2600 Composite mod

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exxos
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2600 Composite mod

Post by exxos » Mon Mar 19, 2018 1:32 pm

cm.jpg
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This board I purchased ready made and apparently is based on the "Longhorn Engineer mod" which seems to be the best mod which people go for. Finding information is hard as there are so many clones and so many "lost" sites it was a epic to work out what this mod is actually doing.

So to start with, it solders direct to the TIA. So the motherboard CD4050 isn't used (which I later removed). The board uses its own CD4050.. which seems a little pointless... Some sites replaced it with a HC device, I would assume as its faster, it would give a sharper initial signal.. Though its not a simple swap as the pinout does not really match other chips..

Basically what we have is the 4050 driving its own resistor ladder and then it is amplified with a transistor. At this point I'm getting confused as the 4050 is outputting 5 V, the resistor ladder is actually dropping the voltage, where then the transistors used I presume amplify the voltage for the next stage. It would have seemed simpler just to adjust the resistor ladder and not have to use a transistor. In any case, I remove the transistor and it made no difference at all to the video quality or brightness. So not really sure why it is even there. I can only assume some original version of this circuit is used to drive the monitor directly from the transistor, later it was modified with a buffer IC after it. Which is fine, we basically have two buffering circuits in that case does not really make much sense.

Another thing I'm not totally sure why, is that the colour pin (TIA pin 9) isn't buffered at all. I would assume it would be better to have all the colour signals all perfectly in phase and buffered.. Is something I need to check on further..

The next stage uses a video conditioning chip generally seems to be the FMS6400. It seems a little hard to find now and is actually obsolete, but it does do the composite signal merge and does buffering also. It also has some internal filters which should help clean up any noise in the signal.

I used pole position my testing.. Title screen with it having a chequered white and black background may be good for testing colour bleeding. The mod seems to do pretty well with this aspect. I did experiment around with it a little and it managed to make it worse :lol: but it seems any sort of load on the TIA is the cause this colour bleeding issue.

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Main game screen with it having a lot of solid colours like the blue sky makes it a good test for any noise in the signals. It is probably difficult to see, but there is vertical bars in the blue sky seem to be every few pixels all the way down the colour blue. This could be down to my fantastic wiring, but I have been unable to find a cause for this otherwise.

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There also seems to be some sort of juddering on the white part of the image. The main school at the top seems to be juddering a little bit, even though the rest of the image appears to be okay. I also see the same in the middle of the title screen flag, where it seems to be juddering slightly. Again I am not sure why that is happening.

Overall, it is not a bad modification that it does seem to have some little annoying niggles. Other than this juddering problem and noise problem, I think this is down to PCB layout issues. Clearly the people designing the circuit and not read the layout guidelines for anything, or even have any experience with HF PCB layout design. So I think if a proper PCB layout is done I think some of these little issues will go away anyway. I could do with doing some more tests, but someone may have blown up something on the board not currently working :roll:

I think the 4050 is not exactly needed, but I think buffering it will definitely not hurt. There is a similar series of chip just does video filtering and buffering, it as a few inputs and outputs, so I think this would be a better choice than the older 4050. I think the transistor can be done away with along with some other parts. I don't see the need in a pre-buffer to the video driver chip itself. The likely I will adjust the resistor ladder values in the hope it will add a little bit more quality to the image.

I need to start designing a new PCB next...
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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Re: 2600 Composite mod

Post by exxos » Mon Mar 19, 2018 2:46 pm

Test PCB done... will order a couple later... :ball:

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4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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rubber_jonnie
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Re: 2600 Composite mod

Post by rubber_jonnie » Mon Mar 19, 2018 2:50 pm

Nice
Collector of old Atari things:
800XL + Ape Warp mod, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg) + Gotek, 1040STFM + Vortex ATOnce + Gotek, 1040STF long button floppy, 4160 STE with Gotek and ROM switcher, 4160STE with 32Mhz booster, ROM switcher and CosmosEx, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

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Re: 2600 Composite mod

Post by exxos » Fri Mar 23, 2018 5:06 pm

I have replaced the FMS6400 chip and I have video again :) though after about 20 seconds video starts to fail and slowly fades to black :roll: leaving the board off for a few minutes and trying again, the video comes back and is fine again for about another 20 seconds.. So clearly something is wrong here.

Trying to follow what is going on with the resistors on the output of the driver chip, it seems they have a 75R in series with the video signal wire, and then 75R resistor to GND... This basically looks how it does in the datasheet examples circuit below only minus the capacitors...

657.jpg
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The thing is, the 75R is actually on the opposite end of the cable to the driver chip. This suggests the 75R load is actually the monitor itself, not a actual load resistor appears to be in the diagram above.

I removed the 75R to GND and now the chip gets nowhere near as hot and I have had it on for a couple of moments now as opposed to a few seconds. And the video seems fine. In actual fact the video quality seems to have improved as well fractionally.

