Re: A question about mono monitors

[Tristan Mumford <xtristan.xmumford@xgmail.xcom>]

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.

> 
> 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.

> 
> (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.

Well, sometime in the next week I'll dig out the apple mono monitor. I have
to find some things to get rid of in a garbage sale anyway. So two jobs at
once!

Tristan.


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