Spectravideo SVI-738 MSX2+ Mod

lör 14 september 2013 by kilroy-71

I have two Spectravideo SVI-738 computers. One in mint condition for my collection and one for daily use. The SVI-738 is perfectly suited to be modified as it has plenty of room in the case for modifications and there are also many instructions available to do various mods. It is originally a plain MSX computer but it comes with the V9938 videochip available in MSX2. It is therefore often referred to be a MSX1.5. I planned to upgrade it to a real MSX2, by exchanging the original 16K VRAM to 128k and replacing the ROM-files with MSX2 ROMs. But then I found out that Nyyrikki had managed to do a MSX2+ of his SVI-738 (Home made weird MSX2+) so I decided to contact him and ask how he had done the conversion. He kindly offered his help and the modification described here is done using his instructions.

The mod consists of doing the following:

  • Exchanging VRAM with 4 x TMS4464 chips
  • Exchanging the V9938 with a V9958 chip
  • Exchanging the Main ROM, Disk ROM and RS-232 ROM (I do not think that the disk ROM is necessary to change, but I wanted full 720k floppy support)

I have also done the following mods at the same time

  • Exchanged the internal 360k drive with a new PC drive to be able to use 720k disks.
  • Exchanged the very stiff and easily breakable flat cable between the keyboard and the motherboard.
  • Exchanged the wires between the motherboard and the power supply with cabels usings Molex connectors to make it easy to separate the PCB:s
  • The FM-PAC-, SCC-mod to make the sound with these extensions to work properly

VRAM Exchange

The VRAM on the motherboard is easily exchanged by desoldering the four TMS4416 (IC22-25) and soldering back IC-sockets and adding two 0.1uF capacitors (C17 and C18) in front of the IC sockets. Then place the four TMS4464 circuits in the IC sockets. Start the computer after the VRAM is in place and check that the computer starts normally.

ROM Exchange

This is the more tricky part, not only do we need to desolder IC49 (RS-232 ROM) and put a socket in place, but as the new ROMs are of different size and with different contents than the original ROMs we need to do some additional modifications.Some address lines missing; A13 and A14 which need to be connected on the EXT/RS232-ROM (previously just RS232-ROM). The original ROM was 8k (IC49 2764), but the new ROM is 32K (27256). I chose to connect the A13 line on the underside of the PCB to A13 on IC50.

The A14 line was connected to A14 on the Main ROM (IC51).

Next problem is that we need to change the inverted output enable (/OE) signal to IC49 to make it work properly. The combined EXT/RS-232-ROM that I used had the RS-232 ROM in the upper half of the ROM (address #4000-#7FFF) and the EXT-ROM in the lower half (address #0000-#3FFF). The original RS-232 ROM was activated by /CS1 and the /SLT30 (slot select) signal. The problem is that the combined output is only activated for the memory range #4000-#7FFF. This means that the EXT-ROM will never be visible. The trick Nyyrikki uses here is to use A15 combined with /SLT30 as the /OE signal to IC49 as A15 is low in the memory range #0000-#7FFF. So what needs to be done is to disconnect the /CS1 signal from pin 10 on IC62 and replace it with A15. I did this by cutting the trace to pin 10 on IC62 and taking the A15 signal from IC40. (The cut is marked with a red circle in the picture below)

The disk ROM (IC50) was originally a 16K 27128, but the ROM I used was a 27256. The A14 line on the 27256 controls which half of the 27256 ROM is used (when only using 16k) and it is originally connected to VCC on the motherboard. The Disk ROM needs therefore be placed on the upper half of the ROM or alternatively on both the upper and lower half to make it work with both high and low signal on A14.

Start the computer when everything is soldered and in place and test that it starts up correctly.

V9938 Exchange

The V9958 chip must be prepared before it is connected to the IC37 socket. Pins 4, 21 and 27 are bent upward. Then connect pins 4, 20 and 27 together with a wire that are soldered to the pins. These are connected to ground through pin 20. Solder also a wire between pins 21 and 58. This connects pin 21 to 5V.

FM-PAC/SCC-mod

This is simply done by replacing the 4k7 R2 resistor with a 150k resistor so that the sound volume is lowered. Here is before...

and after...

Disk Drive Exchange

I found a PC TEAC floppy drive in my stuff that had a jumper for setting DS0/DS1. I changed the jumper to DS0. The floppy cable in the SVI-738 is very short and pin 1 on the new floppy drive was on the wrong side so I needed to exchange the cable to a longer one. I used a normal PC-floppy cable, but connected the drive using the B-drive connector, thus avoid using the twisted part of the cable that cannot be used with the SVI-738.

An inconvinient thing with the original floppy drive is that the screw holes are not in the standard locations. I mounted the floppy on a frame from a PC that I cut and tried to drill holes at the correct locations, but I did not manage to do it properly. So what to do? Yes, hot glue to the rescue! It works, but I do not think that it will hold the floppy in place for very long. I plan to take two steel plates and drill holes in the right locations and mount them between the floppy and the stands.

The original floppy drive was a little bit higher so there is a gap in the case now, but it looks ok anyway.

Keyboard Cable Exchange

The cable between the keyboard and the motherboard is known to break easily and it cannot be disconnected either. I wanted to have the possibility to disconnect the keyboard when working with the motherboard so I replaced the cable with a 50-pin SCSI cable. I desoldered the old cable and soldered a straight male 22-pin header on the motherboard. Then I soldered a similar on the keyboard PCB. Don't do this!

The problem is that it makes it impossible to close the case. Use instead a 90 degree angled connector.

You can also take advantage of the fact that the keyboard has been opened and clean the gold plated parts for all keys with alcohol:

Here is a picture of the backside of the keyboard:

And here is everything connected:

Power Supply Cable Exchange

As I was exchanging the keyboard cable, I decided to also exchange the power supply cables. I use two PC power T-connectors that I had. I wanted to use the correct color coding so I switched some of the wires.

Final Touches

Now everything is connected and the computer is ready to be run for a final test.

Push the power switch and wait:

Great! Lets try the keyboard and the floppy:

Excellent! We are done! Or wait a minute... There is one last thing, the MSX2+ logo...

That is better!

TODO

First off is to add a Real Time Clock-circuit to the motherboard. My SVI-738 is of the older model so it has already a part of the PCB dedicated to a RTC.

... and then I would also like to add an internal memory mapper. Unfortunately I have not found any instructions how to construct one, especially for the SVI-738.

Credits

All credits for this mod goes to Nyyrikki. Thank you very much for all your help and your patience answering my questions!

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