[GreenKeys] Model 33 Q1 transistor replacement

[Paul Newland, ad7i]

Terrific photography!  And an amazing project.  That TTY cleaned up really
nicely.

Well done!

Paul, ad7i


On Tue, Jul 29, 2025 at 1:06 PM Erik Bruchez <erik at bruchez.org> wrote:

> I have noticed this and I keep it unplugged when not in use, but I
>> wondered WHY it was designed this way? Is there a good reason?
>>
>
> I don't have an answer to that question, but at the very least it is a
> UI/UX issue: one would expect that a button in the "Off" position would
> turn off power entirely, while here it only turns off the motor's power.
>
> This said, I have success to report: I managed to get the magnet energized!
>
> I resolved to transplant the Q1 transistor from one SMD board to the
> other. This did not appear to change anything, and I figured that maybe
> neither of my boards were good.
>
> I tried adjusting the "rheostat", to no avail. Output voltage varied
> between about 2 V and 3 V (but see below).
>
> Then I decided to measure the resistance of the magnet, and I measured 6
> Ohms. Finally it clicked: I had measured about 3 V on the output of Q2,
> which I had assumed was incorrect (IBM machines have solenoids which work
> at 24 V or even 48 V). Good old Ohm's law says that the current should be
> 0.5 A, which is exactly what the schematic says. So, with the swapped
> transistor, my driver was working, but still the magnet was not energized!
>
> It turned out that the issue was the way the magnet was adjusted: there
> are two screws which allow positioning the magnet assembly. I hadn't put
> them back perfectly, so the magnet was actually energized, but not well
> enough that it could pull the metal plate. One simple adjustment later, and
> it worked!
>
> After swapping back my transistors, the previously working SMD board
> stopped working. So my original Q1 transistor is definitely toast.
>
> All in all I had 3 issues so far:
>
> - dead electrolytic capacitor
> - dead Q1 transistor
> - misadjusted magnet
>
> But I can now move on to the next set of issues ;)
>
> I'd still be interested in finding a replacement Q1 transistor, so I can
> have a second working SMD board for my other Model 33 (eventually).
>
> If anybody is interested, I have pictures of the work in progress here:
>
>     https://photos.app.goo.gl/S2CtApKF8CMEmwm5A
>
> Eventually I'll write a blog post or two about it as well.
>
> -Erik
>
>
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