SEGA CD glitchy screen

[NeWmAn]

My old SEGA CD (actually a V1 PAL unit), that used to work perfectly, has started showing glitchy/blocky images during the initial boot logo/intro and some FMV videos.
I've found that most of the little 10uF and one of the bigger 100uF capacitors on the PCB were leaking, so I've replaced all of them, but that did nothing
Does someone know what could be the cause of the problem?
Thanks!

[l_oliveira]

Video decompression and the zoom/rotation/scaling is done in the big (208 pin) SEGA LSI. There, a special DSP processes such effects and then put the data on a special memory called "shared ram". Then the Mega Drive reads that information either using the CPU or the DMA controller on the VDP (Video Dysplay Processor) chip.

It's more than likely that program code is transfered normally using the CPU for data copying while graphics are transfered through the VDP DMA.
If there is dirt on the MEGA CD connector it could be possible that the bus is not being released by the CPU during DMA cycles, causing the graphics only problem.

It happened once with my mega CD and was easily solved by cleaning the board connections on both the CD unit and the console.
Keep in mind that since your unit is a model1 there's a second internal connector that binds the U connection board with the main board. Clean the connectors there too.

[Tiido Priimägi]

its just odd that MD actually runs code off the ROM in MCD, and yet it won't crash, but you have graphical corruption, which usually gets solved by just cleaning the contacts...

[l_oliveira]

It's just a matter of *which* pins are having issues ...

[NeWmAn]

The problem was in one of the control lines, probably b22 (/UDSW) but I'm not sure and I have already reassembled the unit.
The connectors were already clean, the problem was elsewhere:  a ceramic capacitor on that line was causing trouble and the signal appeared lower than what it should have been.
Without that capacitor the Sega CD worked perfectly 

[l_oliveira]

Good to know that you found the real culprit.  Still all Genesis consoles I've seen had a cylindrical capacitor on that line, soldered right on the connector for the SEGA CD.

Also some units with the 315-5433 bus arbiter  chip have a track cut on the /AS line of the 68000 and a 220 ohm resistor placed in series.
Without that modification the unit can crash often.  Typical post production fixes. Still I wonder why a "fix" on the board would cause you trouble with the SEGA CD ... Was the capacitor damaged in any way ?

[NeWmAn]

Good to know that you found the real culprit.  Still all Genesis consoles I've seen had a cylindrical capacitor on that line, soldered right on the connector for the SEGA CD.

The capacitor was on the underside of the CD mainboard, right next to the connector and it didn't looked damaged 
So the Genesis had one too? That's explains why I had different (more or less severe) glitches trying different consoles with that CD!

[Tiido Priimägi]

It's just a matter of *which* pins are having issues ...

Its not like when you transfer GFX data, some other signal gets used... I actually don't really know what are the few control signals on the MCD connector... I know there's quite a few address strobes.