Epson HX-20 - Collective brain help required :)

[rubber_jonnie]

Morning all.

So quite a while back (Possibly years) I was doing my round of checking stuff out and found my Epson HX-20 would no longer power on. It beeps, and sometimes the screen shows something, though not very often.

It's a weird machine, the PSU only charges the battery and doesn't power the machine, with the battery doing the heavy lifting, but I rebuilt the battery pack and all had been well for a long time.

Then suddenly all dead. I hooked up a bench PSU to the battery connectors at 5v but still the same. The battery should be 4.8V BTW, it's 4x1.2V NiCD cells.

I just managed to download the technical manual for it and the voltage supplied by the battery can be anything between 4.5v and 6V.

Obviously I have a lot of testing to do, but if anybody has any experience of these machines and what can fail, I'd really appreciate the help as the manual is nearly 400 pages long and It'll take me a bit to read through it.

Better go and find some data sheets too, it has a weird dual processor setup.

[HigashiJun]

@rubber_jonnie

Here are some pages in Japanese (google translation will be your friend):

https://note.com/machatan/n/nb35babe2e8e1

(5 pages)

https://pocketgriffon.hatenablog.com/en ... /01/010211

(5 pages)

Don't know if that will help you though, but certainly give you some clues.

Good luck !

[stephen_usher]

Those machines are a bit of a pain to get into aren't they?

I'm afraid that I can't really comment about fixing as I've only redone the battery and fixed the tape drive on mine.

I would imagine that it could well be one of the static RAMs going up the creek.

P.S. This is the battery pack I used.

IMG_3720.jpeg

[rubber_jonnie]

@rubber_jonnie

Here are some pages in Japanese (google translation will be your friend):

https://note.com/machatan/n/nb35babe2e8e1

(5 pages)

https://pocketgriffon.hatenablog.com/en ... /01/010211

(5 pages)

Don't know if that will help you though, but certainly give you some clues.

Good luck !

Thanks, I'll have a look :)

I think the luck may well be needed!!!

[rubber_jonnie]

Those machines are a bit of a pain to get into aren't they?

I'm afraid that I can't really comment about fixing as I've only redone the battery and fixed the tape drive on mine.

I would imagine that it could well be one of the static RAMs going up the creek.

P.S. This is the battery pack I used.

IMG_3720.jpeg

Yeah, they are not the best, and pretty complex, though I suppose they did manage to stuff a lot into it for the time. could be RAM I guess, might be worth leaving it powered on and seeing what the thermal camera sees.

Thanks for the battery pack info, interestingly the one I built is almost identically specced but 1700Mah instead.

[rubber_jonnie]

OK, so I did some fairly basic checks.

I know there is power, as there are test points, so that is good. I plan to take this further when I have some IC datasheets, but for now it seems OK. All the logic IC's seem to be Toshiba branded, so I can't tell if they have similar pinouts to 74LS, so datasheets needed for them too.

There are 3 oscillators, 1 for each of the CPUs as best I can tell plus one for the clock. All of those work. Also, I left the battery connected and the charger plugged in. The battery did charge up somewhat, though a good deal less than the 4.8v it needs.

Given it beeps on reset and power on, that suggests something is working.

However, I fed it a stable 5v from the bench PSU via the battery terminals and left it, and came back to look at it with a thermal camera. One of the RAM ICs was warm and everything else was cool bar what I think is CPU number 1, which was also warm, though that could be because it's unable to access the RAM and it's just sitting halted.

So off to look up the RAM and CPU pinouts next and to see if the Epson branded RAM has any sort of a direct replacement.

[rubber_jonnie]

Interesting, the RAM pinout looks the same as a 6116 and I may have one in my parts drawer. Fitted it, no different and the same RAM spot gets hot. Voltages good across all the RAM and ROMs.

It's been doing some extra beeping at a lower tone too, not sure what it means but I'll take it as a sign of life.

[rubber_jonnie]

Wow, I've been looking at the schematics for the HX-20 and they are insane!

Also, I'm having to get the CPU pinout from the schematic...

[rubber_jonnie]

OK, so I'm going to make my own pinout based on the schematic and I'll drop it in here when done.

In the meantime I think what's happening (Though it doesn't necessarily explain one warm RAM IC) is that it's stuck in a sleep mode. That would explain why the current draw is so low.

Bit of a whacky architecture this one.

[rubber_jonnie]

OK, so with a combo of schematic and a very poorly scanned datasheet, I have the pinout for the CPU's in the HX-20.

Epson_6301_pinout.jpg

Datasheet is also attached, remove the txt file extension to put it back as a PDF.