Advice about repairing an IBM 5151

tony duell ard at
Tue Sep 1 09:59:13 CDT 2015

> > You need to trace on from pin 8 to transistors on the main board,
> > right up to the flyback transformer.
> >
> I did. The 18KHz signal is present when it enters the board from the
> wire, it is also present on the collector of the following 2
> transistors (the TR21 and TR22 which are part of the brightness
> limiter and horizontal driver respectively), also at the output of the
> T501 transformer and at the collector of the TR23 transistor (which is
> labeled as Horizontal out). I then traced at every pin until the
> flyback transformer input (pin 4). According to the schematic and the
> board it seems that is the signal path from the moment it enters the
> logic board until it reaches the flyback transformer.

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?

> However the flyback transformer is definitely not outputting the
> voltages it is supposed to, as the manual tells me to measure 56.8V at
> the cathode of diode D504 which is basically connected to the output
> pin 6 of the flyback transformer and I only measure few mV.
> Could a winding of the FBT be burned somehow?

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


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