Trying to salvage a B&K ST140

[jwv3]

I have a B&K ST140 with one channel's driver board burned up. I'd like to know if anyone has a schematic for this circuit board. I'm also wondering if I can drive all four output transistors from one driver circuit. Just make a mono subwoofer amp out of it. It uses a K135 and a J50 output transistor, push-pull, on each channel. Any help would be appreciated.

[djmiddelkoop]

jwv3,

sorry I have no schematic for this amp, but I can tell you that the K135 and the J50 are Mosfets specially designed for audio amps.
The original Hitachi are however obsolete.
I just repaired a Sphinx project 14 which has a lot of them inside.
The Mosfets I got from Farnell.com who has them in their catalog, although they were manufactured by some unknown source, they turned out to be OK.

Altough Mosfets in general have a high DC input impedance, the AC or signal impedance can be quite low due to their input capacitance. The driver stage has to be designed for this job.

good luck,

Dick.

[mcp]

Hi jwv3

I have a schematic of the ST140. It is not very sharp, but enough to make out the parts. If you like a copy, just send me a mail.

Cheers

[rob hubby]

hey guys i know this is an old post but i"m experiencing the same problem with my b&k st140. one channel is dead, any help would be graciously appreciated.

[nigelwright7557]

Originally posted by jwv3
I have a B&K ST140 with one channel's driver board burned up. I'd like to know if anyone has a schematic for this circuit board. I'm also wondering if I can drive all four output transistors from one driver circuit. Just make a mono subwoofer amp out of it. It uses a K135 and a J50 output transistor, push-pull, on each channel. Any help would be appreciated.

You could use different MOSFET's but this would also mean realigning the bias circuit.
Applying the wrong bias can destroy the output transistors.