sx 950 050 board removal?

I've decided to try to repair this burned up spot on my 950.
If anyone thinks this is not a good idea please let me know.

I've been only using the preamp section and have recapped the PS, Equalizer,
and Tone control boards already.
Had this for a long time and knew it was burned up but after closer inspection and maybe having a little time right now it doesn't look that hateful. Nothing on the solder side looks burned at all so I thought I'd take a chance and replace what looks burned.
Biggest problem is getting this board out to work on. I've gotten the screws out and taken the tension out of the bottom wires but it doesn't have very much give to it. I assume I'm going to have to disconnect all the top wires to the outputs.
Will I be able to tilt the board and work on the parts I need(see pics)or is there a better and smarter way?

Hope the pics are ok to see the bad spot.

I hope others don't hesitate to reply if they see I posted, as I haven't had one in my hands yet, but do you see the little two-leaded components going to the heatsinks? there are two of them, they are the FRAGILE bias (thermal) diodes, the wires WANT TO BREAK.... be very very careful with them. Remove their holddown screws, and without bending the wire to component body connection AT ALL, tuck them into a safe area on the printed circuit board.

Second if the UPPER wires are impeding you I would explore removing the power transistors from their sockets on the heatsink, that will free up the sockets and then the heatsink should be able to be removed to give room.

You can test the power transistors when they are out, I suspect several are blown from the burns I saw. Get ready to get more mj21193g's and mj21194g's unless I miss my guess (I'll check later).
edit: The 2sd427 and 2sb536 output transistors are 120v 8a 80w (5mhz pnp 7 mhz npn) to-3 power transistors and the mj21193,4's at 250V 16A 250W 4mhz will do quite nicely.


NEVER open up those "spiral" wire wrap connections you were contemplating opening.

Lastly there is a burned area between those two white 0.5 ohm power resistors, but there is a resistor on the mirror image other half of the board that looks burned too, is it?

Take the panel off the inside of the back that covers the business side of all those RCA jacks. Unbolt the heatsink, remove the thermal diodes from the heatsink and gently bend them out of the way. Unbolt the galvanized plate that supports the driver board, and work it out of the unit. Now you can flip the whole heatsink to the front of the machine to get to the back of the driver board.

markthefixer said:

I hope others don't hesitate to reply if they see I posted, as I haven't had one in my hands yet, but do you see the little two-leaded components going to the heatsinks? there are two of them, they are the FRAGILE bias (thermal) diodes, the wires WANT TO BREAK.... be very very careful with them. Remove their holddown screws, and without bending the wire to component body connection AT ALL, tuck them into a safe area on the printed circuit board.

Second if the UPPER wires are impeding you I would explore removing the power transistors from their sockets on the heatsink, that will free up the sockets and then the heatsink should be able to be removed to give room.

You can test the power transistors when they are out, I suspect several are blown from the burns I saw. Get ready to get more mj21193g's and mj21194g's unless I miss my guess (I'll check later).



NEVER open up those "spiral" wire wrap connections you were contemplating opening.

Lastly there is a burned area between those two white 0.5 ohm power resistors, but there is a resistor on the mirror image other half of the board that looks burned too, is it?

Click to expand...

Learned my lesson on bias diodes on my 2230 that you guys are helping me with on one of the other forums.
That was my point about those wire wrap connections. They don't look very appetizing!!!!!!!
Roger on checking the outputs.

EchoWars said:

Take the panel off the inside of the back that covers the business side of all those RCA jacks. Unbolt the heatsink, remove the thermal diodes from the heatsink and gently bend them out of the way. Unbolt the galvanized plate that supports the driver board, and work it out of the unit. Now you can flip the whole heatsink to the front of the machine to get to the back of the driver board.

Click to expand...

Thanks EW. I appreciate you sharing that. As I told Mark I've lived and learned on those diodes. I'll probably give it some time in the AM. Getting late on the East coast.

Thanks again.

Ok, the burned resistors are R37 and R39, 150 ohm 1/2 watt resistors that are intimately involved with the (Q9) (2s)D381 and (Q11) (2s)B536 transistors mounted on those shiny u - shaped pieces of aluminum, they are the "drivers".


With transistors Q9 & Q11 damaged, Q5 and Q7 could also have been damaged. D7 and D9 should be checked as well, but damage is less probable because they are 1 amp big epoxy cased diodes and not little glass signal diodes.

There is an IMPORTANT circuit that includes Q5, Q7, the thermal diode (D5: STV4H) and the bias (VR3 100 ohms) adjustment potentiometer. Some fault in this area can and probably DID produce your smoking ruin. This part of the circuit must be stringently tested before applying power again to keep the smoke in and the parts co$t DOWN.....

edit: here's a list of substitutes I've been working on lately:
q1,3 2sa726 bce mouser part # 512-KSA992FBU (low noise)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
q5 2sa818 ebc mouser part # 512-KSA1142YSTU to-126 ecb 180v .1a 1.2/8w 200mhz 100-320hfe $0.28 ea
q7 2sc1628 ebc mouser part # 512-KSC2682YSTU to-126 ecb 180v .1a 1.2/8w 200mhz 100-320hfe $0.31 ea
q9 2sd381 bce mouser part # 512-ksc2073tu_q to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.53
q11 2sb536 bce mouser part # 512-ksa940otu_Q to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54
------outputs------
2sd427 to-3 mouser part # 863-MJ21194G to-3 NPN 250v 16A 250W 4mhz 8-75hfe $4.10
2sb557 to-3 mouser part # 863-MJ21193G to-3 PNP 250v 16A 250W 4mhz 8-75hfe $4.10

Check the outputs and emitter resistors. Drivers simply do not fry without dead outputs.

I really appreciate all this good instructtion. I'm probably not going to find the time today but will get to it as asap. I bought this unit with a pair of speakers about 5 years ago for $5 knowing it had been fried as told by the owner so I can't really say what happened. I''ve got some parts ordered and some on hand so I thought I'd give it a shot.
The pre section is great and it's such an impressive looking item with some good power stats so it's hard to resist trying to fix it.
I'll post any progress and I'm sure to need help when I get the parts.
Thanks
John