Sega Cd mod1 video issue

my sega cd model 1 has some very odd video issues..ive replaced some caps but no change.for a split sec when the planet image shows on screen right before the "sega" graphic is displayed its 100% clear,but as soon as the dancing sega logo pops up the screen box goes just like the picture.

after you start a game,the sega logo with sonic is perfect..

 

Looks like a ram corruption.

 

sol??

 

Replace ram with similar components.

 

it seems if it was ram it would do it on the entire screen.and like i said a couple of the screens it doesn't do that on.

 

No, that does look like ram. Chips can also fail as they heat up.

It looks like the vram and if only a certain part of the ram has failed or one address line has failed it would only happen in certain places and work fine in others.

 

well crap.any clues and which chip it may be,and or a lead to find a supplier?

 

seems that your right.as sonic cd plays fine outside of the movie.so anytime fmv is displayed is wheb the problem happens.

it seems i have two vram chips with a zip pinout.
TC511664BZ-80 9221HBK .and a TC511664BZ-80 9212HBK

i found a seller on ebay with a picture of the TC511664BZ-80 9221HBK but lists it as a TC511664Z chip.will that one work? and also can i just get two of the 9221hbk chips for both or do i need to get a 9221hbk and a 9212hbk how it is now??

 

Note that there are 3 ram chips there, one 512k main ram, and two 128k (I think?) chips for "word ram", the latter two being responsible for communication between the CD and the Megadrive.

I've a Sega CD 2 that looks like that too, starts out normal then over a few minutes it starts throwing more and more glitches, eventually throwing random reboots or hanging up. Always in random ways too, so not just an address error.

It started doing that after I installed a socket for the bios, I used a hot air station but on the back of the board, as I did not want to accidentally melt the huge EXT connector, which is forming an alcove over the rom chip. It was a very painful process that took way longer than it should, and since the RAM chip is so close, I'm pretty sure I ended up burning that out. I've been meaning to replace it for a long time now... it's a 512k DRAM chip, a HM514270AJ8, I want to use one of the similar DRAM chips from a dead Saturn as a replacement (the sound ram in the Saturn is the same chip).

However it could also be the TC511664 being the fault on my end too, since that's what handles communication between the two units, which includes transferring graphics from the CD into the Megadrive VDP.

 

You could probably fault find that by hitting the chips with freeze spray. Sounds like it gets worse as they warm up. Being cold might make it work and also ID the correct chip

 

This is a good recommendation.
If cold is not available, you can also use heat the same way.
The glitches will change if you hit the right IC.
Just don't cook it

 

well that wasn't it.i replaced the vram at ic7 and ic8 and no change.

 

Check out the Sega CD Repair thread on Sega-16 forums. Amazingly useful.
Also, it sounds silly but...have you tried another Genesis console to rule out that issue?
Video RAM is what it sounds like to me as well, but it could be an ASIC pin corrosion issue too. If it boots fine and THEN gets corrupted it is very, very, very likely to be RAM though.
Save yourself some headache and buy a broken one with a bad laser but good video output. Will run you about $20 on ebay.

-Segasonicfan

 

That is broken traces between the ASIC and the WORD RAM chips. Leaky capacitors are the likely cause.

Word RAM are the large ZIP chips near the leds at the front.