Sega Mega 1 CD VRAM problem

Hello.
I'm working on a PAL Mega CD mk1 that I suspect has a memory problem. The system boots but starts displaying static vertical lines on the logo screen. Also when the spinning logos are moving the lines move with them. The CD player interface is perfectly fine. Memory manager is garbled and games will not start.

I have fully recapped the main board and I see no leakage damage.

I guess that this is a VRAM problem and tried to locate any broken traces along with the schematics on Console5, but some of the IC have different numbers (IC7,8,11,12) than the PAL unit here. I found no trace problems. I marked the areas in green that I checked and linked to the 315-5548. All ok.

My question: Is this the correct area I should be checking? any other problems that might cause this issue. I did clean the contacts of the ribbon, and the CD to Megadrive connections..
Does it sound like one of the zip chips have a fault internally?

 

You could try reflowing the legs on the 315-5548 too, you can find the legs seperate from the board, but when you press with meter probe, you make the connection so it tests OK.

 

Did you notice that the chip is actually 315 5632 not a 315 5548?
Principal is the same though reflow it I have seen the solder joints crack on several model 1 units.

 

You could try checking the temperature of the RAM chips, overheating chip (ie almost burns your finger on touch) can be a telling sign of failure.

 

You can spray them with freeze spray too. Find the problem goes away when you spray the right chip and comes back as it warms up.

 

you were right Bad_Ad84. Loose connection on one of the pins. You can see that the pad is much longer than the legs. when I tested I tested from the pad and not the pin legs. I went over all the connections with my finest soldering iron tip, and it was working after. Not sure which leg was causing the problem, but somewhere between 17 and 65 i think.

 

Kewl!