Service manual

Wells-Garnder Color Vector Monitor Guide
Page 49 of 73
FINAL TWO-MINUTE WARNING
Make sure that none of the wires of this modification can come into contact with the other board
components [easy to do; there is a lot of bare PCB in this area; you may want to tape/glue the
leads down], especially the brown ground wire located to the right of R700. If the modification
hits this brown wire, you can consider it a fumble and you just blew your lead and the game.
Common Ground Connections
From: John Robertson <[e-mail]>
Newsgroups: rec.games.video.arcade.collecting
Subject: TechTIP: How to make VECTOR MONITORS very RELIABLE!
Date: 22 Oct 2001
It’s been a little while since my last Tech Tip, but this is something that’s been on my mind for a
while now, and a posting in the Vector mail-list got the following response from me...:
Vector monitors blow up because the ground reference for the monitor drifts relative to the logic
boards (MPU and video) when the power supply connections overheat. This will then bias the
input signals offset enough to overdrive the outputs. Hence my argument for chucking the
original power supply and putting in a switching supply. I started doing that about ten years ago
and have not lost a single Electrohome/Sega monitor since. I assume this also kills Tempest/Star
Wars/Major Havoc/... monitors etc. Those pesky grounds get a few ohms resistance and all sorts
of nasty things happen.
I first discovered this on Gottlieb pinballs over ten years ago-the ground for the regulator would
overheat the pin/wiper contact which would become a small resistor and thus the ground of the
MPU would drift up relative to the cabinet ground, which also happened to be the ground path
for the driver transistors. When the MPU ground would change to about 0.5 to 0.7VDC above
cabinet ground the base of the transistors would then start to conduct as the MPU would be
trying to turn off the transistors, but the Emitters are tied to the cabinet ground. Hence the
transistors would start to conduct... You will recall that transistors generate far more heat when
they are used at the beginning of their working range rather when they are switched completely
on and off as in regular vector monitors (or solenoid drivers, etc.). So in a little while, it croaks.
No obvious cause...replace the transistors and everything works. For now...
So get VERY GOOD GROUND (COMMON) CONNECTIONS BETWEEN THE MONITOR,
MPU AND POWER SUPPLY for reliability!!!!!!!!!!!!!!!!!!!!!!!! Solder fat conductors with
nasty heavy gauge connectors between each component in the system. Put in healthy
SWITCHING SUPPLIES!
John :-#)#