Paul, are you sure it isn’t the big resistor? It sure looks toasty.

On Mon, Jul 21, 2025 at 12:16 PM Paul Newland, ad7i <ad7i@ad7i.net> wrote:

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.

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

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.

Paul, ad7i

On Mon, Jul 21, 2025 at 10:22 AM Erik Bruchez <erik@bruchez.org> wrote:

Hi all,

I am trying to troubleshoot a Model 33 selector magnet circuit. I don't know for sure yet but there is a chance that it's the Q1 transistor on the board. I am wondering if anybody here has ever replaced this transistor, and if so with what (possibly more modern) part. The original is Motorola-branded and labeled 181671. The schematics call it a "high gain" transistor.

-Erik

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