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Retro_Game_Lover96

RGB Mod Explanation

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13 hours ago, Retro_Game_Lover96 said:

Ooh-de-lally! It works like a charm after I added the 560 ohm resistors, so I finally got color via TMS RGB!!! :D

I guess that unlocks the procedure for adapting a TI-99.  Though it sounds like different revisions will have different levels of needed changes.  But @Falonn should now be able to add some docs to his page on this.

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18 hours ago, Retro_Game_Lover96 said:

Ooh-de-lally! It works like a charm after I added the 560 ohm resistors, so I finally got color via TMS RGB!!! :D

 

F52E619D-8F29-48C5-9F0A-E7056334B47A.jpeg

Can you make a "How to...!" on this?

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How to add RGB on TI99/4A?

Requirement tools/parts:

A working TI99/4A Computer

TMS-RGB Mod Board (Assembled)

8 Pin Mini Din Female connector

8 Pin Mini Din Connector Board from OSHPark (Optional)

8 Pin Mini Din Scart RGB Cable

3x 560 Ohm Resistors

1x TMS9928A Chip

Some wires

A Phillips Head Screwdriver

A decent Soldering Iron

Some Soldering Wires

Desoldering Braid, Pump, or Hakko Desoldering Gun (If you want to do no-cut method)

An OSSC, a Framemeister, a PVM RGB Monitor, or any Scart adapter (Required for RGB)

 

Step 1: Unscrew all of the screws from the bottom of the TI99/4A and then carefully open it.

Step 2: Unplug the power supply from the motherboard and unscrew all of the screws from the motherboard. And don't forget to take off the cartridge connector and take out the RF Shield, as well as the keyboard connector!

Step 3: After you open the RF shield from the motherboard, you will see the TMS9918A Chip, so you need to remove it. If it is socketed, it is very easy to remove it. Then, replace it with the TMS9928A Chip.

Step 4: You need to solder the TMS RGB Board onto the bottom of the VDP Chip like this:

F439F11B-0C40-48DD-B4E9-2C201CD3DD70.jpeg

And solder the wires to the chip:

0F089C84-B350-48B6-8204-081684F78421.jpeg

And don't forget to feed the wire for the audio which is Pin 3 of the TI's Video connector.

Step 5: Solder the three 560 Ohm resistors to those places like those:

Screen Shot 2020-08-25 at 6.02.32 PM.png

Step 6: If you want to do no-cut method, simply desolder the original video connector off from the board and feed all of the wires to the 8 pin mini din connector.

Step 7: Test is to make sure it works before putting the TI back together. (If it works, then move on. But, if it doesn't work, check your work and try again.) Make sure you mount the RGB connector!

Step 8: Carefully put the console back together, and then enjoy!

Edited by Retro_Game_Lover96
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One correction is that the red and blue lines represent jumpers to the DIN connector.  The red line is what you'll find in 9918-based TI's, and the blue line is where a jumper is connected to send component video to the DIN connector.  Changing the jumper lengths is unnecessary.  Only the yellow and green lines need to be resistors.

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3 hours ago, ChildOfCv said:

One correction is that the red and blue lines represent jumpers to the DIN connector.  The red line is what you'll find in 9918-based TI's, and the blue line is where a jumper is connected to send component video to the DIN connector.  Changing the jumper lengths is unnecessary.  Only the yellow and green lines need to be resistors.

Alright

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Nice work.  That output looks great.

 

Can you confirm whether the audio still works on the machine?  I haven't been following the thread closely, but last I saw, the audio subsystem relied on a clock output from the '9918A that isn't there on the '9928A.

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1 hour ago, Falonn said:

Nice work.  That output looks great.

 

Can you confirm whether the audio still works on the machine?  I haven't been following the thread closely, but last I saw, the audio subsystem relied on a clock output from the '9918A that isn't there on the '9928A.

The audio works on my TI while it is connected from the audio pin from the original connector to the 8 pin din mini connector’s pins 1 & 2 because the TI has a mono sound just like the ColecoVision’s.

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4 hours ago, Falonn said:

Nice work.  That output looks great.

 

Can you confirm whether the audio still works on the machine?  I haven't been following the thread closely, but last I saw, the audio subsystem relied on a clock output from the '9918A that isn't there on the '9928A.

It likely depends on board revision.  Some of the audio chips take the clock from the 9918 chip while some take it from the system clock, according to the schematic.  If it has the TMS9919, SN94624, SN76494, or SN76494A, then its clock came from the 9918.  SN76489, SN76489A, SN76496, or SN76496A come from the system clock.

