[GreenKeys] Model 33 Q1 transistor replacement

[Erik Bruchez]

Paul,

It's probably not all that difficult to find a working replacement, but in
> a different form factor.   Can you show a larger portion of the schematic?
> I'd like to see if this transistor is operating as an on/off switch, or if
> it's trying to work some magic to regulate the voltage on the magnet for
> fast pull in and then low current idle.
>

I attach the whole thing. There are explanations in the schematics.

I measure about 0 V on the base of Q1, but also about 0 V on the base of
Q2. My understanding (which could be wrong) is that when one is on, the
other one should be off, and vice versa, which means that 0 V on Q1 should
lead to about 4.7 V on the base of Q2. In any case, I am not getting any
voltage to the selector magnet through Q2 (the large transistor on the heat
sink, separate from the board), which is the original problem I am having.


> Some basic DC measurements might give you some insight into the health of
> the transistor without removing it from the circuit board.
>

This is what is leading me to Q1: it seems that there is continuity between
emitter and collector, which I don't think there should be! I have another
instance of the board where this is not the case. (However, that second
board also doesn't seem to work, but there might be something else wrong
with it.) I could desolder the transistor on that one and transplant it to
see if that gets my first board working, but I was trying to avoid doing
that immediately.


> I'd be curious to know the voltage from base to emitter when the
> transistor is "ON" (or supposed to be on).   Also (can't see that part of
> the schematic) but if there's a resistor in series with the base I'd be
> curious to know the voltage across the resistor (and the resistance of the
> resistor), and then use ohms law to determine the base current.  Same thing
> for the selector magnet current, if that's possible.
>

-Erik
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