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flyingtele 21st February 2012 02:16 PM

2SD427 transistor for Rotel RA-812, need info please!
Hello all, I am currently repairing an old Rotel RA-812 and have struck a problem (well, hopefully not a problem). Having tested it etc, I discovered one channel had 37 Volts or so on one of its outputs, which is not good.
I traced the problem to the driver transistors, one of which was open circuit and the other was fused closed circuit. But then I still had DC on the output, to the tune of 13 Volts. After some more prodding I discovered one of the output transistors had no emitter connection and had a resistance of 100 Ohms between the collector and base. Something strange about the design is that it uses two of same transistor per side, rather than a complimentary NPN/PNP pair.

Now here is my problem, the output transistor is a 2SD427 and none of the suppliers here stock it, the best they can do is a couple of weeks wait at a cost of $35 which I am not too thrilled with. Can I use a 2N3055 in its place? Obviously if using a different transistor then it would be best to replace both the transistors in the channel, or even the amp which is not a problem for me as the 2N3055 is readily available and very cheap. Any info or help as to whether it will work satisfactorily would be great. A bit of a long story for a simple question, but hopefully it was interesting :)

I'm looking forward to hearing this amp, as inside its built like a tank and has lasted well with just a couple of niggles to sort out. Has anyone had any experience with this amp?

Pars 21st February 2012 02:40 PM

B&D Enterprises (which is a good US source for many Toshiba and other 2SA/SC etc. transistors) shows MJ15015G as a replacement. You would need to verify that that is indeed a suitable replacement.

There is also a thread here on replacements:

jaycee 21st February 2012 04:39 PM

MJ21194 would probably be your best bet for output transistors. If you have one blown, you should replace both.

Using all NPN is not that strange - quasi complimentary output stages were very common in those days, as PNP devices were not as good and usually a lot more expensive.

flyingtele 22nd February 2012 04:33 AM

Thanks for your suggestions, I have ordered two MJ21194 transistors :)

Bibliophile 22nd February 2012 05:35 AM

2SD427 is available from WES Components, Sydney, $8 each trade price.
Catalog I have is a couple of years old so things may have changed.

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