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Thread: Help with RGB Mod (Based on 8-Bit Guys video)

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    Kirby (Level 13) megasdkirby's Avatar
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    Default Help with RGB Mod (Based on 8-Bit Guys video)

    Long shot post, but who knows...some text was meant for another forum...Im just lazy as hell.

    Hello everyone!

    I've been checking out the forums, particularly the RGB mod section, as I have many CRTs I would like to mod. The first unit to test out was a Curtis Mathes 19" CRT, model CM19011BX. I found the jungle chip and it's a TN1282N. The chassis is a K15A AA41-11037C. The OSD chip is SZM-370TH1. Looking at the schematic, it's almost identical to the CRT modded by the 8-bit guy on his Youtube video a few years ago, with the only differences been the size of the unit (his was a 13" while this one is 19"), the model of the MICOM and Jungle Chip, and finally some capacitors and diodes here and there. While this may seem like everything is different, the reality is that, by looking at the schematic, it was eerily similar, so I decided to start with this one, as I have a reference to go by.

    Using the methods in both videos, I did manage to RGB the unit and it looks BEAUTIFUL! Crisp, detailed image...so gorgeous. MarkOZLAD helped me quite a bit to understand some of the reasons behind certain steps, so an awesome THANK YOU for the help!

    However, there are issues. The least important one being that after the mod, the screen in RGB mode looked too bright. MarkOZLAD recommended that I lower the brightness on the flyback, which I did, and now it looks fantastic. Problem solved for that issue. The main issue that still haunts me is an OSD issue. For some reason, even when the 5V and all the wires are correct and the switch is configured correctly (as in 8-bit guy's second video), the unit will keep displaying the "VIDEO" and "PLEASE INSERT AV CONNECTION" on the screen. It happens in RGB mode, non RGB mode, and even while playing a game. Even the remote display button doesn't help. So something is a miss. I rechecked all connections and everything is in order. Made sure I used the correct sections for voltage and ground, as well as jump the resistor for the CCD, and that is ok as well. As a last resort, I disconnected the three pins 22-24, the RGB pins from the MICOM, and that helped make the message disappear while on RGB mode, but in non RGB mode part of it remains. Odd.

    So, refering to the schematic pics above (please note they are not from the unit but from a nearly identical unit. I could never find the service manual for my CRT but managed to find a unit using the same chassis and ICs, which helped immensely), I did the following:

    -Insert the RGB lines between the exiting resistor and the 75ohms resistor and grounding it (as was done on the video with the "mini triangles".

    -Jump RU10 680ohm resistor to cancel out the CCD

    -Contrary to the picture of the schematic of the jungle chip, the diodes shown were not available on my unit.

    -Found the 5V and GROUND on CN901 (Adjustment Port) and wired the 5V on ther third leg of the switch and the ground was left on standby for the time being (not used until testing the SMS)

    -Lifted the leg of the resistor (R919) closest to the MICOM and soldered a cable to the leg to the middle pin of the switch. Then I inserted another leg in the now available hole and soldered that to the remaining leg of the switch.

    -C919 and C916 are not found on my unit

    With this setup, the mod works as mentioned above, but the text kept appearing. So I desolered the leg closest to the MICOM chip for resistors R916-R918, breaking the pathway. This made the onscreen text disappear once and for all.

    Granted, by adjusting the flyback it made non RGB mode waaaaay to dark to even be considered playable, but RGB is just perfect and beautiful. Tehnically, it seems I'm pretty much done, but if I can make the OSD menu appear and disappear at will while on RGB mode, that would be fantastic and would put an end to this mod.

    Any ideas? Thanks everyone!
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  2. #2
    Pac-Man (Level 10)
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    wish I could help but I think you know way more than I do.about this
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    Great Puma (Level 12) jb143's Avatar
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    I did an RGB mod on a Trinitron, but it was a few years ago so the details are fuzzy. However, I didn't use a switch like I see a lot of people do, but instead made a simple circuit to mix the existing OSD signal with the injected RGB and it works great. So volume and whatnot is displayed as you expect. Since sync is on a different line, it seems as though you should be able to adjust the rgb signals on the inputs so you wouldn't have to mess with the flyback, but I'm not 100% sure. Try looking up video attenuation schematic. Or video brightness schematic.

    I'm not sure about using the existing inputs in conjunction with the rgb, since mine is in an arcade cabinet and only used for that, but I would think that if there's no RBG signal, then it should just display... since the rgb is basically being shown as an overlay.
    Last edited by jb143; 05-19-2021 at 04:54 PM.

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    Kirby (Level 13) megasdkirby's Avatar
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    Quote Originally Posted by jb143 View Post

    I'm not sure about using the existing inputs in conjunction with the rgb, since mine is in an arcade cabinet and only used for that, but I would think that if there's no RBG signal, then it should just display... since the rgb is basically being shown as an overlay.
    Any idea on how to create a cable from a jamma video connector to the RGB RCA inputs I made on the TV?
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    Great Puma (Level 12) jb143's Avatar
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    I know people have done such things. I'm using VGA in a Mame cabinet with the video card configured to output the correct frequency. It lets the games run at actual arcade resolutions and looks great. Almost as good as an actual arcade monitor but much much less expensive. The main difference between that and using an actual arcade/jamma video (as far as I remember) is that the voltage levels are different....maybe something with the the sync signals. So the circuits are very similar. But it was only a handful of parts.

    AS far as the circuit that goes in place of the big honking switch that people use so flip between RGB and OSD... The schematic is buried in the shmups forum(around page 68,69 or so it looks like).

    I wish I could be more help but I only did this once about 3 years ago.
    "Game programmers are generally lazy individuals. That's right. It's true. Don't let anyone tell you otherwise. Since the dawn of computer games, game programmers have looked for shortcuts to coolness." Kurt Arnlund - Game programmer for Activision, Accolade...

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