750 Series

Details: Published: 14 June 2025

I've managed to restore an almost complete set of the 750 series machines. Here are some of the collection:

Left-To-Right: 750Cs, 750P, 750C Grey, 750, 750C Black

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IBM External Floppy Disk Drives

Due to the different IBM labs and companies developing IBM portables and the time period over which they existed before USB, there are a lot of different types of IBM floppy drive.

Whilst we're only covering external here, there are at least 3 types of connector:

26mm / fattest: Found on the 701c

21mm: Found on the PS/notes, 850, 7xx tablets. 360, 365

17mm / Thinnest: Found on the later models which favoured optical drives built-in such as the 755CD, 760, 600 and various 3xx series. Ultraslim floppy drive.

21mm

This early floppy drive 66G5069, also known as "UltraBay Diskette Drive enclosure" actually contains the original multibay floppy within it's large outer shell. There are also fixed standard external 21mm drives.

These ultrabay floppy drives came in various models from various manufacturers, so externally each 66G5069 drive may look the same, but internally it could be totally different.

TEAC FD-05HG

Here we have a TEAC FD-05HG drive, which has one eletrolytic cap, which as far as I know should only be replaced with an electrolytic. I have had drive read issues by replacing these with a solid capactor. It's a 100u / 10v capacitor seen below:

You can get away, like I did with a 7.7m through-hole capacitor with the legs splayed outwards:

Whilst it's open you may (or may not) want to clean the magnetic heads with a little IPA and check the corkscrew spindle which moves the heads forward and back across the diskette is moving smoothly.

Even though this 7.7mm tall capacitor is longer than the original surface mounted cap, it still just fits under the floppy drive chassis - although you cannot go any more than this.

Sony MPF720-2

And here is another 1.44Mb ultrabay floppy drive - the Sony MPF720-2 (weirdly suggesting it's a 720Kb drive, but it's a 1.44Mb drive):

This Sony drive is similar, but you do not need to remove the little PCB in order to replace the 47uf capacitor:

In this case, small 6mm through hole capacitors with the legs splade are a good replacement:

Translating PS/55 note Japanese keyboard to English keyboard mapping

If you connect a western keyboard to the earliest PS/55 and PS/55 N23 SX notes, the keyboard mappings are all messed up. Here is a rough map of what key is mapped to what real keypress:

IBM 5535 Multistation

IBM Japan had a range of Kanji-character-enabled desktop computers, called the Multistations. This was part of that family with the addition of a greyscale LCD display. Although 5535 is the 'laptop' range of machines, With the sheer size and weight (7kg), I think you would consider it more like a 'space saving desktop', much like the IBM Convertible. It has a sibling called the 5535-S which has a slightly different design and is 'less boxy'.

IBM PS/Note N45 SL (Zenith)

This is one of three N45 SL machines I have in a box. I previously gave up a day and ended up with one complete boxed one, a second unboxed one as a nice working companion and a third 'parts' machine. The N45 came in two models sporting different CPUs - the SL and SX.

You'll notice the case badge has similarities to both the 2141 (a PS/1 type) PS/note 182 and the PS/1 case badge - unlike the other PS/note and IBM Thinkpad oval badges, you'd think it is architecturally closely related to the (same year) PS/Note 182. However this machine feels Zenith-made through and through - much like the Thinkpad 300 which usurped it as the 2615. It shares a similar case plastic, power brick, display, keyboard and battery. Perhaps IBM were briefly marketing the N45 as the low-end (outsourced) notebook and the N51 as the high end (IBM-built) notebook, which eventually became the Thinkpad 300 as low-end and 700 as high.

Putting it another way, IBM were obviously toying with what to do about the cost-reduced 'low end' and had the 2141 "Lexmark Lexbook AR-10" PS/note and the Zenith-made N45 SL then "Zenith Z-Note 433Lnc+" 300 all coming out at once. You'll find some very reputable sites not even acknowledging the 2141 or 261x machines because they do not follow the 'true bloodline' of the IBM notebooks. The N45 SL may well have a Zenith step brother, but I haven't found a reference to it.

My first machine amazingly has a i387 upgrade chip (a maths coprocessor for the 386) fitted.

IBM PS/2 Model CL57 SX

This is a massive, stange, outlier in IBM's journey into portable personal computers. CL for 'Color Laptop' and 57 for....... fuck knows. The type is 8554 and it runs MCA bus. It's almost completely unique although it shares a look similar to that of the possibly-vaporware, Japan-only PS/55 Power (PC) Laptop EWS - which also feels like a skunkworks project.

Some sources state this machine started as a Model N27sx - a 'for fun' IBM Japan design for domestic japanese market. I can't find much source material that this is true though. Apparently, the CPU was downgraded from a 486 to a 386 so as to not compete with other PS/2 machines. Supposedly there are some hidden clues on the machines that despite it's international 85xx model designation, the plant ID is 97 which is IBM Japan Entry Systems factory in Fujisawa.