Pioneer Sx-535 Blowing 4A Fuses

[W.T. Holt]

Working on a nice SX-535 right now... came to me with both 4A fuses on the Power Supply Circuit board blown. Replaced both of them right off, powered the unit up and they blew again immediately. As it stands right now there is signal from both channels albeit quiet, heavily distorted signal. Left side is slightly stronger than the right.

So far I have:
- Cleaned all switches and knobs thoroughly (always my first step!)
- Checked all the diodes on the power supply circuit. No Shorts, all resistance values are up to spec.
- Checked all transistors on the power amp board - Found Q12 to be shorted (right side output transistor) Everything else seemed to be functioning.
- Checked for bad solder joints on the power supply and power amp boards. Nothing Immediately obvious (more thorough check can and will be done).

All these measures have left me with a few questions:
- Is it possible that the bad right side output transistor could be causing both fuses to blow? Seems odd but I suppose it could be possible. If that's the case I'll postulate this is the cause of the poor signal as well.
- Said transistor must be replaced regardless, but a cursory search for the part leads me to believe this will be a tough one to source exactly. Is there an acceptable replacement for a Sanken L1986 transistor?
- Is there something I may be missing/notorious root cause for this problem? I have read a few threads where 535's have had similar fuse issues, but none of them seem to have a positive ID on whats causing the problem and/or how to fix it.

Thanks for reading, Hope someone can help!

 

[larryderouin]

The original outputs for the SX-535 are
2SC789A (2SD313R)
2SA489A (2SB507R)
The transistors in paren being sub's and used interchangeably.
Here's the replacements for them. The info is Mouser's.
to-220 outputs
q11,12 2sc789
512-KSD526Y npn to-220 bce 80v 4a 30w 3mhz 15-240hfe $0.64ea
q13,14 2sa489
512-KSB596YTU pnp to-220 bce 80v 4a 30w 3mhz 15-240hfe $0.59ea

Replace any and all 2sa725 or 2sa726 transistors.They are on the top ten wanted list for transistors going bad. Rebuild the power supply (caps, transistors, check Zener's repl if needed.)

On the Amplifier board, if the Q1 thru Q4 transistors are the 2sa763, these are generally fine. But if 2sa726 change them. Make sure they match for hFe as they are part of the BIAS circuit.
http://audiokarma.org/forums/index.php?threads/latest-sx-535-cap-and-transistor-list.515082/

It's possible that the Variable resistor (pots) either shorted or went open, causing Q12 to open up. Check and if necessary replace with Bournes 100 ohm 362 series pot. Generally the pots are junk and cheap enough to just pre-emptively replace.

You can search the individual transistors in the PIONEER FORUM. Note that a transistor may have 3 or 4 different replacements, depending on what circuit and how heavily it's being used. When I use the search function, I also look for the same unit, or series(SX-x2x,x3x,x50,x80) and the board it's in. Sometimes you strike out, but most times you can find anything in a transistor that pioneer used along with it's replacement. Get the newest replacement usually.

 

[W.T. Holt]

Thanks for the reply! I have ordered all the replacement transistors for the amp board based on your advice, once they arrive its time to dig in. If that doesn't do the trick I suppose its on to the power supply