Blast City / MS-2931 Interference on VGA Input

[clam_wattson]

I recently "finished" a Blast City restore (including a complete recap of the chassis) and finally got around to testing out the 31kHz support on this cabinet. I removed the NeoGeo board from my JAMMA harness and connected a beefy VGA cable with ferrite chokes. I've searched around the web and had a hard time finding anyone describing a problem similar to mine, so I'm wondering if anyone else can shed some light on this!

I tested both an XBOX360 set to 640x480 and a Mister with analog board output. With both of these devices, there is visible interference on the screen. Hard to describe, but here's a crude depiction of the interference pattern that gets overlaid on the image:

I immediately suspected some sort of grounding issue. I had completely disassembled and reassembled this cabinet, so I checked all of the grounding wires and connections throughout the cabinet and on the chassis. I'm relatively certain that everything is connected up properly, but it is tough to thoroughly inspect the whole cabinet in its assembled state.

As an experiment, I decided to disconnect the JAMMA/15khz signal that breaks out of the 15-pin AMP UP connector on the chassis and the interference went away:


I have since determined that it is far more convenient to remove the monitor bezel and disconnect the JAMMA video signal cables from the front of the cabinet, but this is still not 100% ideal for me to have to do any time I want to switch between JAMMA and VGA.

Here's a video of me running 240p test suite on the Mister SNES core with this connector disconnected initially and no interference. I then connect the JAMMA signal to the chassis from the front and the interference returns:
https://photos.app.goo.gl/d8PuiNgkrUuMqFA78 (note: if you view this soon after my post was published, the video might not yet be available at 1080p. the interference is hard to see at low res).

Is this normal behavior? I did not test VGA input upon receiving the cabinet or before I recapped the chassis. I assumed that you should never leave a JAMMA board connected when using VGA, but I think it's kind of strange that you'd also have to disconnect JAMMA video from the chassis as well. Seems to undermine the convenience of auto switching.

The one last piece of this puzzle is this page of the manual. While I don't speak Japanese, I do have Google Lens on my phone which kinda translates this snippet. The translation I get does seem to suggest that removing this connector is instructed when using VGA input. However, in the physical copy of the manual I have, multiple makes of monitors are referenced, so I don't know if this instruction varies by chassis.

Thanks for reading and for any information or ideas you can share.

 

[adgenet]

I have no experience with blasts, but I can help with your manual.

It says:
The monitor is factory adjusted, but when switching games, you might need to re-adjust, and to use your own judgment whether or not it's necessary by using the game's screen test mode.
Adjustments are performed using the board you can find by opening the control panel base.
When the sync rate or input impedance or input levels are not matched, you may experience display problems. Double check the settings when swapping boards.

The red box says:
When using D-sub mini 15 input, remove the AMP 6P connector located here.

 

[clam_wattson]

that is so incredibly helpful. thank you!! it does seem as if what I'm experiencing is expected. I just find it so strange that I've never seen anybody mention this. that being said, I'm relatively new to the hobby, so this might just be common knowledge that I've never picked up.

 

[BlinG]

I had to use some kind of VGA amplifier to prevent a shaky screen when I used my 360 on the Blast. I my case I had a sega jamma to jvs IO laying around, so I used that and the screen stopped shaking.

 

[clam_wattson]

I had to use some kind of VGA amplifier to prevent a shaky screen when I used my 360 on the Blast. I my case I had a sega jamma to jvs IO laying around, so I used that and the screen stopped shaking.

I did pick up an Extron 160xi in anticipation of receiving my blast (since I didn't know what chassis I'd receive). I understand that the 2930 can have run into issues with the 360's sync. I haven't seen a need for it yet with the 360, but it would be worth seeing if it makes any difference with the JAMMA video signals left connected.

 

[Slypty]

I've a very similar issue with my Grundig CRT, it uses the same tube as Blast, so I ended up here looking for Blast interference issues.
I've seen this type of S-shaped squiggle overlaying the screen (see to the left of woman's nose in pics) on an RGB modded CRT that went away when I removed some interconnects, so the problem does seem to generate around sync involvement or timing issues.
On my Grunding, it has an unusual board called U/FOC. This board has a 120M variable resistor with an adjustable POT.. but I'm not sure what the POT actually does. If I adjust too far above 60M, I lose Sync completely, and the screen looks like 'only S-Squiggles' are present with no latching.
The screens a gem to me and it's a Porsche designed CRT, so I'd like to fix it at some point. If anyone has a guess I'm open to anything at this point, having also re-capped and checked grounds, etc.