Pioneer SX-939 Recap

Discussion in 'Pioneer Audio' started by adrian78m, Mar 17, 2017.

  1. adrian78m

    adrian78m AK Subscriber Subscriber

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    Hello all,

    I want to do a full recap of my sx-939, including transistors, and whatever other parts that should be replaced.

    Can someone point my in the direction of a full list of the parts I should get for this? What should be replaced?

    Thanks.
     
  2. tsd71

    tsd71 RIP Tom Petty Subscriber

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    Check the attachment
     

    Attached Files:

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  3. adrian78m

    adrian78m AK Subscriber Subscriber

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    Thanks! Do I need to replace all of that?
     
  4. adrian78m

    adrian78m AK Subscriber Subscriber

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    I noticed that there are some part numbers in your list that are used for multiple caps with the same capacitance, but with differing voltages. Is that ok? In other words is it ok to replace caps with specs of: 1uF/25v, 1uF/10v, 1uF/50v, all with caps of 1uF/50v?
     
  5. tsd71

    tsd71 RIP Tom Petty Subscriber

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    Yes caps voltages can differ. That is a complete list of all parts. It's up to you how far you really wanna go.
     
  6. adrian78m

    adrian78m AK Subscriber Subscriber

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    I heard that it might be a good idea to replace certain transistors as well. Does anyone know which I should target?
     
  7. adrian78m

    adrian78m AK Subscriber Subscriber

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    There are multiple sheets in the file, one called "Caps"
     
  8. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    The Main transistors that are replaced without fail (Must do a pre-emptive 1st strike on) are the following. These are on the AK TOP TEN WANTED LIST of Failing or Failed parts that must be replaced without fail.

    1.) 2sc1451 ((Main amp board) check for quantity on board I think 2 per side)) replace with KSC3503. Suffix doesn't matter.
    2.) 2sa725 (on most all signal path boards) replace with KSA992. Suffixnot a problem.
    3.) 2sa726 (SAME AS #2).
    These are supposed "LOW-NOISE" transistors that have a history of going noisy, then breakdown causing other problems including part destruction on the board it's on.
    DOWNLOAD THE DATA SHEET to get the correct orientation of the transistor and the leg pin out which is different than the originals.

    If the 939 is operating fine and this is a prophylactic overhaul, then other transistors don't need replacing. The EXCEPTION TO THIS IS any on the POWER SUPPLY, and certain designated transistors by MARK THE FIXER or MATTSD on the Protection Boards.

    Larry
     
    Last edited: Mar 19, 2017
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  9. adrian78m

    adrian78m AK Subscriber Subscriber

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    Just the info I was looking for. Thanks!
     
  10. gdmoore28

    gdmoore28 Super Member

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    Wondering if the same type of list and recommended transistor replacement is available for other Pioneer models - specifically the SX980?

    GeeDeeEmm
     
  11. tsd71

    tsd71 RIP Tom Petty Subscriber

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    Look at the bottom of the sheet you will see "caps" click on it.
     
  12. adrian78m

    adrian78m AK Subscriber Subscriber

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    So, my order of caps came in today. All good except for the filter caps. Now I've got these huge things that I don't think will work...
     

    Attached Files:

  13. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    On the PROTECTION BOARD! Take a 1n4007 and solder it to pins 9 and 10, with the Cathode (banded end) toward pin 10. This diode will prevent back voltage from the relay during shutdown from getting to Q7 and causing it to break down. PIONEER fixed this problem in later units.
     
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  14. adrian78m

    adrian78m AK Subscriber Subscriber

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  15. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    All I get for each is a list of catagories a mile long. If all other parameters are within 10%, I would think either one would be fine. Up to you. Me, I'd go for the $$ in my pocket. But then I'm cheap, as I didn't replace mine when I overhauled my 939. And it's still trying to kick my sx-737's ass. Nothing wrong with it, it only beats the 737 in total watts is all.
     
