the front panel, it says 3500 Mosfet. However, when I disassembled
the amp, I was surprised to find that it has been upgraded
to the newer bipolar output model. In fact, 3500B is printed
on the pcb.
power transistors are four pairs of 2SD2155 and 2SB1429. The
amp looks like it has been repaired before. Further testing
revealed a blown pair of 2SD2155/2SB1429. The drivers were
checking around, it seems like the 2SD2155/2SB1429 are no
longer available. Probably discontinued. That doesn't leave
me much of a choice but to replace them with Toshiba 2SC5200/2SA1943.
Naturally, that would mean I have to rework the entire circuit
(a lot of work). The upside is it gives me an excuse to upgrade
the VAS with beefier TO-126 types. I'm not too much of a fan
in using TO-92 transistors for the VAS, particularly with
rails of +/-88V.