How to Clean MSX Computers?

First, a few questions: how dirty are these computers actually? What is wrong with the keyboard(s)? This will determine how you should clean them and what makes sense.

You can use general PCB cleaning instructions to clean the PCBs; there is nothing special in MSXs in this regard. For example, one of the first Google hits is this: https://www.sfcircuits.com/pcb-school/clean-pcbs-ultrasonic-...

Rule of thumb: use "dry" methods first, such as compressed air (regular vacuum cleaners might create large static electricity potential, so I would stay away from those)! If you can not get a good enough result that way, then you may try something wet. Don't use solvents, unless the PCBs are dirty with something that really needs to be removed and requires some solvent. Also, IIRC (?) these computers have plastic parts which are not easily removable (around the cartridge slot). In case you are going to expose the PCBs to solvents (such as acetone) make sure (test somewhere where it doesn't matter) the plastics can take it. However: don't use them to clean if it is not absolutely necessary. Cleaning agents / solvents can do more harm than good, such as remove protective laquer from the PCBs.

IMO using regular tap water (at least in areas where tap water is not too hard) can be used to rinse the PCBs, but many sources say you would be more safe with de-mineralized water, though. But you need to make sure they dry all the way before re-assembly (use a well-ventilated dry place, perhaps with a little bit of steady heat if you are impatient; for example, I've used the lower step of a Finnish sauna; those who are not familiar with one, a Finnish sauna is a dry place unless someone throws some water on the stove, and the lowest steps are perhaps around 40-50°C - depends on the sauna though).

As for the keyboard: I presume these computers have a very similar keyboard to the AX-350(II). If that is the case, see this thread.

You should be able to get to the membranes easily and be able to clean them; in case the keys actuate properly (which was not the case in my AX-350II), there should be no reason to dismantle the mechanisms, as the problem must be in the membranes in that case.

My AX-350II keyboard was practically filled with sand and dust. The key mechanisms (and caps) do come off but you need to be careful, as it is very easy to break them unless you know what you are doing. See the pics and instructions linked in that thread. I don't think submerging the keyboard parts will do any good, you really need to take the keyboard mechanisms apart in case there is extra stuff inside the mechanisms.

EDIT: LOL@Hamlet

I agree with Meits. Unless there is absolute need to clean insides, I'd just clean the outsides too and leave it at that (a badly working keyboard is a good reason to open one up). AFAIK these machines don't have suicide capacitors inside, and those which have a backup battery, use AAA batteries.

Also, with these machines they may be mostly held together by plastic tabs instead of screws. Be careful with the tabs, they are easy to break! That's one other reason to not open these without a clear need, as even with instructions it is difficult not to break the tabs with these machines! Look at CX5MII service manual for how to open AX-200 (from pictures online looks like it is the same frame, but could be different, too; couldn't find accurate pictures of AX-200 case).

The AX-370 looks different, though, so the CX5M service manual (which can be used to see how to open AX-200 or at least AX-350), and those tips in the link I posted above do not probably apply to it, at least not accurately.

The Sakhr AX-170 I recieved was a complete disaster, with spiders and bugs inside along the clumps of dust and dirt. Every key you press never comes back up. I disassembled the poor thing down to the smallest spring and rinsed the keys in soap water. Rusty parts submerged in white vinegar then rust brushed away. I didn't wash any PCBs though.. just used a compressed air can to blow off dirt in hidden kinks and used a brush with alcohol on parts with dirt stuck on them.
It was a very fascinating sight once it was all re-assembled. Almost new!

Here are some pics of the process:
https://www.instagram.com/p/B5yX4AuguiY/?igshid=1lpbjmdgyora4
https://www.instagram.com/p/B50Y1tfgjQT/?igshid=1s37t716c1i69
https://www.instagram.com/p/B52w6QNgc6N/?igshid=v3rwe7igp0vj

Note this machine uses screws to hold up the chassis. Other machines that rely on plastic clips require much more care as others mentioned. Usually the damage is irreversible.

Thank you guys for the greate replies. For sure I'm going to make use of the ideas and tips mentioned here and linked to.

I'll be taking pictures as I'm proceeding with the cleaning and will post them here. The pictures of course won't show the helper suggested by @hamlet