They probably left off the centering because it isn't normally needed in a
black and white monitor-based game. You just use the centering rings on the
CRT neck instead. Plus adding a DC component for centering does add slightly
more power usage/heat to the output transistors.
If the amount of centering you need is beyond the range of the centering rings
may I suggest you have a board or monitor problem that is causing a DC offset.
In my opinion (for whatever that's worth) the actual problem should be fixed
instead of applying the band aid of centering pots. But I do have to admit
that whatever gets the job done probably isn't going to cause further problems,
UNLESS you are adding offsets to counteract misadjusted centering rings.
tom mcclintock wrote:
> Just in case Matt McCullar forgets, this is what he posted to RGVAC
> regarding centering pots on a Battlezone board.
>
> --------------
>
> I'm not 100% certain about this, but I'm looking at a _Battlezone_ board
> and _Red Baron_schematics.
>
> It looks like Atari did plan to include X and Y centering pots during
> the engineering stage; why they left these out, I don't know. But it
> appears easy enough to add.
>
> X channel first.
> The pot value at R101 (XCENTER) should be 50 k.
> You must also add a 0.1 uF cap at C102.
> Also add a 220k resistor at R100.
>
>
> Now for the Y channel.
> Pot value at R91 (YCENTER) should be 50 k.
> You must also add a 0.1 uF cap at C101.
> Also add a 220k resistor at R90.
>
> And that's it.
> What you're essentially doing is adding a DC component to the output
> signal. The pot's leads are connected to +15v and -15v, with the wiper
> in the middle mixing in the voltage-divided signal with the video signal
> at the final op-amp output.
>
> ----------
>
> tom :^)
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