Re: A question about mono monitors

["Michael J. Mahon" <mjmahon@aol.com>]

Tristan Mumford wrote:
> Michael J. Mahon wrote:
>
>
> > Tristan Mumford wrote:
> >
> > > Hello!
> > >
> > > This is a semi A2 related post. It's about something I have noticed but
> > > cannot explain.
> > >
> > > Recently I tried what is apparently an exact clone Videx 80 column card
> > > in my //e. It sent my 1084S berzerk. I didn't think too much of it at the
> > > time.
> > >
> > > Today I built a fully accessorised XT. I chucked in a card I found with a
> > > composite out, figuring it was just a standard CGA card. It was a Persyst
> > > BoB, which is apparently some kind of super CGA card.
> > > Getting back to my point. I tried it with the 1084S. Same thing happened.
> > > So I dug a Micron 3 mono monitor out of the shed. It went berzerk too.
> > > After fiddling horizontal alignment until I got a stable image I
> > > concluded that it seems like the HSync is too fast.
> > >
> > > Would the Apple mono monitor have the same issue with the cards? It's the
> > > type with the internal vertical pivot for the CRT. I can't say the exact
> > > model because it's kind of buried in the shed right now.
> >
> > 80-column cards use a CRT controller chip, and its parameters are set
> > from the firmware in ROM.  If the parameters are wrong, the sweep
> > frequencies will be wrong.  Of course, it's unusual for a ROM's contents
> > to change.  ;-)
>
>
> That's very interesting. I have to wonder if that's the cause. I also
> suspect the wires soldered onto the board for the RCA out. It seems as
> though a previous owner overendowed with good intentions and thumbs added
> their own version of the videx video switching system.
> IIRC the wires are soldered on before a resistor leading to the our prong
> connector. Note the fifth prong has a solder pad but was never added.
> Problem is I don't know what it should be connected to. The scan of the
> documents I found didn't offer a hint.

Composite video is just a signal and ground, so nothing you need.

Bypassing the resistor may just be causing the video inputs to be
overdriven.  Try connecting with the resistor in the circuit, as
intended.

> > I used to have a Videx clone that produced some "flagging" of the raster
> > on the upper few lines, so I tweaked the parameters slightly and burned
> > another ROM to fix it.
>
>
> That'd do it. I'm impressed you went to those lengths to fix it.

I built the card myself, and the flagging was pretty annoying.

There was no setting of the horizontal hold that worked well for
both native (40-column) and Videx 80-column video before I made
the change.

> > (BTW, I think I know what caused my $E8 ROM to be shorted--I think that
> > someone before me put it into its socket reversed.  *That* would be
> > enough to short a ROM.  ;-)
>
>
> Yes. That would short it alright. Chips don't like being plugged in
> backwards, not that I would know eheheh eheh ehhh... It's amazing how fast
> they heat up when plugged in like that.

It's the only likely mechanism I can think of that would kill a ROM
like that.

-michael

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