Adapt Spectravideo SVI.728 for second cartridge slot and improve sound

From personal communciation by Lauri Mela 

My only cartridge slot is full and I can't use SCC-carts at the same time with 3.5" drive.
The SVI-707 however connects to the back of my MSX, expansion module interface 
(50-pin ribbon cable). 
I found out that signaling to the backside is the same as with the regular cartridge slot. 
My next project is to add another cartridge slot using the backside connector. 
Should be pretty easy if it works straight as a regular cartridge slot.
 
I got another cartridge slot working, I found a scsi-cable with 
50-pin normal scsi connector and 50-pin connector identical to the cartridge
connector. Only problem was that pin 1 would be pin 2 and vice versa 
for all pins. Then I found a another scsi-cable with normal connectors and it 
was about 2cm in length. Between that and the first cable I put a connector

The real one has of course 50-pin on each side. I only needed flip the 
2cm cable and then pin problem was fixed.

Note that this modification for a seond slot can be made on other MSX-1 computers as well
such as the Toshiba HX-10. Note and measure the need for reversing top and bottom rows as
described above.

Sound modificion

Another modification was to replace on resistor on the motherboard, 
because
PSG sound was too loud and SCC couldn't be heard. At this page there 
are instructions for 738, but information for 728 model couldn't be find on
internet.
With the 738 instructions and 738 schematics it was figured out which resistor 
should be replaced on 728 model. The resistor is R53 and is located near the 
soundchip. It is to be replaced with 150K resistor (original 4K7) as with 
the 738 model.