12 13. -13.1v
    Pin 7. -37.6v
    Pin10 -34.8v
    Pin 9. 34.9v
     
    Last edited: Jul 4, 2017
  4. Motnick

    Motnick AK Subscriber Subscriber

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    142
    Location:
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    AWH-033

    Pin 6. 35v
    Pin 10. -13.2v
    Pin 22. 35.3v
    Pin 25. -35.5v
    Pin 28. -35.4v
    Pin 33. 35.5v
     
    Last edited: Jul 4, 2017
  5. Motnick

    Motnick AK Subscriber Subscriber

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    142
    Location:
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    AWM-025

    Pin 2. -13.2v
    Pin 12. 7.48v AC
    Pin 11. 35v

    Pin 1. 80 mv
    Pin 3. .325v. Very unstable.
     
    Last edited: Jul 4, 2017
  6. Motnick

    Motnick AK Subscriber Subscriber

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    Location:
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    Please advise on next steps
     
  7. Motnick

    Motnick AK Subscriber Subscriber

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    Currently I have the offset good enough that the amp will play. Maybe this is good enough to begin the recap?? I still have some snap crackle pop.
     
  8. Motnick

    Motnick AK Subscriber Subscriber

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    I think recap the power board, then the protection board, followed by the amp board?
     
  9. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Great advice from moodydan.

    Probably a good idea to rectify the offset issue completely before you recap l think, some KSA992's to replace the 2SA725/726's as the amplifier input differential pairs should sort the DC offset issue out. These will be the two pairs of small signal transistors closest to the front of the receiver on the main amp board. You probably won't need to match them as they are normally very close in hfe.

    When you go to recap, make up a transistor list also as there are plenty in there that are iffy as moodydan pointed out, mostly the 2 2SC1451's on the main amp and the 2SA725/726's and the 2SC1312/1313's throughout.

    Little Wima MKS2 film capacitors work and fit well to replace any electrolytics 4.7uF and under. Nichicon UKL to replace any other low leakage capacitors listed as CSSA or CEANL.

    All the best with it :thumbsup:.
     
    Last edited: Jul 5, 2017
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  10. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    The trim pots may be getting a bit dicey also, prob pay to add some Bourns multi turns to your list and also some new driver transistors, power supply regulator transistors and a new protection relay.

    If you spend the time to do all of the above you will end up with a VERY nice 737 that will sound much nicer than it does now, l am sure you will not be disappointed.
     
    Last edited: Jul 5, 2017
  11. Motnick

    Motnick AK Subscriber Subscriber

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    Location:
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    I have built a Mouser order which includes all the capacitors, transistors, protection relay, and trim pots. It sounds like my best bet is to go ahead and rebuild the AWH-033 power amp board first, replace the transistors and trim pots on the amp board, and see what happens? Then move onto the other boards.

    Is it possible to narrow down which transistor is having the issues first?
     
    Last edited: Jul 5, 2017
  12. Motnick

    Motnick AK Subscriber Subscriber

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    Location:
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    I did a bit more probing at the transistors on the AWH-033 board. My measurements are in parenthesis.

    Left
    Q1. .6v (.63v) seems ok
    Q3. .6v (.63v) seems ok
    Q5.-1.2v (-.99v) unsure if this is ok
    Q7.1.2v (1.15v) seems ok
    Q9.-1.2v (-1v) unsure if this is ok
    Q11.36v (35v) seems ok
    Q13.-36v (-35v) seems ok

    Right
    Q2. .6v (.36v) not looking too good
    Q4. .6v (.36v) not looking too good
    Q6. -1.2v (-1.27v) seems ok
    Q10. -1.2v (-1.29v) seems ok
    Q8. 1.2v (.95v) unsure if this is ok
    Q14. -36v (-35v) seems ok
    Q12. 36v (-35v) seems ok

    Based on this it seems like something is definitely up with Q2 and/or Q4? Please let me know if you guys can notice anything based on these measurements. I am still new at this and attempting to learn how to go about narrowing down an issue like this before replacing any parts.
     
  13. loweran

    loweran AK Subscriber Subscriber

    Messages:
    875
    Location:
    Austin, Texas
    So you have the DC offset/bias under control? I would just move on to do a full rebuild. That means both transistors and caps. Start with the power supply and the protection, then do the amp board then the control and EQ. Also you will want to fully clean the pots when you have the control and EQ board out. You may also want to pull and re-grease and replace the mica in the transistors in the heat sink. Replace any burnt out lamps and your in business.

    Be sure to test it after each board on a DBT.

    Yes, it's shotgunning, but you have it running. You will be glad you put the work into it.
     
  14. Motnick

    Motnick AK Subscriber Subscriber

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    Location:
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    I have been able to get the left channel to zero out using the adjustments on the board. I have noticed that when I first turn it on cold that the offset starts at about 20mv in the left channel but comes down to zero within a few minutes and stays there.

    The right channel is a lost hope however. I have gotten it adjusted down enough that it stays out of protection, but I still have about .200 - .350mv offset in the right channel. The right channel seems to drift around between those values. I'm guessing my issue with this channel has to do with one of the transistors.

    I have gone over the switches and pots twice so far with the deoxit.

    I have also ordered all of the parts to do the rebuild. My thought was to start with the amp board once the parts arrive to see if that will address the right channel offset issue, before going onto the other boards. What do you think? Any input you guys have for me is appreciated.
     
  15. loweran

    loweran AK Subscriber Subscriber

    Messages:
    875
    Location:
    Austin, Texas
    You need to get the power stable. You can start with the amp, but might still have issues with voltages. Since it's running, you can start with the power and work your way down. I would replace the trim pots too.
     
  16. Motnick

    Motnick AK Subscriber Subscriber

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    Location:
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    I did include the trim pots on the order. I plan to replace them as well.

    Is the DBT required here? I'm not familiar with how they work.
     
  17. loweran

    loweran AK Subscriber Subscriber

    Messages:
    875
    Location:
    Austin, Texas
    Yes. Don't attempt this without it. It will save you a lot of heartache. You can build one for less than $10. I dont have a link, but do a search on how to build it. It's basically a light socket and switch. The lightbulb takes the voltage in the event of a short and saves the electronics from damage.
     
  18. Motnick

    Motnick AK Subscriber Subscriber

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    Ok, I'll build one. So basically I want to use it to power up the receiver after each time I replace anything on the circuit boards. If the bulb glows brightly then I have done something wrong?

    What happens with the bulb if all is well with my rework? Bulb stays off?

    What wattage bulb should I use?
     
    Last edited: Jul 6, 2017
  19. loweran

    loweran AK Subscriber Subscriber

    Messages:
    875
    Location:
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    If all is well, the light will go bright and then dim to a faint glow. If it stays bright, you have a short. In that case, do not move to line power until you resolve the short. This is why I test after each board. I know where to look for the mistake.

    You can use a 75 watt incandescent bulb.
     
  20. loweran

    loweran AK Subscriber Subscriber

    Messages:
    875
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