My m72 pcb stopped running for some unknown reason. I ordered a bunch of parts to replace stuff on the m72 and test hardware on it. While the m72 is on the bench a m92 ninja baseball batman board I bought showed up today. The version I bought is one of the irem korean boards with a clone yamaha fm chip. I swapped the game board onto my original irem board which uses a real yamaha.
One problem with the irem m92 board is that the board outputs darker rgb than standard arcade boards. All my arcade boards output the same brightness rgb except for my m92 boards which are much darker. I now have two m92 boards, gunforce and ninja baseball batman, both of these boards are equally as dark so it's definitely not some random flaw they're built this way. In an arcade machine you'd just turn up the screen brightness since the board would exclusively be attached to that moniter. I use a supergun where I change arcade boards like cartridges so I want them to all run on the same setup without having to adjust anything.
The obvious solution to this issue is building a rgb amp before the rgb reaches the jamma connector. I have an extra ultimarc rgb amp from a commission so I decided to use that. It's good practice to wire up the ultimarc amp since I'm using it in an up-coming commission. I wired up the amp and it made the picture brighter but the colours became messed up. The specs on the ultimarc amp is it's designed to amp 1vpp to 4vpp. This board outputs rgb that's fight brighter than 1vpp so I needed to weaken the input strength of the rgb. Using a series resistor alone didn't help. Tiido saved me yet again by telling me to use a voltage divider which worked.
Here's the voltage divider for the rgb lines:
One series resistor for each of the three lines (r, g and b) followed by one resistor on each line to ground. I think I wound up using 10 ohms series followed by 300 ohms to ground. Here's the amp wired up and installed on the pcb:
How the picture looked before, ninja baseball darkman:
How the picture looks now with the amp installed:
Win. I guess the lesson to learn here is when you want to weaken a signal and a series resistor alone doesn't work right, try using a voltage divider. I found the same rule applies to audio too, voltage dividing on audio sometimes is the only way to make an audio signal quieter. This pcb the sfx samples are very low quality and completely unfiltered, I'll try adding some low pass filtering later to give the sfx more bass and mask some of the distortion.