RGB signal from MK4 board is very weak

[Hugo86]

The RGB output from my Mortal Kombat 4 board seems very weak, resulting in a very dark screen.

 

The seller demonstrated that it worked (watched it run at his house) and the MK4 system test which automatically runs when turning the board on also seems to indicate the board is fine. 

 

I tried to solve the problem by using a RGB signal increase board from Axunworks (https://www.axunworks.com/h-product-detail.html?goods_id=669521). This greatly increases the brightness, but the colors still look very pale.

 

Does anybody know how to solve this?

[Turbo-Torch]

I'd start off cheap and simple by replacing those two radial capacitors.

[Hugo86]

1 hour ago, Turbo-Torch said:

I'd start off cheap and simple by replacing those two radial capacitors.

I’m not super technical, but they seem to be in pretty good condition? Is there any way to check or measure before I replace them?

[BDW]

I had this same issue with my MK4 board on a Wells Gardner 19" monitor. I threw some 330 ohm resistors on the RGB lines and and I had decent video. This guy on arcade-projects had the same issue and solved it the same way: https://www.arcade-projects.com/threads/mortal-kombat-4-dim-video.16956/

Edited December 17, 2021 by BDW

[Hugo86]

41 minutes ago, BDW said:

I had this same issue with my MK4 board on a Wells Gardner 19" monitor. I threw some 330 ohm resistors on the RGB lines and and I had decent video. This guy on arcade-projects had the same issue and solved it the same way: https://www.arcade-projects.com/threads/mortal-kombat-4-dim-video.16956/

Thanks for the link, this does seem like the same problem! 

 

Is it possible to show me some pictures of your board and the modifications that you made? This is my first board and I’m a little bit hesitant to modify it.

[BDW]

3 hours ago, Hugo86 said:

Thanks for the link, this does seem like the same problem! 

 

Is it possible to show me some pictures of your board and the modifications that you made? This is my first board and I’m a little bit hesitant to modify it.

No need to modify the board. Go grab a jamma extension harness on ebay. Find the 3 RGB lines, cut them, and then solder the 3 resistors in. No board modifications necessary.

[Hugo86]

330 ohm resistors did not help, only decreased the signal further. Does anybody know how much mV each RGB line should output? currently with 330 ohm it produces 275 mV, without around 400 mV.

 

Wondering if the problem is with the board or with my supergun/setup.

[BDW]

Just in case you're still around (or if anyone comes up with this issue in the future) this problem seems to be monitor based, not an issue board to board. I was finally able to track down a Wells Gardner 25k7XXX monitor and the MK4 board I own works flawlessly with that monitor. I tried it again with my 19" Wells Gardner and I ran into the same issue that OP had in the original post. I also tested the MK4 board with a spare Nanao MS9 I had and I had the same video issue as before, except the resistors didn't fix the problem like it did on my 19" monitor. There's something about the video signal with MK4 boards that works better on some monitors and worse on others.