Colecovision DRAM Replacement Question

[bradd1978]

Hi Everyone,

 

Thought I'd check in and see if anyone could help me with an issue I'm having with my Colecovision.

 

Long story short, I've done all the typical repairs on the Colecovision (power switch, bad solder joints on power supply) and i'm still getting glitchy, scrambled graphics on my system. I know this can be a DRAM issue. I went ahead and ordered some 4116 Ram but I'm not sure what ones to replace. I could go ahead and change all 8 but would rather pinpoint which one(s) are faulty.

I looked up the Colecovision service manual and it showed example screens of screen glitching and the appropriate Ram to change (U10 - U17). I did a check on my console without a cartridge and my screen problems didn't quite match anything in the service manual....

 

So......

 

I'm posting a picture of my screen without a cartridge in hopes someone with a bit more knowledge can steer me in the right direction. In the end, if I have to replace all 8 ICs, then so be it, but if I can avoid it, that would save my mental health

 

Thanks in advance

Bradd

[Kiwi]

They're all flawed. They are cheap VRAM that can currupt on impact or vibration. It usually clear up when disconnecting the power cord and plug the cord back in. 16KB of RAM back then are expensive so I think Coleco cheap out on this area of the hardware. I would replace them all with good quality VRAM.

 

Just like the Game Gear surface mount capacitors, they only last about 20 year til they dry out.

Edited March 5, 2013 by Kiwi

[+nanochess]

It looks like it's lowest significant bit, that would be D0 from Z80 point-of-view, but Texas Instruments wanted to do something different, so its VDP is marked in reverse, so the corresponding pin is 3 (TMS9918A) named AD7.

 

You should check with a multimeter if there is connection between pin 3 of TMS9918A and corresponding data pin in DRAM memory, other signals you should check for continuity are RAS, CAS and R/W.

 

If every line in PCB is fine then you should proceed to replace DRAM chip.

 

I hope this helps.

[JamesD]

If you look at the numbers in the copyright you'll see the leftmost bit seems to be stuck on.

That probably indicates a DRAM at one end or the other of the row of chips but I don't know which it would be.

Based on what I've seen I'd yank them all, put in sockets and do a 4164 mod so you don't have issues again.

[bradd1978]

Success!! Well I ended up starting at U17 and replaced the Ram. Still had the graphic glitches but it improved and I could see it was an issue with U11 according to the service manual. Replaced it and we have a clean picture :-)

Thank you everyone for your help and comments

Have a great day

[+nanochess]

Success!! Well I ended up starting at U17 and replaced the Ram. Still had the graphic glitches but it improved and I could see it was an issue with U11 according to the service manual. Replaced it and we have a clean picture :-)

Thank you everyone for your help and comments

Have a great day

I'm glad your Colecovision now works fine.

[SKELETOR68]

I know I could not help, but still glad it is working.

[max1227]

Where do you find replacement chips for the colecovision and ADAM?

[+evg2000]

Where do you find replacement chips for the colecovision and ADAM?

I haven't bought the ram upgrade from them, but I've made other purchases from them and amy very happy with service/product. Looks like they currently have a10 shipping delay, maybe vacation?

http://console5.com/store/kits/memory-kits.html

[max1227]

Ok, great thanks!

[bradd1978]

I ordered my replacements off ebay. You can still get them on there from a seller in Europe. Shipping was super quick, like a week, and I'm in Canada :-)