28 Sep
2026
28 Sep
'26
8:11 a.m.
On 9/26/26 08:01, Rob Jarratt via cctalk wrote:
Some time ago I posted asking for help with my VT100
monitor board and flyback. This is the monitor board that
uses Germanium transistors and for which there is no known
printset available. The conclusion was that the flyback
had failed, proved by the burning smell and poor ring test
results, and described at greater length here;
https://robs-old-computers.com/2026/03/30/vt100-video-display-failed-flybac….
I have managed to get a NOS replacement flyback. The ring
test shows that it is good. However, I have enough
suspicions about the two Germanium transistors (Q414 and
CR406) that I don't want to just slap it in and hope it
works, especially as I have little chance of ever finding
another flyback. The primary coil of the flyback, driven
by these two transistors, is the one that is shorted in
the bad flyback and I don't know if the flyback failed
because of these transistors or not.
Are there any tests I could do on the monitor board prior
to fitting the new flyback? Could I source any kind of
modern inductor to act as a sacrificial part that would be
enough to prove correct operation?
I would just replace the transistors with Silicon ones,
designed for flyback use. Generally, you can replace Ge
with Si quite nicely. Yes, it is possible that the primary
can be substituted with a beefy inductor, but there might be
other windings of the flyback that would absorb excess
energy, and without those it could develop excessive voltage
and cook the transistors.
Jon