Re: VECTREX

Heads up Paul, we're getting there; read on:

> > So, basically this means that only the beam on/off signal is corrupted here,
> > since the even characters are all on the same scanning (rasterized) lines.
>
> That is correct. And yes, I think the shift register is in the 6522. If Z
> comes from the same sample and hold circuit as the XY signals, is it possible
> that it just isn't holding properly?

I took another look at the scematics, and verified some things with my o'scope.
The DAC is only generating the intensity, not the on/off cycles of the beam. It
actually only generates a reference voltage for Z (intensity) which is then
switched on/off by the 6522. One of its outputs on the second peripheral bus is
used for this purpose (it is called /BLANK). So it can't be the DAC.

> Suppose (and this is totally unfounded
> wild speculation) that the text display code refreshes the Z value every
> other character (it's a tight loop after all) and for some reason it works
> for a second or so on power up :-) Like I said, wild guess. Focus on every
> component that affects the Z signal - this includes any caps in the S/H.
> And do tell when you figure it out.

You are probably exactly right here. When I fix /BLANK to 5 volts, all vectors
are drawn including the 'black ones'.
When I change the S/H capicator (bigger one in parrallel) only the brightness
varies (like vectors fading to dark, pretty cool actually).

This leads me to the idea the 6522, which I replaced, is not fast enough. I put
in a 6522, while originally there was a 6522A in it. Maybe the same as with the
6502A microprocessor, which runs at 1.5MHZ, unlike the normal 6502 which only
runs at 1 MHz.

Then I changed the system clock speed to 500 Khz (the max my audio generator can
deliver), which resulted in very large vector due to timing problems in the DA
section, but the texts WERE COMPLETE. I'll try to find the 'A' version and let
you know, but I think changes are pretty good.

Thanks for aal the hints so far!

Mendel
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Received on Thu Sep 14 14:33:28 2000