Advice about repairing an IBM 5151

tony duell ard at
Sun Aug 30 15:29:59 CDT 2015

> > 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

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