Re: Space Duel Board X-Axis Deflection Problem

Dave,

If you're getting deflection to only the RHS of the screen, there are few
good candidates to check.

1) Make sure this problem is your game board, and not your monitor. A
faulty deflection circuit in the monitor would give you this behavior. If
you have a scope, take a look at the output of the X-axis on the game board
and see if the voltage swings both positive and negative relative to ground,
or if all voltages are positive. Likewise, you could check this by putting
your scope in XY mode and this would let you see if the screen looks the
same as it does on your vector monitor. Obviously, if the screen looks
correct on the scope, or if you are getting both positive and negative
deflection on the X-axis from the game, your monitor has the problem.

2) Monitor is good, so now make sure that all the right voltages are going
to the DAC and the op-amps that follow. If you were missing the negative
voltage input at any point in the output stage, you'd be able to generate
only positive voltage swings, i.e. those on the RHS of the screen.

3) If the voltages check out, then make sure the most significant bit going
to the DAC is toggling. The most significant bit (the tenth bit in this
case) has the property that it selects the from the right or left quadrant.
If this bit were stuck in one position, everything would be drawn to one
side of the dead center of screen. I'd have to check the schematics to be
sure, but I'd expect that if it were stuck high, that would generate points
on the right hand side only. Either way, the direction isn't too important,
it's the state of this input bit that matters.

4) If all three items check out, then socket and replace the op-amps in the
output stage.

5) Then if you're still not getting X-axis left hand side video, replace the
DAC with a new one.

Joel-

----- Original Message -----
From: "Dave Langley" <robotron@ntlworld.com>
To: "Vector List" <vectorlist@vectorlist.org>
Sent: Saturday, July 19, 2003 4:41 PM
Subject: VECTOR: Space Duel Board X-Axis Deflection Problem

> Hi ...
>
> I'm trying to get a Space Duel Board running from my newly acquired game
...
>
> Initially the board was dead. I started substituting socketed components
> and discovered that the AVG chip was bad. This was replaced with one of
> clays replacement chips (Thanks Clay!) and the board now runs with no
> X-axis deflection at all.
>
> Probing around with my scope I failed to see any DAC output on the working
> board, what do you look for?
>
> What I did notice was in the 'bad' X-axis of the non-working board there
> were spikes coming from the DAC output. This was different from the
> working board so I changed it .... and I now have X-axis deflection but
> only on the RHS of the screen ...
>
> It's worth saying that the DAC (AM6012) I fitted was a used one that may
or
> may not have been working, but I have similar problems with a Star Wars
> boardset too!
>
> Where do I go from here?
>
> Thanks in advance for any help!
>
>
> Dave Langley
> www.robotron-2084.co.uk
>
>
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Received on Sat Jul 19 21:55:49 2003