Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Sun Aug 30 18:36:26 CDT 2015

Hi Tony,

I have checked the horizontal signal from the MDA using an
oscilloscope. It is pin 8 like you mentioned. I get a regular 18KHz or
so signal out of that. Pin 9 gives the vertical signal at roughly

Then I traced that on the board and it seems ok (as in I can see the
18Khz signal on other components). One thing that I noticed is that
Pin 6 of the CRT neck comes pretty much from one of the pins of the
flyback transformer (via some diodes and resistors). So I checked the
voltage on the pins of the flyback transformer itself, so here is how
it looks

1 and 8 - Not connected
2 and 4 (I think those are inputs) - 15V
3 - GND
5 - 15V (this one goes to pin 6 of the video board where you mentioned
it should be hundreds of volts)
6 and 7 - about 200mV.

Does that look correct to you?


On 30 August 2015 at 16:29, tony duell <ard at> wrote:
>> > Do you mean no picture or no raster? To me the 'picture' is the video
>> > modulation (that makes different bits of the screen light or dark). If you
>> > turn up the brightness and contrast controls do you get any illumination on
>> > the screen?
>> The screen is entirely black no matter what I do with brightness and contrast controls.
> OK.... Given the CRT voltages you give later, I would expect that.
>> > The CRT pins (from memory) are as follows (All voltages guessed wrt ground):
>> >
>> > 1: Control grid (10's of V, +ve or -ve)
>> 225mV
>> > 2: Cathode (10's of V +ve)
>> 170mV
>> > 3,4 : heater. One is ground, expect 12V or so on the other
>> 3 : 12V
>> 4: Ground
>> > 5: Control grid (see pin 1)
>> 225mV
>> > 6,7 : I call them anodes, you call them grids :-). Expect a few hunded volts on each pin.
>> 6: 7.33V
>> 7: 10.29V
>> >
>> > What voltages do you measure?
>> I checked all those with respect to ground ( pin 4).
> Well, all the electrode votlages are very low.
> They all come from the flyback transformer in the horizontal output stage. Now the 5151 is
> unusual in that it doesn't have a horizontal oscillator. The horizontal signal from the MDA
> card goes straight to the horizontal driver which, IIRC, is transformer coupled to the
> horizontal output transistor.
> So, start by checking you have a horizontal signal from the MDA card. I think it's pin 8 of
> the DE9, if not it's pin 9, the other being the vertical sync signal. If that's present and correct
> check through the driver and output stages, there are not that many components.
> If it is transformer coupled, then I have had those driver transformers go open-circuit in
> monitors -- not in the 5151 yet,  but it could happen.
> -tony
> Regards,
> Vlad.
>> -tony

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