So I tested the SVIDEO socket on the back of my monitor on resistance, and I measured 75R there! So it seems my suspicions about this resistor seem to be correct and that is not a actual load resistor as it appears in the circuit diagram.

In light of this, I have no idea why so many of these circuits have sold and nobody has mentioned this issue before :shrug:

The datasheet says..
A single video load consists of a series 75Ω impedance matching resistor connected to a terminated 75Ω line. This presents a total of 150Ω of loading to the part. A dual load would be two of these in parallel which would present a total of 75Ω to the part.
But in my case I actually have 75R with a termination resistor on the PCB, along with 75R termination on the monitor itself. Resulting in 37.5R! This in series with 75R results in 112.5R load (a fair bit under the quoted 150R load). The datasheet says maximum output is 40mA per output. So from 5V that is 5/0.040 = 125R. This explains why the chip is incredibly hot and dies in under 20 seconds! :roll:

I also tried a series capacitor as illustrated in the diagram in the datasheet, but I did not see any difference at all with or without the capacitor in place. So I assume for this project it is not needed..

So time to tweak my PCB layout now I have learned more..

UPDATE:
Seems when I turn up the "colour" on the TIA pin 9, the colours get bolder, but also seem to get sharper.. which actually I think is bad. As while testing pole position.. the top half of the screen has a pixelated pattern on it.. also on other places.. So I think a little "colour bleed" will actually make the image look better..
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: 2600 Composite mod

Post by exxos » Fri Mar 23, 2018 11:13 pm

Decided to re-design it again.. To save on "wires" this one is done to just solder a header strip onto the TIA.. then solder the PCB on top..

Likely won't have time to finish this new layout until next week now.

r2.JPG
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4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: 2600 Composite mod

Post by rubber_jonnie » Sat Mar 24, 2018 2:00 pm

Nice Chris, hopefully I'll get some more testing done on mine now I have a Pole Position cartridge that I can do comparison tests on with yours.

Mine is a much simpler mod from Tynesoft, as you know, and removes some parts needed fro the modulator, and adds a transistor and 75R resistor.

Seems fine, though I have yet to try turning down the colour with the pot as yet, see if it helps with some of the colour artifacting.

Will post more soon. Also need to do a comp mod on my 7800, which should be very similar.
Collector of old Atari things:
800XL + Ape Warp mod, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg) + Gotek, 1040STFM + Vortex ATOnce + Gotek, 1040STF long button floppy, 4160 STE with Gotek and ROM switcher, 4160STE with 32Mhz booster, ROM switcher and CosmosEx, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

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Re: 2600 Composite mod

Post by exxos » Mon Mar 26, 2018 11:01 am

rubber_jonnie wrote:
Sat Mar 24, 2018 2:00 pm
Nice Chris, hopefully I'll get some more testing done on mine now I have a Pole Position cartridge that I can do comparison tests on with yours.
Be interesting to see how it looks with your setup.
rubber_jonnie wrote:
Sat Mar 24, 2018 2:00 pm
Mine is a much simpler mod from Tynesoft, as you know, and removes some parts needed fro the modulator, and adds a transistor and 75R resistor.
I would try removing it. The 75R "load" is the monitor, there shouldn't be a actual 75R resistor on the board.
rubber_jonnie wrote:
Sat Mar 24, 2018 2:00 pm
Seems fine, though I have yet to try turning down the colour with the pot as yet, see if it helps with some of the colour artifacting.
It seems when the colour and brightness hit a certain spot, there is some odd pixelization appearing.. its like the image is "to sharp" but its like the colour is out of phase with the image on alternate lines.. Will try and get a screenshot of it...
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: 2600 Composite mod

Post by exxos » Mon Mar 26, 2018 11:02 am

ALPHA 3 layout..

Will get a couple made soon.. then the real fun can start :twisted:

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4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: 2600 Composite mod

Post by exxos » Fri Apr 13, 2018 10:14 pm

PCB's arrived :) Now just to find some time...

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4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

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rubber_jonnie
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Re: 2600 Composite mod

Post by rubber_jonnie » Fri Apr 13, 2018 10:52 pm

Cool!

I have a bit of time off now, hopefully I can get back to looking at this. Just got to solder in another RAM upgrade and blitter to an STFM and I'm good to go :)
Collector of old Atari things:
800XL + Ape Warp mod, 2x 1010 cassette, 1050 + Happy mod, 65XE (128k) & XC12, SIO2SD, 2600jr, 7800 and Lynx II
Atari 520ST (1Meg) + Gotek, 1040STFM + Vortex ATOnce + Gotek, 1040STF long button floppy, 4160 STE with Gotek and ROM switcher, 4160STE with 32Mhz booster, ROM switcher and CosmosEx, not to mention various bare ST boards for testing including a PAK 68/2 :)
Plus the rest..
Amiga stuff, Mac stuff, Sinclair stuff etc...
www.electronicnothingness.co.uk

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