Trace Elliot AH-300 repair help

[Brouilly]

Hello,

A friend asked me to look at his AH-300 that had a pint spilt down it. It had a dangerously high DC voltage at the output, so the output transistors were replaced, now there is no DC at the output... good. But It still doesnt work.

The negative PS cap looked like it was damaged, it was reaching -70v but it never discharged, so I replaced it. Now the replacement doesnt discharge at all, or takes a very long time.... not sure about that.....

I have traced the signal to the first BF423 in the power amp section, where the signal stops. So my question is, can transistors fail even if they havnt shorted?

Any help and comments are really appreciated.

Thanks

Charlie

 

[mrfeedback]

Output transistors usually fail C-E short, B-E measuring ok.
That said is is not uncommon that they fail with all three legs shorted.
C-E intact and base open circuit i don't think I have seen.

Eric.

 

[indianajo]

Transistors can burn the little wire from the leg to the die, like a fuse. Usual BJT check is double diode test. Pull base out of circuit, use meter on ohms BE then CE forwards (good is 4-600 ohms usually) then backwards (good is 1999 ohms). Forward direction depends if NPN or PNP. My analog meter works on all scales but the Sears DVM seems to not see forwards on 200 ohm scale, needs 2000 ohm scale.
eFET's seem to be dead all ways until you get high enough in voltage to trip the zener protection (if any) so check your catalog when checking to make sure you don't have one of those. For more details on devices & meter use, see "Electronic Devices, the electron flow version", by Thomas Floyd, a community college text I got for $2 (obsolete problems). Maybe your community colleges there use some other useful text.