Hi, can anyone supply me with a circuit diagram for an AW B92 stereo system, circa 1971? A friend's got one he'd like me to fix, but there's parts missing from the amplifier section; the previous owner's looted the power transistors.

If it's this model, then the transistor complement is listed here, if that helps:

http://www.radiomuseum.org/r/amalgamate_awa_b92y.html

OK, thanks. So it used four R.C.A. 40050 power transistors. Maybe if there's nothing else missing I can put it back together.

Also replace the bias diodes or transistors as a precaution. Most common reason for output stage failure.

The amps themselves should run at very low power without the output transistors fitted. That is a good check that all is well before committing a new pair of transistors.

You should consider replacing the germanium 40050s with silicon 2N2955s. They are physically compatible, more readily available and much tougher. You will need to adjust the bias of course but that's easy.

These units actually sound quite nice. A son-in-law has one.

Right, that explains why he could hear the radio with the output transistors missing! I've got the circuit now. How would I adjust bias for silicon transistors, increase the resistance of the 39 ohm output transistor base resistors? 82 ohm perhaps? Would any silicon diodes do for the bias diodes?

If you email me the circuit I can tell you.
It's interesting because that design used germanium outputs but silicon drivers. It there are temperature diodes they will probably be silicon.

Circuit emailed to you Ian.

Thanks Sue

As I said in my email, probably just adding an extra diode would do. If not, use a 1k trim pot and set the bias to the point where the output transistors just start conducting. You can measure this across the 1 ohm resistors.

That's an old trick with this kind of amplifier. Get it running on the drivers before you fit the output transistors. Very useful with large, high-power amplifiers where otherwise you'll let the smoke out very quickly.

By the way, if for any reason they meet with an accident and you need to replace any of the other transistors, an AS147 is the same as a BC547. An AS157 is a BC557. But those AWV transistors were quite reliable if a little fragile in the leads.

I've got the chassis out, but it's bad news; apart from the output transistors, both the phono preamp and the amplifier boards are missing. I've got a spare phono board in the junk box, but not the amp board. Does someone have one they don't need? It's approximately 5 by 7 cm, has eight small AWV transistors, and is marked 726B5 on the copper side.

Continued in the Wanted and Tech threads.