Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Sat Aug 29 20:24:14 CDT 2015

I have a normal IBM PC 5150 (not a 5155) with the usual MDA card
inside. So when the computer is on, no picture comes on the monitor.
So I am trying to diagnose that.

(on a side note I do have a few of both monochrome cards as well as
another IBM PC XT - 5160 as well as 2 other 286 based machines I can
try, just in case the issue is NOT with the monochrome monitor but
with the card/PC assembly).


On 29 August 2015 at 21:17, william degnan <billdegnan at> wrote:
> There is a 2nd connector for the video card of the 5151. I can't quite tell
> if you mean you have the video card that comes with the 5155 and you're
> trying to test with a card that came from somewhere else or something else.
> I am unsure if the 2nd connector pertains to the internal or external port,
> but you want to be sure you have this in place.
> .
> On Sat, Aug 29, 2015 at 9:07 PM, Vlad Stamate <vlad.stamate at>
> wrote:
>> Hi all,
>> I am trying to identify why my IBM 5151 display has no picture when
>> connected to a normal MDA card in a IBM PC 5150. So armed with an
>> oscilloscope, digital multi-meter and the SAMS Computerfacts for it I
>> started to investigate. First, the card does send out signal and that
>> signal does reach the board inside the monitor. I checked the power
>> part of the circuit, all diodes and the transistor check fine. I
>> probed some of the vertical and horizontal transistors and there is a
>> signal there too. Then on the video board both TR19 and TR20 have on
>> their collector and emitter (respectively) a signal (95Khz). What else
>> can possibly be wrong?
>> The SAMS manual talks about "injecting a video signal" at different
>> pins however I have no idea exactly how to do that.
>> So there is current going out of the power part and there is signal on
>> the video board attached to the neck of the CRT.
>> Regards,
>> Vlad.
> --
> Bill

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