Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Questions about the RaspberryJAMMA adapter.

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MarkOZLAD
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Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by MarkOZLAD » Tue May 14, 2019 11:04 am

Hi all,

Just thought I'd share my experience getting RaspberryJAMMA working with my RGB modded CRT TV. (I scratch built my cab with a consumer CRT that I had modded for RGB)

The process was really quite simple. The Jamma Composite Sync line was fed into the AV port via a 1000 Ohm resistor. The 1000 Ohm resistor works with the 75 Ohm termination resistor in the TV to divide the voltage and produce a sync level of around 0.3V (assuming 5V sync from RaspberryJamma - which I never measured!, I came up with this number empirically) I suggest anything from about 470R to 1000R would do the trick.

For the Jamma RGB lines I added 500 Ohm potentiometers to each line. Again, with the 75 Ohm termination resistors in the TV this creates a voltage divider that attenuates the signal. I found that turning the pots all the way up to 500 Ohms got me the best picture which was not what I was expecting! It was the same for my Wonder Boy arcade PCB - 500 Ohms was perfect.

If you were to try and build an adaptor for scart you could use the same techniques for sync and RGB and you will also want to:
- feed a 5V line into a 180 Ohm resistor and then into pin 16 to force RGB blanking
- for Scart pin 8 use a 12V to force the switch to AV.
- for Audio you could connect the RaspberryJAMMA's line out put to pin 2 (right) /6 (left) and audio ground to 4.

Of course with scart you can possibly buy an adapter on eBay.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by MarkOZLAD » Tue May 14, 2019 11:06 am

https://www.geocities.ws/podernixie/htpc/cables-en.html

This page is a great resource for building adapters. Check out the Jamma to Scart.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by dee2eR » Tue May 14, 2019 3:10 pm

Good topic. Adaptors like this are great projects. As are scratch built cabs.

I built a similar circuit JAMMA to Scart converter once before. I used trimpots rather than resistors on the RGB lines (like you did with your RGB mod) but was much the same idea as that circuit otherwise. Quite simple to DIY and the results are great. Was a long time ago so I don't remember the reference I used.

At the time I had a video chain of JAMMA RGB wired to Scart RGB then converted to Component with an off the shelf converter then into TV. Sounds silly but works well enough, I didn't have a TV with a Scart input so it was the easiest way to use arcade boards with good picture quality. Back then I wasn't confident poking around in a TV to try hacking the RGB directly into the TV... It sounds like you've done it in a much nicer way.

That later became my first scratch built cab too. I took the TV apart and removed the speakers and cut the casing down at the sides to make it around 3 inches smaller in width. Then I made a stand that held it at about the right height and angle and added panels to it all for the speakers, coin door and sides. It's an oddball arcade cab with what is essentially a free standing chassis holding the panels up rather than the traditional cab. It's all very hacked together as I went and still unfinished many years later as I got it playable... I should try and finish it one day, it's still raw wood and could use some T-moulding (and if I start working on it again I'll probably want to properly decase and remount the TV & maybe RGB mod it too). Plays great as is though.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by MarkOZLAD » Wed May 15, 2019 4:56 am

All good art remains unfinished!

Other options for the RaspberryJAMMA include super guns. There are plenty on the market and often very reasonable prices.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by Cjoli » Thu May 16, 2019 10:17 pm

MarkOZLAD wrote:
Tue May 14, 2019 11:06 am
https://www.geocities.ws/podernixie/htpc/cables-en.html

This page is a great resource for building adapters. Check out the Jamma to Scart.
Thanks for the info, I have recently been thinking about this but have never done it before, I have 3 Sharp 20 Inch TVs from the mid 90s that only have RF in, so I need to do some more research but I assume they can be moded.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by MarkOZLAD » Tue May 21, 2019 5:44 am

That page is about building adapters.

If you want to RGB mod consumer CRTs that is a whole other story. I’m one of the foremost experts at it and I am happy to help. If you can provide the TV model/chassis number and preferably schematics I can advise.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by Cjoli » Sat May 25, 2019 11:47 am

MarkOZLAD wrote:
Tue May 21, 2019 5:44 am
That page is about building adapters.

If you want to RGB mod consumer CRTs that is a whole other story. I’m one of the foremost experts at it and I am happy to help. If you can provide the TV model/chassis number and preferably schematics I can advise.
I would love a little help, I have been trying to find a service manual for the sets I have but have had no luck. The model I have is a Sharp 20F-S40, I can get you any other info that you would need.

I also have a couple of 25" flat glass CRTs that have component inputs, is there a easy way to go from a Pi to the component inputs?

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by dee2eR » Sat May 25, 2019 3:58 pm

Cjoli wrote:
Sat May 25, 2019 11:47 am

I also have a couple of 25" flat glass CRTs that have component inputs, is there a easy way to go from a Pi to the component inputs?
I actually bought an HDMI to component adaptor at some point and never got around to checking it out... now I wonder where I've put it.

HDMI to RGBS has sub single frame delay and while I never measured it I don't think RGB to component has any more delay than that (but does have slightly worse image than the original RGB source, only really noticable near the edges of the screen). I don't think skipping the middle man would work any worse... a single conversion is better than two. The big question for me was would it handle 240p, not all DACs handle it properly.

This is the sort of thing I bought: https://www.ebay.com.au/itm/1080P-HDMI- ... A0FVXd7RQA It's not the one I got, I don't know if it's any different on the insides, I never tried mine and don't know if I recommend the product at all but they're pretty cheap if you want to give it a go and find out. Even if mine worked great or didn't work at all it probably wouldn't mean anything at this point, it's been a couple of years and cheap eBay gizmos seem to evolve over time. If you try it post the results.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by Cjoli » Sun May 26, 2019 12:00 am

I have a couple of these https://www.cs1.net/products/audio_authority/9A60.htm I paid way too much money for when I was experimenting with grooveymame before I switched to the Arpicades. I was also wondering about something like this http://www.arcadeshop.com/i/773/rgb-to- ... e-v4.1.htm, the biggest problem I have found trying to use CRT TVs in the past is they don't have the proper controls to make adjustments to the screen for things like sizing and centering the image.

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Re: Using RaspberryJAMMA with RGB Modded CRT TV (and possibly scart)

Post by MarkOZLAD » Mon May 27, 2019 12:54 am

If you're using RaspberryJAMMA and have an RGB modded set you won't need either of those adapters, you will just need to attenuate the RGB and Sync signals as I have discussed in this thread.

As for the position and size controls, depending on the age of the set you usually have size controls as pots on the chassis and/or in the TV's service menu. In my experience consumer sets often do not have a horizontal size control (although many do) but will almost always have horizontal position, vertical position and vertical size. Luckily when using the RaspberryJAMMA we can adjust horizontal size in the config files (I have posted about this previously).

If you need to get into a TV's service menu you will need a remote and a service manual. There are many great sites to find these, https://elektrotanya.com/ is one of the best. Others include https://www.electronica-pt.com/ and https://www.manualslib.com/ are also great resources.

If you're a hard core arcade monitor person you're probably not going to like it but for me it is my main hobby. I've never worked on an arcade monitor that wasn't one of my modded consumer CRTs.

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