Tech: Omega Race - X deflection only positive

Hi all
        
The Problem: X deflection is only positive. During the attract screen,
it looks like the letters being written start at 0 volts deflection and
proceed to the left, for probably half of the sentence, then starts over
at 0 volts and overwrites the second half. Writing always proceeds from
left to right. Almost like you take the right hand portion of the
screen and overlay it with the left.

So far: Looks like the output of the TL082 at BC8 is only positive.
Since I have a working set, I compared and on the working set the output
spans 0 volts.
        Y looks fine. I swapped all the chips (dac, 81 and 82) from x
to y, and the problem stays on x. I did find a problem with x1 going
throught the 74374 at A5, so I replaced that chip and that signal is
better. Still, x is only positive. Voltages to the DAC, and op amps
appear fine. I'm starting to think I have a problem with the X1 to X11
signals. Perhaps the data going to the DAC is buggered. Course, it's
been 20 years since I have forgotten op amp circuit design, so I could
have issues with supporting circuitry. What the hell is the 4016 near
the xy outputs? Screen reverse is an input to this. Don't think this
is my problem, as I can clearly see only a positive voltage at the
op-amps.

On an unrelated (possibly) note... Here is what I did to get this far:

Watchdoggin'

Repair acid damage: Replace 245 at S1, and some pull-up resistors.
Replace RAM sockets as needed.
Put in machine. Get flashing buttons...6, 9, 14. Reseat ROM at F1(14),
replace ram at H1(9), then ram at R3 and RS2 (6)
9 and 14 cleared up, but 6 remains.
Plugged in 9010a, ram test fails at 4400, indicating R3 or RS2, but
result is FF on a 00 write, so I suspect chip select or write enable.
Write enable is common, so I looked at the RAM1 signal. Scoped fine, so
I put the logic comparitor on 74138 at PN2, and told the 9010a to loop
on a write. Sure enough the comparitor hated pin 14(RAM1). Swapped it
and I appear to now have the cpu running well.

Using my scope, I saw a lot of noise on the Y. I first pulled the DAC
and put in another, but no-go. I replace the LM082 at BC8 and that fixed
it up. I think the scope was the tool of choice for this problem.

At this stage, I am not sure if my daughter board is 100%. Could it be
telling the CPU board lies about screen inversion, causing my X problem?
I am not thinking so, but who knows.

Nuff Ramblin for now.
Any insight are appreciated.

Mike Ranger
 

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Received on Thu Oct 23 08:47:41 2008