Hi,

I'm quite new to arcades but not to electronics. I had some problems with
two monitors and now I want to understand what's going on !

First, I have a JAMMA Mortal Kombat 1 pcb wich works fine. Since I don't
know a thing about monitors and isolation, at my first attempt to plug the
pcb to the monitor I didn' use an isolation transformer for the monitor. I
plugged the monitor in the AC outlet of my house (without grounding the
frame), my pcb WAS isolated by the isolation transformer inside my cabinet
(the switcher power supply is powered by the secondary of the isolation
transformer).

I plugged the video signals on the monitor and it was working fine. I even
played MK1 several times with great video.
For a totally unknown reason to me, one time I had everything disconnected
on my cabinet. After I plugged everything back as it was when working, a
short happened when I plugged the video signal to the monitor, busting the
video ground pad on the JAMMA connector pcb. Also, the fuse of the monitor
blew and it was blowing each time I replaced it.

Ok now, I thought that the monitor (which was quite old) just busted a part
and "almost" fried my PCB. I decided to buy a new, bigger one. Both
monitors were Wells Gardner K7000 series (19" and 25"). Since I thougth
that the monitor was the problem, I did the same bloody thing with my new
monitor... you guess what happens ! But It didn't happen at first since I
didn't repair the ground pad at first. So I plugged the video signals
(without the video ground), I could si a very distorted image and input
resistors began to heat and to smell fried electronics. When I plugged the
video ground, the fuse fried. Since I paid more money for this new monitor,
I repaired it by changing 2 rectifier diodes on the AC regulating stage of
the chassis. Now my monitor works back.

Ok, now that you know the story here are my questions, I wand to understand
to avoid the same problem again.

I now understand that these monitors have no "ground" reference and that the
frame has a potential. I also understand this is why we need to use an
isolation transformer.

Now, what I don't understand is what happened with my monitor. It looks
like when I plugged the video ground to the monitor, I was shorting the AC
power... busting the rectifier. Is it what you guys also think ?

Do you think my MK1 PCB could be the source of the problem ? When I first
plugged the video signals without the ground, there was current sinking from
somewhere and I can't understand why (video signal connector was very hot
and input resistors began to change color). Any ideas ?

Now, I've seen everywhere on the internet that only the monitor has to be
isolated for security reasons. On my Midway cabinet, everything is isolated
(powered by the secondary of the isolation transformer), the PCB, the neon.
If I plug my pcb video ground on the monitor while both are on the same side
of the isolation transformer... will I have the same ground problem again ?

I know it's a long question but I can't find the info I'm looking for on the
web. Thanks to all of you that can help me understand.

J-F

Now, what I don't understand is what happened with my monitor. It looks
like when I plugged the video ground to the monitor, I was shorting the AC
power... busting the rectifier. Is it what you guys also think ?



That's exactly what you're doing. If you look at the schematic, "ground"
(the frame of the monitor) is connected directly to the negative output
of the bridge rectifier so by running it without an isolation
transformer you're creating a very dangerous situation where the entire
frame and any metal parts of the cabinet are floating at over 100V above
true ground. Many times the power supplies are not fully isolated, so
the video ground is also close to earth ground, if you connect it to the
monitor ground it's expected that you'll blow the rectifier in the
monitor and you may blow traces off the game board or damage the power
supply.

In short, get an isolation transformer for the monitor, they're cheap
(under 10 bucks from Bob Roberts) and are an absolutely essential safety
component. If you're unsure about anything relating to this, hire
someone who is, you or someone else using the game can be injured or
killed as well as a very real fire hazard if this is not done correctly.