Tech: Midway Space Invaders

[John Bennett]

[ukvac] Tech: Midway Space Invaders
Hi Guys,

I have a direct short on my cpu board of the above, everything on the
board "LOOKS" o.k., i've buzzed it out with any socketed chips out
with no difference.

Has anyone come accross this fault before, or do i start pulling
components one at a time until i hit the little bugga!!??

Failing a solution has anyone out there got a schematic for it, just
the CPU BOARD diagram will do, but all i can find on the net in the
way of SI drawings are of the pinouts and power supply board.

I'm pulling my hair out here, and i'm getting close to putting
unprotected 12 volts on the board and seeing what bursts into
flames.....

Cheers in advance,
Nick 'Gadget' Nethercott.





DATA Imported from archives: originally posted by (n.nethercott@virgin.net)

 

[PaulSwan]

Re: [ukvac] Tech: Midway Space Invaders

n.nethercott@virgin.net wrote:
>
> Hi Guys,
>
> I have a direct short on my cpu board of the above, everything on the
> board "LOOKS" o.k., i've buzzed it out with any socketed chips out
> with no difference.
>
> Has anyone come accross this fault before, or do i start pulling
> components one at a time until i hit the little bugga!!??

Yep, a few times. First, look for any long leads on components that may
be bent over and touching something else. Most modern games had cropped
component leads but some older and bootleg ones don't.

You could try and burn out the short if it's in inside a chip but you
would more than likely just end up with a board with a fault that is
harder to trace if you succeed.

The way I've done this in the past requires a "constant current source"
and an accurate digital voltmeter. What you do is attach the constant
current source and measure the voltage at various places over the board.
The closer you get to the short, the lower the voltage will be. Also, if
something gets hot that might be a hint

Another method I've seen used takes advantage of the way the power is
distrubuted. You will see on the board that there are power rails that
run in columns from the edge of the board. You can make a small cut on
each column and isolate it from the main rail. This allows you to
isolate the short to a particular column. From there you can cut each
lead on the chips within that column until the short disappers.

>
> Failing a solution has anyone out there got a schematic for it, just
> the CPU BOARD diagram will do, but all i can find on the net in the
> way of SI drawings are of the pinouts and power supply board.
>
> I'm pulling my hair out here, and i'm getting close to putting
> unprotected 12 volts on the board and seeing what bursts into
> flames.....

If the short is on the 12V rail then it may be easier to trace as only
some of the chips use the 12V supply. On the Space Invaders CPU I think
it may be limited to the RAM and CPU.

Schematics are available on the WWW I would imagine.

Good luck,

Paul.

>
> Cheers in advance,
> Nick 'Gadget' Nethercott.
>
>
>

 

[John Bennett]

Re: [ukvac] Tech: Midway Space Invaders
Paul,

Thanks for the advice, your right in that the CPU and RAM does use the 12
volt rail unfortunatly only the CPU is socketed there does also appear to be
a number of passive devices on this rail. I was going to start on these
first seeing as they are a damn sight easier to remove than DIL devices, but
i reckon your idea of breaking down the power rail sounds a winner.

I've spent hours looking for a schematic for the board with no luck, spies,
restoration sites.....blah de blah.....great sites but no schematic.

Thanks for your time,
Nick 'Gadget' Nethercott.



DATA Imported from archives: originally posted by Nick Nethercott (n.nethercott@virgin.net)

 

[Adrian Purser]

Re: [ukvac] Tech: Midway Space Invaders
Hi,

I had exactly the same kind of problem on my SI board. I found the problem
to be a bad cap. I narrowed it down by cutting a couple of tracks to isolate
sections of the board.

Ade

=================================
e-mail: ade@arcadestuff.com
web: http://www.shadys.demon.co.uk
=================================

 

[John Bennett]

Re: [ukvac] Tech: Midway Space Invaders
In article <95s02k+r247@eGroups.com>, n.nethercott@virgin.net writes
>I'm pulling my hair out here, and i'm getting close to putting
>unprotected 12 volts on the board and seeing what bursts into
>flames.....

Or power it up from a current limited supply and see which chips getting
hot by feeling them with the back of your right index finger. Sounds
rough, but works well.

--
Clive Mitchell



DATA Imported from archives: originally posted by Clive Mitchell (clive@emanator.demon.co.uk)

 

[John Bennett]

Re: [ukvac] Tech: Midway Space Invaders
Thanks for your help guys,

I've found the dead cap on bank 15 on the board, blue jobby +22 @ 25v
(22mf????? i guess)

Boy that was fun!, not only did they feed it to each RAM chip from the power
rail but they linked it from chip to chip as well!.

L8er,
Nick 'Gadget' Nethercott.



DATA Imported from archives: originally posted by Nick Nethercott (n.nethercott@virgin.net)