IBM 5151 (MDA display) - common faults / gotchas?

Brent Hilpert hilpert at
Sun Nov 9 04:19:28 CST 2014

On 2014-Nov-08, at 9:03 PM, drlegendre . wrote:
> Hey Brent,
> I guess I'm still confused.. and for the purposes of my inquiry &
> discussion, let's stick to what's going on with the Osborne / Zenith setup
> - OK?
> It sounds like you're saying that the horiz. osc. in the Zenith monitor is
> synced in both phase & frequency to the horiz. input signal provided by the
> computer motherboard. But if the horiz. signal were to disappear for some
> reason, the osc. would continue to run... is that correct?

That would be the idea, you'd still have HiV, and you'd still have some semblance of a ..raster.

> So if the osc. continued to run in absence of the horiz. signal, how would
> it stay on-frequency?

> And much more importantly -  If it's capable of
> setting its own operating frequency, how does the horiz. signal input
> override that?

Speaking vaguely, each sync pulse restarts the oscillator to the beginning of a cycle.
But the free-running F of the oscillator should be near the target F so that the sync pulse only has to 'pull' it a little ways (in time).
When you adjust the H-sync control of an (old) TV/monitor you're adjusting the H oscillator F, to bring it into the capture range of the sync control.
Electronically, it can be accomplished in a number of manners, from a simple 'reset' of some sort to PLL style phase detection.

> And why didn't Osborne "trust" their hardware to provide a
> horiz. signal - while IBM and (as you say) Commodore PET clearly did -
> ultimately, what did this amount to, if anything?

Might be just a matter of who they sourced the monitor from.
It was the beginning of the period of transition from 'TVs' to 'computer monitors'.
There are some other benefits to having the raster and HiV there when sync is lost, such as the visible raster may give some clues as to a problem.

In the Zenith case, how much difference is there between a 555 astable and monostable? (A: not much).

> Sorry for so many dumb questions. Oh, and here's another.. about the
> meaning of 'raster'.
> When we talk about  the presence or lack of a /raster/, do we mean the
> 'lighted rectangle' on the CRT screen? Or is the raster the horiz & vert
> travelling beam that 'paints' the lighted rectangle? I ask, as some video
> t-shooting guides have determiners like "Raster present / No raster?". I
> ask, as I was taught that the scanning beam itself is the 'raster'.. but it
> seems more like the evidence of that beam - a lighted rectangle - is what's
> actually meant.

I've always taken raster to mean, as you say, the lighted rectangle, drawn by an electron beam in a progressive line by line scan.

The mere presence of a drawing beam would encompass many/all other types of CRT displays besides raster displays.

> And yes, I know the derivation of the term. Raster is from Greek 'rastrum',
> essentially meaning a rake.. which makes some sense, of course, as the H/V
> systems produce a pattern on the screen much like a rake dragged across
> sand.
> On Sat, Nov 8, 2014 at 10:26 PM, Brent Hilpert <hilpert at> wrote:
>> On 2014-Nov-08, at 7:24 PM, drlegendre . wrote:
>>> Tony,
>>> Ok, then I guess I +really+ don't get it.
>>> Why does the Zenith +need+ a horiz. osc. when it also requires the horiz.
>>> input signal - which just happens to be at the exact same frequency
>>> (15.7KHz) as the horiz. osc? What is the horiz. input used for, then, if
>>> not to set the frequency of the 555 output?
>> No, in a strict sense, a TV or monitor doesn't 'need' a horizontal or
>> vertical oscillator, it just needs the V & H sync pulses to start the ramp
>> generation at the proper time.
>> Historically however, a TV had V & H oscillators so that:
>>        1. A raster would be present when there was no signal, in part so
>> it didn't burn a hole in the center of the screen.
>>        2. The raster would ride through sync pulse dropouts on weak or
>> marginal signals, rather than collapsing sporadically.
>>        3. The horizontal oscillator needed to keep going, consistently,
>> so the CRT HiV would be generated and stable.
>> (One might argue 3 mitigates 1, no HiV, no burn).
>> Once you have the oscillators, you still need the sync pulses to keep the
>> raster scan ramps in phase sync with the video picture information.
>> Early computer monitors were just slightly hacked/improved versions of TV
>> monitors and old design habits die hard.
>> As mentioned,  some designs did do away with the oscillators, when the
>> sync could be relied upon to be present and stable. IIRC, the Commodore PET
>> with it's built-in monitor was another of those.

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