The project continues. After discussing the options for on-board RAM replacement (with 2 megabyte board by Andrew Koryakin) with our guru (Caro), it has been decided to remove all mapper control ICs from the board. They will be replaced by sockets, so that the upgrade could be reverted to the original state at any time. Four logic ICs were removed (314, 319, 322 и 359), also the via on the board was cleared to solder the pin to connect MSX Engine's SLT32 signal to the RAM board.
Also, as the preparation for the upcoming MSX2+ upgrade several solder pads were prepared to host the diode (slot 0 expansion) as well as the pins for connecting the FMPAC/F4 board. Also a transistor and a resistor were removed from the CSYNC circuit to prepare for the CSYNC out modification.
To my great surprise, I could desolder the v9938 VDP using the ZD-915 station without damaging the chip and the board. That was way easier than I previously thought. So I saved the VDP chip for future repairs! Also, as it appeared, the ZD-915 is a very good instrument to clean the pins of the desoldered chips from the remaining solder.
What remains now is to write MSX2+ and FMPAC BIOSes into five 27C256 EPROMs, install the v9958 VDP, a couple of diodes and to check whether this custom MSX2+ upgrade with the on-board RAM replacement succeeded. So far, the prepared mainboard looks like this:
Once again, nice job! This is going to be a beast 
I could not wait for the weekend and within a few hours finished the upgrade of the restored Yamaha and also completed the experiment of upgrading (actually - replacing) the on-board RAM to 2mb. As a result I've got a funny "sandwich" inside the computer - CPU relocation board, RAM expansion board designed by Andrey Koryakin and the FMPAC/F4 combo board. That's how it looks:
The boards sit in each other's sockets pretty well, so this "sandwich" is safe. The vertical separation between the boards is big enough for adequate cooling. The distance between the top of Z80 to the upper case's cover is 3cm. It should be noted that the RS232 board holder on the PSU's case should be bent downwards (see the red arrow) to allow the proper installation of the "sandwich". Otherwise the metal part would touch one of RAM chips:
As a result, at the end of this project I've got the following configutation:
- YIS-805 MSX2+
- FMPAC mono
- 2mb RAM in slot 3.2
- Gotek floppy emulator with OLED screen as drive A:
- 3.5'' floppy drive as drive B:
- SFG cartridge in the side slot
- improved video board (composite video, S-Video, RGBS)
Fom now on this restored and upgraded computer will be my number one instrument to develop and test various MSX projects. One of such new projects will be soon announced on this forum.
Now I would like to share some details about the replacing of the on-board 128kb of RAM with 2mb of RAM on the separate expansion board designed by Andrey Koryakin. The board's project can be found here:
https://hansotten.file-hunter.com/do-it-yourself/memory-mapp...
To install this RAM expansion board into YIS-805 (not necessarily MSX2+) and similar computers you need to do the following:
- install a socket at the CPU placeholder
- assemble the Z80 relocation board ( https://github.com/RBSC/YIS805_Adapter )
- remove all 4 RAM chips
- remove 4 mapper and register control chips
- install a pin to connect the board to SLT32 signal
I would like to mention that this upgrade is easily reversible - to restore the original configuration it's enough to plug back all RAM and logic ICs. So, no Yamahas were harmed during this upgrade. :)
THE PROJECT IS 100% COMPLETE! Thanks to everyone who followed the progress and helped to get this done!
Though the restoration project was considered as completed, I could not stop thinking that I hadn't restored the broken floppy drives to 100% of their original condition. The write protection was still not working because of the lack of proper microswitch. After re-reading the Russian forum I noticed a post from one user who mentioned that similar microswitches had been used in some old PC floppy drives, namely Mitsumi and Samsung.
To my surprise, I found two Mistumi D359T5 and one Samsung floppy drives in my closet. One Mistumi drive was missing the front panel, so it was a good candidate to become a donor. After opening the drive I realized that it actually had 2 of those microswitches instead of one. And I had 2 Matsushita floppy drives to fix. What a coincidence!
I desoldered both microswitches and after a couple of hours I could design and print the 3D model of the proper holder for them. Then the microswitches were installed into both drives and they were tested in the restored and upgraded Yamaha YIS-805 MSX2+ computer. Both drives were correctly detecting the presence of the diskettes and write protection worked like it should. So the drives were finally fully restored!
Here are a few photos of the microswitch replacement process including the wiring diagram:
The 3D model of the holder for the microswitch can be found here:
And, finally, I decided to replace the rotten foil shield on the back of the flat ribbon cable that goes from the front slots board to the mainboard. I don't think that this shield actually does anything useful because of the short length of the ribbon cable, but I could not stand the rotten foil. I had to desolder the ribbon cable from the front slots board to be able to repair it. At the same time I've made a clone of this cable for my spare YIS-805 front slots board form the SCSI cable and 2 new connectors, just in case...
