STAX SRM-Xh (early Mosfet version)

[Ulfar4]

Hello all,

I have this unit, but it stopped working so I opened it up. It seems that in right channel are shorted mosfets on output stage. But LEDs are not lit on both channel so I need to check it more. 
I started drawing schematic from the board. Since this is only one sided pcb, it was easy. But there is one thing I don't really understand. It is the cascode pair of 2SA1699. It should be that emitter is connected to the drain of the 2sj109 and output is at collector. But there it is connected differently. Or there is something I don't see.

If we look on the board, there is drain of the 2sj109 and it is connected to the base of 2sa1699 according to datasheet.

 

Maybe someone has full schematic of this board or can point me out for what is wrong.

There are full pictures of this board.

 

 

 

[kevin gilmore]

pretty sure you have the pinout of the 2sa1699 wrong.

emitter connects to source of the jfet. At least it does for srm212 and srm252

 

[Ulfar4]

Thanks, it must be it. Because then would also not work current source for the jfet.
Yeah, my bad, of course to the source, same as for those amps as you say.

Anyway, I updated schematic in first post.

Now I desoldered all 2sk216 and two of them are dead. 
Those mosfets were on one side of the right channel. I will disconnect amplifier part from power supply and see if the rails measure ok. Any other ideas what else should be wrong?

[Ulfar4]

The power supply rails are ok as it measures around +-230V without load.

[spritzer]

I've fixed at least five of these over the years, all with bad mosfets so they just fail.  Once swapped out the amps spring to life. 

[Mach3]

I know caps have about 20 years life span, didn't know that applied to mosfets too. Or is it longer than 20 years?

 

[spritzer]

No, not at all but they are very sensitive to static which is the most likely culprit