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I just wanted to +1 this thread as I did the RGB mod this weekend to my TI and it was really straightforward with those tips on the 560ohm resistors for getting it to work.  The hardest part was the mini din connector in my amateur attempt to do a no-cut mod after removing the existing 5-pin din connector.  Man those things suck to solder too!

 

Also, I checked and I had a SN94624 chip on the board but didn't have to mess with the jumpers or switch to different chip when I dropped in the 9928 to get the audio to work. 

 

Very pleased with the crispy RGB signal I get now from my beloved TI.  Thanks for listing out what to do here.

 

ti994a.jpg

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Posted (edited)

I followed your guide step by step and got beautiful RGB from my NTSC TI-99/4A!!!

 

I replaced the TMS9918A chip with a TMS9928A from eBay (A socketed IC--what a breeze!). I soldered on the TMS-RGB, and added the 560 ohm resistors on the lines where indicated.


Where I marked here, there was a resistor (22 ohm?) already populated on the board, with its left leg going into the left throughhole (orientation matching the image). I removed it and replaced it with 560 ohms, with the left leg going through the throughhole to the right of it this time, to match the picture. I should note--it was picky--it truly had to be 560 ohms. I had tried with a higher resistance at first (I didn't have a 560), and the monitor could not find sync. My fix was to replace it with two resistors in series to sum to 560 ohms, and now it works like a charm.

 

Since I'm not using the stock DIN connector, if I've read the other posts right, I'm safe to ignore the red and blue lines.

 

71696908_ScreenShot2020-08-25at6_02_32PM.png.6650d98eed2fb150a618517bd24b04c6.thumb.png.b9de68469f45b985cba60d8797853d3b.png

 

 

Edited by mdfk
I only meant to include one image.
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Where can I find the right 8 Pin Mini Din Scart RGB Cable to connect the output to an OSSC, ideally from a US seller?

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I chose a Sega Genesis 2 jack (purchased from Console5, with breakout pcb designed by mobiusstriptech) because, unlike the 8-pin mini din used with the NESRGB and Micomsoft Framemeister, the Sega Genesis 2 cable includes audio, and quality, shielded RGB cables for it are fairly easy to find. I will need to find a way to secure this jack much better than it is now. I put some kapton tape on the top there so as to not short anything where the original jack was located (there is 12v and a ground under here).

I circled the 560 ohm resistors that I installed, and you can see that I replaced the TMS9918A with a TMS9928A.

I also took a shot of how it looks on my PVM (forgive the moire pattern).

pvm.jpeg

sega genesis 2.jpeg

tms-rgb.jpg

560 ohms.jpg

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I was planning to do this mod but I'm struggling with figuring out how I'm going to connect this to the OSSC.  I ordered a fully assembled TMS-RGB board from mobiusstriptechnologies.com - and that board has the 75 ohm output resistors on it.  According to https://tms-rgb.com/guideOutput.html that means I can't use the Genesis 2 type cables because they add additional resistors - I have to use the XRGB 8-pin mini DIN, but I can't seem to find anyone in the US who sells a cable for that connector - retro-access.com seems to be MIA.  How are others wiring this up?  Thanks!

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Okay, I bit the bullet and ordered the cable from the UK, and it shipped surprisingly fast.  However, I ran into a frustrating issue preparing to do this mod.

 

I have 3 consoles, and after spending an hour disassembling and reassembling each of them, they all had the SN94624 sound chip, which gets its clock from the 9918, so as I understand it, if I were to complete this mod, I would lose sound.  Even though all three TIs looked different on the inside in terms of how the power supply was connected, they all had the same sound chip. 🤬

 

So, is there any way to reroute the clock to the SN94624?  Is it possible to swap it for a different chip?  Help!

 

Thanks,

-Mike

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Oh, LittleLarrySellers said he has the SN94624 and sound worked fine after the mod, so maybe this will work?  Is what is written in post 33 backwards?

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6 hours ago, webdeck said:

Okay, I bit the bullet and ordered the cable from the UK, and it shipped surprisingly fast.  However, I ran into a frustrating issue preparing to do this mod.

 

I have 3 consoles, and after spending an hour disassembling and reassembling each of them, they all had the SN94624 sound chip, which gets its clock from the 9918, so as I understand it, if I were to complete this mod, I would lose sound.  Even though all three TIs looked different on the inside in terms of how the power supply was connected, they all had the same sound chip. 🤬

 

So, is there any way to reroute the clock to the SN94624?  Is it possible to swap it for a different chip?  Help!

