Re: A question about mono monitors

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

Tristan Mumford wrote:
> Michael J. Mahon wrote:
>
>
> > 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.
>
>
> But the issue is which two of the four header pins do I connect to? As near
> as I can tell it's identical to the original card.
>
> Overdriving would explain the unpleasant squeal from the monitor.
>  
>
> > > > 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.
>
>
> Ah right. So your mod removed the need to fiddle screen settings between
> switching modes.
>
> There was one thing that didn't make sense to me about this person's wiring
> of their own switching system. It would have been a lot neater if the
> person used the internal video header for standard 40col. It would have
> saved having an extra cable feeding out just to get the RCA 40col output.

True--a lot of "switching" 80-column cards were designed to plug into
the external video connector instead of using the header.  I haven't
checked the schematics, but the header may bypass the video level
control on the main board, which was handy for "balancing" the 40-
and 80-column modes.

> > > > (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.
>
>
> A massive surge or ESD may do it. Maybe.

Yes, but it's either much less likely to happen or less likely to
cause the effect.  Inserting it reversed in the socket is a 50/50
error, and is quite likely to fry it.  Just my best guess...

-michael

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