Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Tue Sep 1 12:56:32 CDT 2015

On 1 September 2015 at 10:59, tony duell <ard at> wrote:
> What do you see on the collector of TR23, which should be a small power
> transitor (TO220 package I think). If it's working properly there would be some
> very high voltage spikes (kV) there, but under these conditions probably not. What
> is the average DC voltage there?

Hmm, you are right, I am not seeing any spikes in voltage there. On
the collector of the TR23 all I am getting as DC voltage is around
110mV. The oscilloscope tells me I am getting Vpp of about 4.4V with
Vmax at 2.6V and Vmin at -1.8V and an average of 105mV (which matches
what my multimeter tells me too).

That sounds too low doesn't it?

> Could be, You have a schematic, the flyback windings should be very
> low resistances. Check them with the ohmmeter.

I did measure the resistances across pins of the flyback transformer
on the same windings and I am getting (what I think are) good results:
1 or 2.2 Ohms (for the 2 windings left and right, not the upper one,
as I do not want to touch the high voltage stuff for now).

> -tony

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