Posted on Jun 12th, 2013 at 12:44 PM by Eric Wright

Back around November, I found myself repairing a few arcade cabinets and/or boards. Having to disconnect and reconnect the board inside the cabinet many times during the course of troubleshooting can be quite annoying and time consuming. Some people are lucky enough to have an extra monitor chassis laying around that they can use, I sadly do not. One question that I have seen come up many times on arcade forums and what not is the following: "can i use a TV as an arcade monitor?". While this question has been asking many times, there does not seem to be a definite answer and most of the time the person asking the question is left with information on swapping tubes or buying an expensive converter. Having a spare TV that I got from my sister, I decided to give it a try as a "proof of concept" type of thing. The basic theory is as follows: If we connect the composite Sync to the TV's Video In connection, the TV should be able to separate out the H Sync and V Sync signals to drive the TV. To get our RGB picture to show up on the tube, we will drive the RGB transistors on the TV's neck-board directly from the arcade game.

Lucky for me, I was able to find a detailed schematic in the service manual for the TV (Sharp 13n-m100b) and was able to pinpoint where to make my modifications. I located the harness connector that fed the RGB signal to the neck-board and followed it back to IC201. After studying IC201 for a little bit, I noticed a few things that made me happy. Between the RGB output and the neck-board, there were three resistors that I could remove and connect the signals from our arcade game to. It turned out to be even better than having an easy way to disconnect the RGB out from IC201, on the board itself there was a spot for three diodes that were not used on this model. I now also had an easy way to connect the games RGB signals too.

Next I needed to find out where I could connect the composite sync to, in my first test I simply connected it to the Video In connector on the front of the TV. The problem with this is that the TV needs to be on Video 1 to get the proper sync.... and trying to put the TV on Video 1 can be a pain in the butt when you have garbage on the screen. I once again got lucky, IC201 has two video in pins, one for the external video in (pin 42), and the other for tuner in (pin 44). Both of these pins are connected to their video sources via a component in series (R901 for external video in, C416 for the tuner in). I removed these two components, jumper-ed pins 42 and 44 together, and then connected this jumper to our composite sync. The TV will not use the games sync no matter what channel it is set on. Yay!

To make it easier to connect and disconnect the TV from whatever arcade game I was working on, I added some mono connectors on the back of the TV. one for Red, one for Blue, one for Green, and one for Composite Sync. You can see from the two videos below that the monitor does sync up and will display the game, but it is lacking a proper drive circuit and there is no way to adjust the drive or cutoff for the colors.

Like I said earlier, I did this modification back in November and I am not sure why I did not do a writeup sooner. I have recently been thinking about continuing with this project and building a new neck-board for the TV that will let me adjust the drive and cutoff for the three colors. I have also been looking for a bigger and newer TV that I can hopefully sync to a game 24kHz horizontal refresh.

6-14-2013 Update:

I decided to change a few things up and see if i could inject the video over the RGB lines normally used for the On Screen Display (OSD). There are four inputs used for OSD, R,G,B, and /BLK. I assumed that /BLK was to make the background black for the OSD so I tied it to GND. It seems that I got this backwards and it needs to be tied high (as you can see in the first video below). After pulling /BLK high, I gave it another go. This time we had color, though sadly these inputs apear to be working as TTL inputs rather than analog. It appears that I will end up making a new neck-board with drive and cut off after all.



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