[GreenKeys] Model 33 Q1 transistor replacement

[Erik Bruchez]

Chuck, all resistors measure correctly, even though a few of them look
toasty. But that's expected on these boards based on what I have seen!

On Mon, Jul 21, 2025 at 11:30 AM Chuck Rehor <chuck at rehor.com> wrote:

> 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 at 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 at 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
>>>
>>> [image: C6BC2216-1423-4DCB-A6BF-7869D424140C.JPG]
>>>
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>>
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