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 AWA B92 stereo
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 Return to top of page · Post #: 1 · Written at 2:34:35 PM on 5 October 2017.
Sue's avatar
 Sue
 Location: Daylesford, VIC
 Member since 13 January 2011
 Member #: 809
 Postcount: 265

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.


 
 Return to top of page · Post #: 2 · Written at 2:41:17 PM on 5 October 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 3972

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

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


 
 Return to top of page · Post #: 3 · Written at 11:31:26 PM on 5 October 2017.
Sue's avatar
 Sue
 Location: Daylesford, VIC
 Member since 13 January 2011
 Member #: 809
 Postcount: 265

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.


 
 Return to top of page · Post #: 4 · Written at 10:49:13 AM on 6 October 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 608

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.


 
 Return to top of page · Post #: 5 · Written at 11:59:46 PM on 6 October 2017.
Sue's avatar
 Sue
 Location: Daylesford, VIC
 Member since 13 January 2011
 Member #: 809
 Postcount: 265

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?


 
 Return to top of page · Post #: 6 · Written at 12:00:52 PM on 8 October 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 608

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.


 
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