Re: Omega Race - help!

On Wed, 23 Jan 2008 18:21:03 -0600, you wrote:

>> Based on the schematics in the service manual, it does indeed look as
>> if the game can be strapped for multiple different input voltages. If that
>> were the case, though, it looks as if your +12V should be low as well. Have
>> you checked that?
>
>I finally printed out a nice large drawing of page 92 (96 on the PDF).
>OK, yeah, +12 is dragging as well - it's at 10.79 with the pot all the
>way up.
>
>Checking the transformer inputs, it's tapped at 120V, so that's correct.
>Unreg'd input to the power supply board is 8.57V for the '8V' input and
>16.79V for the '15V' input. Hmmmmm.... maybe it's the linear power
>supply board after all.
>
>Am I right in understanding that this game doesn't use the 12V for logic
>at all? Also, where do the vector drive voltages come from?

        In this particular case, no, it doesn't look like the +12V off the
power supply is used on the logic board.

        HOWEVER... (you knew that was coming, didn't you. :) ) Take a look at
page 103 of the PDF file (assuming you have the same one I do, the linked
from the KLOV "Omega Race" page). There are actually *two* power
transformers in this system, and the second one feeds both the vector
monitor (which gets 60VAC via a center-tapped winding, and 6.3VAC via a
separate winding) and the "mother board" which gets 36VAC via a *third*
CT'ed winding. The "mother board" has its own rectifier and DC regulator
section which generates numerous odd voltages, such as +/- 8.3VDC, -15VDC,
etc. -- via U101 - U104, and it is these voltages which drive the DACs and
output amplifier stages for the X/Y signals.

        So I would say if you're having display problems, these voltages need
to be checked as well, and so does the strapping on that transformer.

        As for the low +5/+12 outputs -- you're measuring these right off
connector "D" of the Power Supply board, right? It's very strange that
both outputs are dragging, considering that there doesn't appear to be
anything using the +12V output at all; if anything, you'd think it'd be
*higher* than normal under no load...

        ...OK, here's what I'd do. First, inspect the power supply board for
the obvious stuff like cold or cracked solder joints, especially around
anything that's big and heavy and/or repeatedly stressed (i.e. connectors &
large power transistors). Look for any corrosion or oxidization of the
connector pins. Also look for any oxidization of the casings or mounting
hardware on those big TO-3 pass transistors, or try ohming out the
case-to-board connections to make sure there's no excessive resistance
there. (Not likely, but I *have* seen it happen once or twice...)

        Replace those tantalum capacitors, just on general principles. :) I
don't trust old tantalums; I've had too many of them decide to go FOOM at
inopportune moments. (The nasty thing about them is that unlike aluminum
electrolytics, tantalums have a nasty habit of failing *shorted* rather
than open, which causes them to burn up and sometimes take out other
components with them. My TRON did that not long ago.)

        Scope the input voltages to the power supply board to see if you have
excessive ripple, if you haven't already done so; it's entirely possible
that one or both of those "big blues" in the filter section aren't up to
snuff any more.

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Received on Wed Jan 23 21:28:59 2008