Re: Tempest twisting images...

On Wed, 14 Jun 2006 11:02:46 -0700, John Robertson <pinball@telus.net> wrote:

>Found the problem. Turns out the replacement XY driver board I had
>left (from my parts stock) is a version P327 with the input
>protection circuit. I found that this circuit was the cause of the
>twisting by hooking my scope to the inputs of Q600 and Q700 and
>getting the same twisting, but not at the X & Y outputs on the
>motherboard. Hence something was a problem in this circuit. Replaced
>the two transistors Q804 & Q805, plus two of the diodes D806 & D808
>(Germanium I think) with 1N914s.
>
>Same problem. Poked around for another 1/2 hour...
>
>Gave up and clipped R810 & R811 - inputs to the protection circuit -
>and now the picture is fine.

I *always* remove this circuit (by clipping or unsoldering components), it was a
pretty lame attempt at a protection circuit, doesn't work well, but screws up
the vectors even on version 3 of the ROMs. It's also incompatible with most of
the games the ZVG supports.

It's only function is to kick in when an X or Y input becomes stuck at one
extreme or another. In an arcade it could be days (at least hours) before
someone noticed that Tempest wasn't working, and during that time the monitor is
going to get *REAL* hot! The problem is that it kicks in when it notices even
slight differences in left/right, or up/down symmetry, and then in only slightly
kicking in, causing the vectors to warp. It would have worked if they'd have
added a delay circuit of some kind, like if the symmetry is off for more than 10
seconds, kick in.

At home, if you turn on your Tempest and it doesn't work, and you don't hear
enough chatter, you turn it off, making you a much better protection circuit
than this one. And you, as a biological protection circuit, have the advantage
of not warping the vectors during normal play.

-Zonn
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Received on Wed Jun 14 15:42:18 2006