Pioneer SX 525 and SX 939

freakquincy

New Member
Hello, first time posting here. I have both a Pioneer SX 525 and SX 939 that are experiencing the same symptoms: After a few minutes of use, they develop a loud static/crackle/thunder from the right speaker. From what I've read this may be shorted/bad transistors on the power amp board, as input source and volume do not affect the problem. (I guess this is called the 'main amp' in the 525 manual?) . In any case, I did search a few threads on here, but just wondering if anyone can point me to a definitive repair/rebuild thread for either of these models. I think I may try to tackle the 525 first, due to the simpler design, but on the other hand the 525 used to be my dad's and I don't want to wreck it. But cracking open the 939, man that thing is busy inside.
This may be a dumb question, but on a receiver like the 939, do you have to sometimes de-solder most/all the connections to some of the boards to get enough room to work on them, or are there other tricks to gain access?
Best transistors to sub for 2SA572-4B, 2SC1318-R, 2SA720-R, 2SC789-O... Do I need to replace the diodes and varistors too?
Which caps/components to replace for complete rebuild, but without going overboard? Anyone have complete parts lists for digikey or mouser or...?
FYI I do have some experience with tube stuff (Hammond), but this will be my first venture into more cramped solid state territory...
Any help, suggestions, bashing, and insults will be taken under consideration. :)
 
You don't need to de-Solder any wires to gain accessto the boards. There is built in slack in the wiring. Just undo the wiring from the retainer clips and you'll be able to lift and flip out the individual boards to access them.

Problem transistors: 2sa725, 2s726 and 2sc1451 on the 939. These are replaced as a pre-emptive 1st strike! PERIOD. On All boards. These are the most commonly BAD Transistors on ALL PIONEERS of the Early to late mid 70's. The 2sa725/726 get replaced with the KSA992. The 2sc1451 gets replaced with the KSC3503. This should end most if not all of your symptoms

There is a partial caps/Transistor list on this thread (mine).http://www.audiokarma.org/forums/index.php?threads/another-939-rebuild-thread.494501/ I did not add to MTF list as I did NOT add anything that he didn't list . You'll find that a lot of the 939 boards are the same as the 1010. If you corelate the board numbers between them you should be able to recap the whole thing using the thread list and a 1010 list.

If the 939 is running but noisybeforehand, just replace the noisy transistors as above, test, and if ok, then recap using MTF list.

As for the 525, you'll have to hit up in the guys for ideas and part #'s. IT's a fairly simple unit, and orders of magnitude simpler than the 939.
 
OK, working on the SX-939. the SX-525 is shelved for now.

I replaced the 4 x 2SA726's on the power amp board and about to test. I was going to also replace the 2 x 2SC1451's, but I got worried because the replacement KSC3503's don't look like the originals at all, they look more like Q9 and Q10, which are flat have a hole in them to screw to a heat sync that's mounted to the board. (Edit: there also other flat transistors on the board with this design that don't have a heat sync.)

Also, I thought I understood the pin-outs on the KSC3503, but my meter did not match what was in my head, so for now I left the original 2SC1451s in place. How do the pins of the KSC3503 match to the 2SC1451? The 2SC1451 is kind of odd because it's wrapped in a crimp-on circle heat sync, but even if you look down through the hole in the top, there is no flat spot to orient the transistor when determining which pin is which. Confusion on the pin-outs, not the heat sync question, was what made me hold off on replacing these two transistors. Any tips or hints are appreciated.
 
Hi, l rebuilt my SX-939 about 18 months ago, l would definately get those 2SC1451's out of there as Larry suggested. The ECB screen print on the main amp board will be correct and the pinouts for the KSC3503dstu's are as below. Don't worry about the different package style compared to the originals.

KSC3503-pinout.jpg


I would also replace the complementaries to these while l was in there. The complementaries being the 2SB527 or (2SA809 in some units) mine had the 2SA809's.

The 2SB527 or 2SA809 gets replaced with KSA1381estu. l have also used MJE350's for these and MJE340's for the 2SC1451's, these are a little lower in gain but still work fine.

BTW l am not sure why Pioneer used a 2SB527 as a pre driver and supposed complement to the 2SC1451 as it's specs are NOT similar to a 2SA809 at all, have always wondered about this?

You will be able to replace these transistors by just tilting the board forward and not removing the heatsink as they are at the top of the board.
From memory there are some small bright blue Sanyo caps on this board also that can be very problematic, if value is small enough replace with film caps. Guess you will sort this out when you recap the board later.

All the best with it.
 
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