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PNP where an NPN should be

kwayde

kwayde

AK Subscriber
Subscriber
I've been working on this G-9700 that was stored for decades. Fired it up in FM mode and it took hours for the display to work good and for power to even out. Fired it up again the next morning and the same process began. Decided to rebuild it, new caps, etc. Once it was rebuilt I fired it up and it stayed in protection where I found I missed replacing cap C617 I removed on the AF power supply board. Put a new cap in, fired it up and it stayed in protection, then went dark. Now I get nothing. Started troubleshooting and found a wrong transistor in place on the RF power supply board. Pulled TR602 which should have been a 2sc945 (NPN) but was a 2sa733 (PNP). I'm wondering why the unit worked the first time I fired it up (after a few hours) with the wrong transistor in place.

Screenshot (369)1.png
 
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Unless you have concrete evidence the unit was previously worked on and botched, you would be wise to replace like for like what's on the board rather than use the schematics.

Schematics can have errors. The unit worked with the parts on the board (on the presumption it is original).
 
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whoaru99 said:
Unless you have concrete evidence the unit was previously worked on and botched, you would be wise to replace like for like what's on the board rather than use the schematics.

Schematics can have errors. The unit worked with the parts on the board (on the presumption it is original).
Click to expand...

There is evidence of previous work being done. I think someone put the wrong transistor in. TR602 is listed as a 2sc945.

Screenshot (368).png
 
FWIW reversed bias transistors usually act like a ~7 volt zener - so would expect to see about 6 volts on the output of the regulator instead of 13, and the control pot would have little or no effect. Maybe warming up would lower the reversed transistor breakdown voltage, and let the voltage go high enough for the circuit to work. This is totally off the cuff, so YMMV
Also - Sansui uses that type of regulator circuit in several amplifiers, and that transistor should be an NPN.
 
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