Main > Monitor/Video Forum
Segai M21C XX hourglass screen
Zebidee:
This really does sound/look like a monitor grounding issue. Three ideas to explore.
1) Have a look how your Segai monitor frame is grounded.
Now this is going to be confusing (it is for me), but your logic or DC ground (this is your video signal ground) should not be connected to FG (full ground, or earth ground). FG is at your mains, DC ground is local to your device.
If your FG is also connected to your DC ground, then this can cause the interference you see.
Many people will say you should connect "exposed" stuff like coin doors, metal control panels, and the monitor frame to FG, and many manufacturers do this. Mostly for safety, in case of a short (although not really needed if the monitor frame is normally completely enclosed in a cab). However, monitor frame is usually also connected to the ground wiring behind the tube, which is then connected to the monitor's DC ground!
So, this is a long way of saying: if there is a wire connecting the monitor frame to FG, try removing it, see if it makes a difference.
2) Make sure your home-made VGA breakout cable has all the available grounds connected (pins 5,6,7,8,10).
In particular, make sure VGA pins 9 (5v), 11&12&15 (monitor ID pins) are not connected to your ground.
Make sure that the VGA shield ground is *not* connected to video grounds. This can cause interference. You may get connectivity between video ground and shield ground when cable is plugged into PC, but don't worry about that.
3) It may also be something not connected the right or expected way for the JAMMA. Are you still using the JPAC for connecting the controls?
Someone had a similar issue here recently, swapping game PCBs in a cab, but I can't find the thread. Anyway, I think they solved it by connecting relevant grounds on the JAMMA harness (or maybe the adapter loom, some games use non-standard harnesses).
trick72:
Thanks for your reply Zebidee. I really appreciate it you take the time to help me out here.
There was indeed a ground wire connected from the monitor frame to FG. (There was a metal lip on the "plate" where the monitor system board resides to FG which had a ground wire on it). When I disconnected this the intensity of the lines is reduced by 75%. The scrolling and amount of lines is the same but they are much less visible now on some games but on the blue of metal slug from the video I attached earlier you can still see them. So you definately found the cause of the issue. Thanks for that! Now I assume there is still some path from FG to DC ground somewhere else... because the lines are still there being less visible. I see there is a ground strap (flat metal strap) around the monitor glass tube itself but not sure where this is going to... I think it is going to a ground pin on the monitor system board or neck board . But I didn't dare to disconnect it.
I double checked the pins of the VGA cable and they are as you described. The correct 5 pins are attached to ground, the others are not.
Lastly the JAMMA... I do use the JPAC for buttons. So the jamma wiring harness is still used. It is a version with a PS/2 keyboard cable going from the JPAC to the PC. The grounds of the VGA cable are going straight into the 5 pin monitor cable (together with the R,G,B and sync), so I don't use the JPAC pin14 or JPAC ground pins for this video ground.
The PC power supply and cabinet (crt) power supply are on the same broker line, they share the same FG I guess. Could they interfere with one another?
Zebidee:
--- Quote from: trick72 on August 29, 2022, 04:59:03 am ---There was indeed a ground wire connected from the monitor frame to FG. (There was a metal lip on the "plate" where the monitor system board resides to FG which had a ground wire on it). When I disconnected this the intensity of the lines is reduced by 75%. The scrolling and amount of lines is the same but they are much less visible now on some games but on the blue of metal slug from the video I attached earlier you can still see them. So you definately found the cause of the issue.
--- End quote ---
Part-way there at least :D
--- Quote ---I see there is a ground strap (flat metal strap) around the monitor glass tube itself but not sure where this is going to... I think it is going to a ground pin on the monitor system board or neck board . But I didn't dare to disconnect it.
--- End quote ---
Yes, leave that grounding strap alone. The CRT can get stray voltages around the back, and the grounding strap siphons that off to DC ground. Leave it in place, and leave the wire connected to the neckboard.
--- Quote ---The PC power supply and cabinet (crt) power supply are on the same broker line, they share the same FG I guess. Could they interfere with one another?
--- End quote ---
Maybe, but doesn't seem likely. Especially if you've already removed that FG wire from the monitor frame. Unless there is something else? The cabinet probably has an EMI/noise filter fitted near where the AC power comes in. These limit from noise/interference going in both directions (to and from the cab). If there isn't one, maybe it could use one. No harm done by having one, and they are cheap and easy to fit.
I confess I am running low on ideas.
Try running it with no JPAC disconnected? Just to see if it makes a difference. I'm still wondering if there is something about the way the JAMMA and JPAC are connected together.