Samson Servo 260 amp

[WaveyDipole]

Having recently picked up one of these spares/repairs for pocket money, and figuring that since it uses discrete components that it should be repairable I was hoping for a minor repair. It turns out I got more than I bargained for. On powering up I got the dreaded smoke and so quickly turned it off!. Further investigation showed that the output section on the left channel was seriously damaged. Three of the four output transistors were shorted, one of the driver transistors was blown and 3 of the ceramic resistors were open. There were also other faulty components including one small transistor and half a dozen resistors.

I had a look around on the Internet and found that the output transistors (2SD2155/2SB1429) are not easy to come by, and are not cheap, although there appear to be some available from the States on eBay. I temporarily replaced the blown driver transistor (2SC2238) with something I had to hand of a similar rating but with slightly lower collector voltage. Since the power rails are +51v-0--51v I figured 160v collector voltage still gave plenty of safety margin. The case of a small TO92 packaged 2SA968A was split apart and this was replaced with a 2N2907A which having a close enough rating was available to hand.

Given that the output stage was comprised of two identical pairs in parallel, I decided to test whether the unit was viable by disconnecting one pair from the right hand channel and 'borrowing' the output transistors and a ceramic resistor to fix the left channel. This left me with just one pair of output transistors per channel instead of two, but the protection has now disengaged and I am getting distortion free sound on both channels and the oscilloscope shows clean sine waves on a 1kHz signal.

The amp is probably not  its not worth spending large amounts of money on and I can live with the lower power output, but it would be nice to get the amp up to its original spec provided I can do so within reasonable cost. Does anyone know whether there a more commonly available substitute I can use for the 2SB1429/2SD2155 pair? The spec is collector voltage 180v, collector current 15A, base voltage 5v. A search on Farnell only turns up a single Darlington part and I couldn't find the complimentary part to go with it. Also, since the power rail is +51v/0v/-51v hence 102v top to bottom, why are all of the semiconductor components in the driver and output stages rated at 180v? Could something with a lower rating, say 160v, 140v or even 120v be used?

Secondly, is it safe to run with a single pair as is, or is some modification required to limit the maximum power that might be demanded of the single output pair? I expect that with only one output pair I will get half the power, but do the current limiting resistors need to be increased? For example, should the ceramic resistors be changed from from 2.7ohm to 0.5ohm? I notice that the lower model (170) has exactly the same circuit but uses different value biasing resistors in a couple of places to derive a lower power output.

[WaveyDipole]

I think I have found an alternative to the 2SD2155/2SB1429. RS have stock of MJL3281AG/MJL1302AG devices which are the same spec, except that the collector voltage is slightly higher. I can actually replace all 8 devices for less than it would cost me to replace 4 buying used parts from eBay. I understand that the complementary pairs should not be mixed and matched, but would I need to replace all 8 devices, or could I get away with replacing just the 4 in the left channel and leaving the original 4 in place on the right channel?