Ok so I couldn't resist a cheap multi mega faulty on ebay anyhow it plays audio cds fine when switched off. However when powered on there is only a black screen on the tv I can make out a faint block area near the top of the screen if brightness is turned up. It has been recapped. Previous owner says it just stopped working. The z80 & adjoining chip get warm & also one of the big asic. Could this be a bios or ram failure any help would be appreciated.
Scope the ram and bios, see if any address lines are stuck or data pins look dodgy. You could also pull the ram and test it out of circuit, you can also dump the bios to see if it matches dumps. Have you checked for corrosion etc on the pcb? Cleaned the cart slot?
Make sure the caps are soldered in the correct polarity! NEVER underestimate how FU*&ing moronic people can be! Always do the non destructive test first! CHECK THE CAPS!
Data lines d0 d1 & d2 look dodgy on LC3664BML-10 in comparison to the other one. My scope is at its limits I think lol I will try to upload a pic
If it looks dodgy, best to take it out and test it. Pretty much just using the scope to point you in correct direction. (or if you dont have test equipment, replace the chip with a new one to rule it out)
one bad & one good trace same pin either chip. Stuck ram or short ? Caps are all correct & good there was also a few dodgy asic legs but these have been cleaned & tested for continuity. Also the cart slot is one of the cleanest I have ever seen
Apparently this chips hard to get but I see there is compatibles interesting ram tester program link below http://www.sega-16.com/forum/showthread.php?30044-Genesis-SEGA-CD-multi-tool-homebrew/page2 If only the cdx booted lol
I was implying to test them externally. If you have a universal programmer - a lot of them allow testing of some types of ram. Or just replacing it works, as they are often quite cheap.
I'll admit somebody must do the job once in a while but... Just thinking about a rational human being recapping a MULTIMEGA is bonkers. Even the most gutsy professional engineer has every reason to balk at its design. Then again cap damage so soon is odd enough. Multimega released after the time frame commonly associated with Sega's bad caps.
Rubycon caps are good if they used cheaper we would be in a mess now. Just look at a vestel tv pure crap.
Small update Z80 & adjoining ram removed & tested in a megadrive 2 working perfectly. Looks like it's definitely the 5660 asic then
I've seen this issue before and documented a repair of it on this very site. If you search around you will find a lot of info I've posted for repair of these on the web. See here: http://assemblergames.com/l/threads/sega-cdx-issue-need-advice-or-parts.56914/page-2#post-826051 Your issue is fairly common, often resulting in corrosion coming in from the cart slot. A pin or two will usually go bad on the top of one of the ASICs there and will require some very fine soldering (30AWG or smaller) to repair a trace. Make sure to meter out the wire after you solder it in, since the points are small it would be easy to short if you are not used to SMD work. -Segasonicfan
