Video Interference Cure
Marc Mandeltort
Marco Specialties
Columbia, South Carolina
James Beck
Greater Southern Distibutors
Atlanta, Georgia
PROBLEM
Anyone have the cure for very minor video
background noise on many different kits
we've installed? We've tried grounding every
power supply and video ground connection
in sight. It appears to be computer generated
herring-bone styled trash and is not readily
noticeable unless you stare at monitors all
day like we do. Naturally, like most good law-
abiding citizens, we toss out the RF cages
that come with the kits and I'm wondering if
that habit is coming back to haunt us. Most
recently, our Capcom Bowling is displaying
this affliction. Suggestions?
SOLUTION
The problem is usually caused by the fact
that switching power supplies offer great low
frequency and DC isolation, but poor high
frequency isolation. The actual problem is a
ground loop that is modulated by the high
frequency switcher action. The approved
cure is to add a 100 ohm, 2 watt resistor
between the switcher's earth ground (some-
times marked FG) and the logic level ground.
***
This shorts out the ground loop at the begin-
ing instead of at the monitor. Some people
just add a hardwire between the logic
ground and the earth ground instead of a
resistor. This works even better, but if there
is a fatal loss of isolation, the boards could
suffer great damage. The resistor actually
acts like a resistive fuse. Blowing open under
a high current (short-circuit) condition.
Merit Pub Time Prograni Update
Jim Kelly, Customer Service
Merit Industries
Bensalem, Pennsylvania
PROGRAM UPDATE
This Eprom change updates two bookkeep-
ing functions in the menu. The Games
Played feature (301, 501, Cricket, etc.) was
corrected to count higher than 100 per
game. The Lifetime Coin counts have been
adjusted to match the coins registered in
Coin 1 and Coin 2.
INSTALLATION
This new program (2278-00 U3-2B, U20-2B
and U21-2B) is installed by changing 3
Eproms on the DRT 950 PCB. This update is
available by calling your Meqt distributor.
NOTE
When this program is installed, a Ram Clear
must be performed by using switches 1 and
2 of Dipswitch 2. If there are any questions
about this program update or any Merit
product, please call Customer Service.
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