well I'm very curious what you'll find. The circuit for the rtc is indeed not very difficult, but I'll-might-get-to-that - if the rest finally works. I'll detach the mobo and take some pictures today too.

I have also a Yamaha CX5MI/128 and have made the changes to the VRAM which was on the main board. I have burnt the EPROMs for MSX2 and made the changes by connecting the subrom to address to 0000. I was able to obtain a SONY floppy drive (MSX is very scarce in Canada). The machine boots fine and comes up with the MSX2 Basic and MSX2 Disk Basic and indicates 00KL and 128K memory during the logo on booting, but I cannot boot to DOS. I have tried creating the diskettes but an so far unsuccessful. Do I have to make changes to the memory mapper? I suspect may have a memory problem so does anyone have a basic program that will test my memory. Was there a change to be made to the CX5MII memory mapper to make it work? I have the rom with no clock. I cannot recall where I obtained the image. I have also obtained the SUN IDE interface and am able to boot from it but alas still cannot get into DOS. It sems to hang or loop. Any help would be appreciated.

the CX5MII of my friend are the same PCB

you can see the pictures of the additionnal clock on this french web site

http://www.msxcafe.com/modules/xcgal/index.php?cat=10005

hey, that's cool, once I get the roms done I'll check that out surely, thanks!

the rom are copy of 8280 in this CX5
8250 works also

Well I got it working, the IC# for this board is IC16 for the MREQ signal. Thanks all for the help

grit - could you write a new or updated upgrade guide?

I was planning on that but.... I got a rom which stated 64K main memory so I thought; 'well it works, let's change that too' I put in another rom and now it does NOT work anymore. Problem is I tried various roms to see which one worked and now I don't know which one it was originally . I suspect it were the Lois C Grosso roms from hansotten.com (which had a slightly different keymap by the way) As stated above I also directly 1-1 copied the roms from my 8280 but that was a no go. Maybe because my CX5MII lacks RTC? Checked the address of the extrom when I put in the original MSX1 rom but that all looked fine. When I have a working rom I'll write it up.

Thanks in advance!

Well, I can verify that the Grosso Roms are working but I can't call it 100% yet, the keys are not mapped correctly (-I have a USA version CX5M) and the 64KB main ram is a bit annoying. I think I'll better see where I can adapt the 8280 roms for no-rtc use...

search a breaking VG8235 and desold all clock component
resold on a pcb in your CX5MII
is not easy but after you have a real MSX2 with clock and set screen work
see schema of 8235 and copy

I was just going to ask you where the * you got the Ricoh chip and I was also wondering how you adjusted the trim capacitor on you rpictures but now it all makes sense.

Has anyone thought about a pic chip emulation for the clock chip, since they are as scarse as hens teeth?

Well, I don't know what's tougher, getting a PIC to do this or finding an MSX2 or Amiga to butcher....

Here I've put a little writeup on the conversion:
http://www.cx5m.net/msx2/xc5m_m2.htm