Picked up a couple of Colecovisions, one good, one bad

[marauder666]

NTSC, got them over to the UK for less than a single PAL one would cost here.

Cases on both are in good condition, no cracks or broken bits.

One's working fine.  The other just gets a garbage screen.

 

Disassembled switches and cleaned them up.

Voltages seem fine.  Only got one UK PSU to use.

Swapped the VDPs between them, and its still bad, the other still good.

Swapped the 74LS138 between CPU and VDP, same.

I'm thinking it must be the RAM, or could it be something else?

 

I've got a PAL machine with socketed RAM, so it'll be easy to test, get the bad machine socketed up, just swap the whole lot.

 

Ignore black and white on picture, my TV can't really handle NTSC RF signals.

 

 

Edited June 21, 2022 by marauder666

[+-^CrossBow^-]

If you swapped the VDP and still got the same results that pretty much cinches it to the VRAM at this point. And it is high failure point item. There are +5 RAM upgrades that can be done to replace the original RAM out with like but only requiring the +5 and not needing the -5 to operate. I have that installed into my daily driver CV but it is also the one that happens to give me the most issues regarding compatibility with homebrews for some reason but I've never had an issue with any officially released games and my flash cart works great on it.

 

But yeah... most likely the VRAM at this point.

 

Keep us updated on the progress!

 

[marauder666]

Swapped in the 4116 RAM from the PAL machine and it's working.  Ordered some 4164, its much cheaper than the 4116 over here. 

I've noticed on the schematics that it uses -5V on the joystick ports, possibly for detection of the trackball (maybe!!).  Easy to sort with a simple charge pump circuit, which I've got on a PCB from my guitar pedal building days.

 

Or, just not bother and it'll work for the controllers I'm likely to ever own.

Edited June 22, 2022 by marauder666

[marauder666]

As I've no spare UK PSUs for these machines, been looking into powering them from something sensible, and this appears to work just fine.

Mainly for the machine with 4116 RAM, as the other is getting RGB modded at some point, so will just need 5V anyway.

 

12V DC input, 5V DC and 12V DC output, but also into a separate board that generates -5V.

The blue board is a LM2596S switch mode step down converter, these are dirt cheap, and outputs the required 5V.

The green board is a Road Rage from Madbean Guitar Pedals, must have had this lying around 10 years, it uses a TC1044S chip to generate -5V. Actually -4.7V.  I suspect a LT1054 chip would get closer, but its all within specs.

 

The brown board they are mounted on is the RF modulator PCB from a PAL machine, the NTSC machine has 2 of the 3 screw mounts for it, so thats stable enough, and out of the way.

 

Picture looks pretty good.

 

Only issue is that the -5V needs to be active before the 5V and 12V, to bias the RAM correctly at power up.  The brick power supply always has the -5V active too,  so not much of a problem. and the LM2596S doesnt get hot just powering the charge pump.

 

My own FMS6400 composite video mod in this too.  Started using these a lot recently.

 

 

 

[marauder666]

2nd one modded, I've gone with my own S-video mod on this.  It'll be mine, looks really nice through the BVM.

Some jailbars blue skill screens, but I can live with that.

 

Just need some 2.1mm DC jacks and they'll both be done.