  16. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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  17. adrian78m

    adrian78m AK Subscriber Subscriber

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    Took me a few days, but here's what I've done so far:
    - Replaced all electrolytic caps on all boards except those on the tuner board, and the big filter caps
    - Replaced the 2SC1384 on the protection board with a 512-KSC2690AYS
    - Replaced the heat-sinked 2sc1451's on the power supply board
    - Replaced the 2SA726's on the power amp bard
    - Installed a 1n4007 diode across pins 9 and 10 on the protection board, cathode toward pin 10

    Previous work:
    - Replaced volume pot
    - Replaced power switch
    - Replace protection relay

    The systems works, but has the following problems (tested with headphones only):
    - Goes into protection randomly (more often after the system has been running awhile)
    - In any setting, with volume all the way down, there is a very slight static sound that never goes away
    - When turning volume up from minimum, sound appears in the left channel before the right channel.
    - FM Muting button seems to do nothing

    It's worth noting that these problems existed before I stared doing any work on the stereo.

    I am going to order new filter caps.

    Where should I go with this project now? I really want to solve the protection mode problem.
     
  18. adrian78m

    adrian78m AK Subscriber Subscriber

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    Some pics of the recap process:
    20170326_113506.jpg 20170326_113215.jpg 20170326_113016.jpg 20170325_145610.jpg
     
  19. rcs16

    rcs16 Super Member

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    That confirms to me what I have said before, isolate the problem(s) first before doing a shot gun replace everything/troubleshooting approach as a solution.
    You can start by measuring all the power supply voltages to see if they are okay. if so then do the power amp checks first before any adjustments are done. If adjustments are required, could do this on the dbt first, I am not sure if you changed the old pots, the bias pot could be flaky and cause a problem.
    Did you clean the volume pot with deoxit F4 fader lube/cleaner? I assume the old one was toast and the replacement is a used part. The old part could be very dirty, how would you know its condition, unless you measured/tested it before you installed it.
    You could check the continuity through the mute switch, of course with power off. deoxit if necessary, check the DC voltages on both sides of the switch.
    Determine if the static sound is in the power amp or the pre-amp need to test with another pre-amp/power amp f you have one.
     
  20. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    FM MUTE will only work effectively with an antenna connected, and with you able to get both weak and strong signals. FM MUTE is a cut off for weak signals and inter-station hiss, not a mute switch that you would associate with dropping the volume so you could say answer the phone. It will mute any signal that is below a set threshold when on and also inter-station hiss. It is not adjustable.

    I agree with rcs to check voltages on the Power supply against the scat 1st and address any problems there. Think of the power supply like your car engine. If it's NOT running on all 4-6-or 8, it's gonna run like shite. When doing one of these it's best to do the power supply 1st, then the Main amp, then start at the inputs and work towards the amp. Replace any problematic transistors (2sa726,& 725's and their complimentary partner if any). Trouble shoot the protection board, (look at sx-1010 rehab projects also as the 939 and 1010 share the same protection board. Everything there that applies to the 1010 applies to the 939.)

    When checking the offset and bias on the 939 while on a DBT, when finished, turn them DOWN all the way. You'll fry the amp if you don't due to the increased voltage. then readjust when on full line voltage. Set it for 25ma as per the 1010 update. 50ma is a mistake the PIONEER Engineers made in their math and Mark The Fixer found it and explained why it was high. I would get new pots for the bias and offset. Like RCS said the old ones could be and more than likely ARE flaky.

    Check the old pot with 2 dmm's set up on the correct pins. Rotate slowly and check operation one side against the other. Shoot some DeOxit F-5 (F-5 is the Faderlube not F-4) into the pot and work it back and forth 50 times and then check again. If it's now running right keep it for a spare (sealed in a plastic bag). Do the same for the new one. Note that some PIONEER Volume pots do get offset. See if you can get inside (take it apart) and check the wipers for play on the shaft. You might be able to work out the play, or not. The only place to get factory pots is a parts unit. So you're kind of stuck trying to get and keep them
    working. A lot of manufacturers used the same brand parts as pioneer and have the same problems. So it's not an isolated problem. A lot of people just chalk it up to the age of the unit.
     

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