21-08-2018, 10:06 AM
(21-08-2018, 08:21 AM)Refugee Wrote: T305 readings are showing short circuit B-E.
You should be able to measure it on a low ohms range with the set switched off.
No, Mike has already replaced T 305 and there is no external component to cause such a fault.
Mike, your short circuit IFT theory is a red herring for two reasons:
1. If the IFT was s/c, there would still be a voltage drop across R322, so the collector voltage would still be approximately normal;
2. A silicon transistor needs ~600 - 700mV between base and emitter to turn it on but you only have 100mA, so the transistor is turned off. Thus there is no voltage drop across R322!
Try connecting your meter directly across the base-emitter junction to see that the 100mV figure still exists and is not a myth created from the rounding of the individual readings.
The chances of a PCB powered by a 6V battery breaking down is, I think, highly improbable but a resistance check across the junction will rule this out (assuming there is nothing physical bridging the tracks as you would have surely found that when changing the resistor.
Final thought for now: is C329 leaky and, thus, pulling the bias voltage on T305 base high?
No, thinking on my feet, not that! Is it C331 that is leaky, thus dragging the emitter voltage low - because there is obviously current flowing through R319 - and it's got to be going somewhere!
Actually, having worked all the way through that lot, I think a check of C331 should be your first move!
