400 video and RAM mods done - 99% done

[marauder666]

Just need the DIN socket installing, but here's the gutshots.

 

Modded this for a mate, Mamemeister on Youtube, he's never been able to use it with 16k and the worst RF pic I've ever seen. Luckily, he never paid for it, it was a freebie.

 

Cosmetically its in good condition, but pretty much useless in its current state.

 

First was to check sort upgrade the RAM to 48K, all went well, worked first time.  Tried to test and the computer booted faster than the AVG and SDrive-Max, so I changed the CPU reset timing cap to a 680nf from 100nf, and it gives enough time to the devices to sort themselves out before the 400 is looking for them.

 

Tested with BASIC and some newer cart images and they worked fine, but pretty much invisible in the snowstorm onscreen.

 

I used a FMS6400 chip to buffer and filter the luma and chroma signals from the, needed a 1k resistor to ground on the luma, and a 75R on the chroma to get video to the specified input levels, after that the circuit is pretty much the datasheet one.

Signals are picked up through the existing modulator pin header, apart from chroma which is through a wire with a connector for easy assembly.

 

Audio I used a TLC2272 opamp to buffer the output of both the POKEY and SIO audio.  I used a CDROM audio cable with the pins swapped around to allow easy disconnection on the audio circuit.  

 

Picture only my notoriously bad Toshiba LCD over S-video has some very slight jailbars, but I'm very happy with it, especially on a CRT.  UAV would get rid of the transistion between luma 7/8, but thats pretty much it.  Looks great.

 

I'll add some CRT pics when the DIN is installed.

 

 

 

 

 

Edited April 24, 2022 by marauder666

[marauder666]

100% done.

 

Tidied up the video board installation by using a board the original modulator was mounted to. FMS6400 chip has nice composite and s-video outputs so no need to switch them using the RF channel switch, that hole slightly enlarged, is now a standard 5 pin DIN output.

 

Very pleased with result, only issue is the AVG cart doesn't fit when the shielding is back around the cart slot.

[ZylonBane]

2 minutes ago, marauder666 said:

400 video and RAM mods done - 99% done

So you've done 396 mods?

[marauder666]

8 minutes ago, ZylonBane said:

So you've done 396 mods?

It seems like it on 2600 and 7800s

[_The Doctor__]

GTIA fix for the line down the center mod?

 

397 FTW !!

Edited February 9, 2023 by _The Doctor__