MSX Computers
Introduction
MSX is a computer standard (specification) from ASCII Corporation with Microsoft that became reality in June 1983, based initially around the 8-bit Z80 CPU, with the aim to ensure compatibility between computers. Various computers using the standard were manufactured by a number of companies including Panasonic, Philips, Sony, Toshiba, and Yamaha. Typically Microsoft would provide their version of the BASIC programming language built into the computer, known as 'MicroSoft eXtended BASIC', which is where 'MSX' comes from although there are debates as to the true meaning of 'MSX'.
The MSX computers were sold throughout the world between 1983 and 1993 and were a success in Asia, South America and Europe, but North America never got their hands on the computers, which is strange considering Microsoft's involvement but may have been down to North America already having well established American computers from Apple, Atari and Commodore.
The original MSX line was followed up with MSX2 in 1985, MSX2+ (Japan only) in 1988, and MSX TurboR (Japan only, 16-bit) in 1991, each standard bringing more advanced features. Although MSX did not become the computer standard throughout the world in the way originally envisioned, it was very successful and served as a starting point for many getting into computers for the first time and is fondly remembered by many today with a loyal fanbase.
Please see YouTuber Nostalgia Nerd's video on MSX to complement this introduction:
Sony HB-75B
The HB-75B from Sony is a MSX 1 computer that was released on 1st June 1984 for £299 and is the Great Britain version of the HB-75 (the version intended for Japan), an improved version of the HB-55 from 1983, having more RAM and a better keyboard. As well as the HB-75B revision intended for Great Britain, there were other variants of the HB-75 released in different markets with their own model numbers:
HB-75AS Australia
HB-75F France
HB-75D Germany
HB-75P Other European countries
While this section focuses on the HB-75B, much of the information should be mostly applicable to the HB-75 and the other variations.
The specification for the computer is as follows:
Z80A CPU, NEC version (D780C-1).
TMS9929A PAL video display processor.
AY-3-8910A PSG Programmable Sound Generator.
48KB ROM (16KB for integrated software, 32KB for BASIC).
64KB RAM (in slot 2).
Optionally, a HBI-55 data cartridge could be purchased separately (but was packaged with the Japanese HB-75), which stores data from the built-in computer software, 4KB in total, and can be used in either slot A or slot B. Data on the cartridge is retained by a lithium battery which will run down after about 5 years so it's recommended to transfer data to another cartridge before that happens. The HBI-55 cartridge nowadays goes for hundreds but it is possible to make your own.
When you power on the computer you are presented with a screen called the 'Personal Data Bank' which allows you to access an address manager, scheduler, memo, transfer (or Copy if a HBI-55 cartridge is inserted), and BASIC. There is also a BASIC + DATA CARTRIDGE option if the HBI-55 cartridge is present. The options are selected with arrow key up/down and selected with the return key. I found it a bit annoying to have to select BASIC every time the machine boots rather than going straight to BASIC.
Programs can be loaded from and saved to a tape recorder via the 8 pin DIN with appropriate cable (supports remote control of the recorder). In addition to tape, a software cartridge can be run either from slot A (the top slot, which has priority) with the illustration toward you or slot B (the rear slot) with the illustration side facing up. Slot B also supports a disk drive controller. Note that the Japanese model has just one cartridge slot and an expansion bus at the rear that does not support software cartridges.
The HB-75B was my first MSX computer (although I already had the Sega SC-3000 which is MSX-like) and while looking for an MSX computer to buy I was in particular looking for one with a built-in SCART socket (a rare thing for a microcomputer), thinking it would be an easy way to get RGB from a computer (which I was wrong - see further on). A possible reason why SCART was included on some Japanese designed microcomputers is that Japan has RGB21 (RGB Mult) which is similar to SCART, with the main difference being the pinout is different, so adapting would be quite straightforward.
As mentioned, I thought I could just plug a standard SCART lead into the computer and the other end into the RGB enabled socket of my LG plasma TV only to find that the image continually switched on and off. Trying it on my SCART to HDMI upscaler the image didn't switch on and off but instead constantly shifted position left to right, which was almost as annoying.
Composite video, however, was solid so that led me to believe that if there was a fault it was in the RGB circuit not the video chip itself, which operates independently of composite. Though investigations online and probing the computer I came to the conclusion that the problem was with using modern TVs and other devices on the HB-75B, which are more particular about the video signals. Indeed, the HB-75B switches the SCART blanking signal on and off at about 15.7KHz (the same frequency as the horizontal sweep in a CRT screen), which is strange, as the blanking signal should be steady to ensure the TV is in RGB mode (rapid switching of blanking is a SCART feature that can be used for overlays, take note, but makes no sense in this context since it would be overlaying RGB on composite).
So the solution I found was to place a 220uF electrolytic capacitor into the SCART lead, its anode connected to blanking (pin 16) and its cathode to blanking GND (pin 18), which resulted in a steady image. If you jam the capacitor into the SCART pins where the wires are connected you won't even need to do any soldering. Note that with the capacitor in place it doesn't matter what end of the SCART lead you plug into the TV/upscaler.
Even with a steady image I found there were fairly faint 'jail bars' - vertical streaks across the screen, something I was used to with other retro computers and games machines. Although I wasn't too bothered about it I followed the recommendation online to extract the sync pulses from the composite video using an LM1881 circuit and feed that into the SCART's composite video input but found that made no noticeable difference in video quality.
Resources
AY-3-8910/AY-3-8912/AY-3-8913 datasheet:
https://map.grauw.nl/resources/sound/generalinstrument_ay-3-8910.pdf
Sony HB-55P/75P/75B Service Manual (GB):
https://archive.org/details/sonyhb5575sm
Sony HBI-55 Data Cartridge Service Manual (GB):
https://archive.org/details/sony55hbmsm
TMS9918A/TMS9928A/TMS9929A video display processors data manual:
Resources - all MSX computers
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