I'm not sure what I was smoking when I wrote that.  The 9918, 9928, and 9929 all have the clock output on pin 37.  It's up to each individual TI99 whether or not they tap that clock for sound, or use the system clock.  But changing the video chip will have zero impact on sound, since the clock will still exist in either case.  If it used the video chip clock before, it will continue to do so.  If it used the system clock, it will continue to do so.

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Success!  I followed the instructions in Post 28, namely:

  1. Disassembled the console
  2. Removed the 9918A with a chip puller
  3. Soldered a ribbon cable to the TMS-RGB
  4. Soldered the TMS-RGB (ordered from Mobius Strip Technologies) under the VDP socket on the back side of the board
  5. Soldered a wire to the sound output
  6. Desoldered the video connector and removed it from the board
  7. Desoldered the one resistor that was where the bottom 560 ohm resistor goes
  8. Added and soldered the three 560 ohm resistors
  9. Soldered the ribbon cable and audio cable to a mini-DIN 8 jack following the XRGB wiring diagram (using this helper board)
  10. Bent back the shield near the mini-DIN jack to make enough space for the new connector + wires
  11. Attached the mini-DIN jack (upside down) to the board with solder and hot glue, using a nut as a spacer (otherwise it is too low for the opening)
  12. Double-checked continuity between the pins of the mini-DIN jack and the TMS-RGB and checked for shorts
  13. Cleaned off all the old thermal paste from the shield
  14. Inserted a TMS9928AN (purchased a used one pulled from a working system on eBay) into the VDP socket
  15. Applied new thermal paste to the VDP
  16. Covered soldered wires with electrical tape
  17. Put electrical tape around ribbon cable where it comes near the shield to go around the board
  18. Put everything back together
  19. Connected the mini-DIN jack to my OSSC's SCART connector via the recommended cable
  20. Set the OSSC for AV1 (SCART) RGBs

I powered it up and got the most beautiful crisp title screen.  Audio works fine with the SN94624 sound chip.  My TI has never looked this good!

 

IMG_6053.thumb.jpeg.d01d2fcfefc66f4c175726a3fd820ae6.jpegIMG_6054.thumb.jpeg.86a01c3bb21d9954da99dfa2efc8e37b.jpegIMG_6056.thumb.jpeg.a93c511e80bdfb68142eb83942c2fb64.jpegIMG_6059.thumb.jpeg.6e63b7b5b90d0613c2ba28fe6741d1cf.jpeg

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Posted (edited)

I like the way you routed the connectors up around that screw boss in the cutout on the side there. That's better than how I did it. In re-assembly, I ended up having to cut a lot more of the RF shielding. Hindsight's 20/20. Very nice.

Since I'm using this Genesis 2 cable (with resistors in the SCART head), I'm going to remove the 3 SMD resistors on the TMS-RGB board and jumper them with solder. It didn't look too dark to my eyes, but I'm also sitting right next to the screen, in low ambient light. I will report back.

(I might have also chosen the Genesis 2 cable because the VDP on the Sega Genesis can trace its lineage to the TMS9918 series of chips. The story my head is that the grandchild is taking over the family business, so to speak.)

Edited by mdfk
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21 minutes ago, mdfk said:

I like the way you routed the connectors up around that screw boss in the cutout on the side there. That's better than how I did it. In re-assembly, I ended up having to cut a lot more of the RF shielding. Hindsight's 20/20. Very nice.

It's a tight squeeze, but it fits.  I wrapped that part of the cable in electrical tape to protect it against the edges.

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Posted (edited)

I connected the audio from the stock DIN connector pin 3 through hole, with the short little red connector seen in this image. (You can also see that I'm trying to figure out how I'm going to secure this jack. I thought maybe I could solder it in with some legs, then cover it with hot glue?) I put overcoat pen over the holes for 12v and composite (pins 1 and 4).

20210329_101153.jpg

Edited by mdfk

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One caution on attaching the din connector directly to the motherboard is that at least for me, the case blocked the bottom of the port. I put a small nut under the connector to raise it up a bit.  I did my best attempt at soldering the nut to the board but the solder didn’t want to stick to it. So I put a bunch of solder on the connector and then hot glued the whole thing.  Still worried it will break off at some point though. 

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