Audio power transistor replacement

[Tony Skitt]

Hi

I have a 1975 Pioneer SX-1010 receiver which died on me yesterday - this
came as quite a shock, as I had been expecting it to last forever.

Anyway, the fault was easy to locate - two blown output transistors, and
after removing them the receiver is working again (it uses a total of 4
output transistors per channel, i.e. 2 x NPN and 2 x PNP). Of course I
need to be careful not to turn it up to loud as otherwise the surviving pair
will go too!

Can anyone suggest where I could get some replacements? Presumably I will
need a gain-matched set of 4 components, but I don't think the actual type
can be that critical, as the receiver doesn't even use the type shown on the
circuit diagram!

Circuit diagram: 2 x 2SA747 plus 2 x 2SC1116A
Fitted in receiver: 2 x 2SA679 plus 2 x 2SC1079

Alternatively, could I use single transistors instead of a parallel pair?
Presumably modern devices are available with higher current capability?
Probably better Ft as well. (The Pioneer has +/- 53 Volt DC rails, and
delivers about 116W/channel into 8 ohms). Anyone tried this?

Regards

Tony Skitt

[TIM SCHWARTZ]

Tony,

You want to be careful about fitting higher frequency devices, as ths
can often lead to parasitic oscillations, especially at clipping, so I'd
suggest NOT using transitors with a much higher F(t). I'd also stay
with the paralleled output transistors.

SPECS:
(PNP) 2SA679 2SA747
120V 120V
12A 10A
100W 100W
40-140 beta 60 beta
6 MHz 15MHz

NPN 2SC1079 2SC1116A
120V 140V
12A 10A
100W 100W
40-140 beta 60 beta
4MHz 10MHz

I would certainly replace all of the transistors in the channel, even if
you find original devices. For substitutes I'd consider:

2SB554 (PNP) and 2SD424(NPN) 180V, 15A, 150W, 40-140 beta, 10Mhz, likely
available from MCM Electronics (www.mcmelectronics.com). Another choice
would be MJ15003(NPN) and MJ15004 (PNP).

Don't forget that you will need heatsink compound when you put them in.
Also, it is VERY IMPORTANT to set the bias in the amp. If it is too low
you'll have awful distortion, and if it is too high the channel will run
hot and fail again.

Please e-mail me directly if you need more detail.

Regards,
Tim Schwartz
Bristol Electronics

[Freud]

If I may, I'd like to add just one more thing: I would also check the
drivers for the output transistors on that channel; very often, they fail
when the output transistors do.

Hope this helps,
Pierre Therrien

[Doug King]

Try www.grandata.co.uk they have most of them listed, I would not worry
about matching, you probable will not notice any difference?

Doug

--
Doug