Ms. Pacman Cabaret Scratch Build

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Zebidee:
^^ This seems like sensible approach

Then you can do whatever you want with it :o

Greenman:
Thanks Zebidee and Mike.

Yes, I have a jamma adaptor for Ms Pacman and will be wiring to the jamma standard. Amazingly, I got it locally. I reckon it has probably been in stock at the store for years!!

I want this one to be an upright cab. For some reason, even though I grew up in arcade stores, I never really warmed to the cocktail cabinet. The angle of the joystick and buttons just didn't do it for me. I am waiting on a female jamma interface so I can wire it up and test the board (including that regulator). Snail mail from China so I have to be patient.

Greenman:
So this is the mod that needs to occur on the CRT TV chassis.

Replace resistors 656-658 with 160 ohm and 75 ohm resistors in series, then bridge off between them for RGB out including adding 500 ohm pots along each of the RGB wires to drop the 15khz signal down to @0.7 volts. Then run 5v from IC901 (see pic) to the blanking signal of the OSD. I'll use a single pole, double throw switch to control the blanking signal.

If anyone is looking to RGB mod their old TV, I highly recommend the shmups forum as an excellent resource. https://shmups.system11.org/

Zebidee:
Good luck with your modding project :D I will be keenly following you progress.

The shmups RGB modding thread is great. Just long and convoluted!

For your RGB 500R pots, I suggest multi-turn cermet pots (bourns is popular brand). They are much better quality than the ---smurfy--- carbon film pots you often get, which are flimsy, "crackly" and break easily. Being multiturn, it is much easier to adjust the cermet pots to right values needed.

Then put them into a gang like this:

I used those pots for a JAMMA to TV (component modded) hack I did a little while back. You can read about it here if you like.

You may be tempted to remove the pots once you have the right values, and replace with resistors, and this is fine. I would suggest you keep the pots if you ever want to change PCB later, as different PCBs have different RGB output levels. Investing a couple of dollars for some decent pots that won't wear out quickly makes more sense too.

Also, I found I never needed the pots to be over about 150R (sometimes 75R was good, sometimes zero was good), so you may do just as well with 200R pots.

EDIT: I was using GreenAntz RGB to component transcoder, and it tolerates RGB input up to 2vpp which may be more than your CRT TV, so stick with 500R pots

Your planned approach to the mod looks good. You might need less than 5v for the blanking (2v may be sufficient!), though that will likely work.

I did a similar RGB/OSD mod on a 1980 National TV a year or so ago, and it looked really good too, but I have since moved on from that because the TV did not have a full SMPS power supply (just "partial"), meaning that the video grounds are shared with the main "AC ground" at the power supply. Weird and dangerous. It means I would get zapped (quite powerfully) just from handling the VGA connector!

That TV's only input otherwise was via RF, and that connector was buried deep inside the plastic housing (now I know why). Anyway, due to the dangers, I shelved that project until I could work out a neat way to isolate all the video grounds.

So, take extra extra care if modding early/old TVs.

Greenman:
Thanks Zebidee. I hadn't considered multi-turn cermet pots. I'll definitely be replacing the pots with resistors when I establish the best resistance required.

So the CRT TV chassis had a cover over part of the board that I had to remove to complete this mod.

Once I had full access to the board, I installed the 75 and 160 ohm resistors in series and bridged off between them for the RGB signal.

Connections for blanking switch

This is the 5v source for blanking.

I have no idea if it will work yet. I'll do some tests on it in the coming week. :dunno

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