Help me understand monitor functionning.


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.


Reply to
Loading thread data ...

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.

Reply to
James Sweet

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.