Epson HX-20 Repairs

[dutchacorn]

Recently I got 2 HX-20s off eBay. My curiosity was triggered by the "Mitutoyo DP-20" labeling on one of them. Neither of them work, this thread is to document their repairs. It can take a while because time is scarce but it may help others track down faults. It will probably help me document what I've tried. It was quite obvious a previous owner had worked on them. The keyboards were swapped between the units, batteries removed and some of the parts were in a separate bag.

The Mitutoyo is a custom machine, more pictures information on the ROMs and expansion module posted on the stardot forum here.

Symptoms and analysis so far:

Mitutoyo

• Older MOSU board, several patch wires, BASIC version 1.0
• Keyboard / LCD unit (top part) is faulty. Has marks of battery leakage on the FPC cable between the keyboard and LCD PCBs.
• When using a known-good keyboard, it starts up, BASIC works. When using the soft-switch to switch it off it often hangs or falls into a TRAP. A reset brings it back.

Epson

• BASIC version 1.1
• Keyboard / LCD unit (top part) is faulty. Has marks of battery leakage on the FPC cable between the keyboard and LCD PCBs.
• When using a known-good keyboard, it starts up with the beep-beep but has no video output. BASIC works (can enter a SOUND statement). When using the soft-switch to switch it off it also seems to hang. A reset brings it back.

Mitutoyo MOSU board



Epson MOSU board

 

[dutchacorn]

Looking at the 2 FPC cables that connect the MOSU board to the keyboard, I noticed they were stained /corroded. Cleaning these made them both fully functional again. So the battery leaks on the cables between the keyboard and the LCD board does not seem to have led to broken tracks.



For the MOSU boards the first thing I'll do is replace the electrolytic capacitors. This may improve the display on the MOSU board for the Epson. Replacement caps are on order.

I did not bother trying the cassette drive, the belt would need replacement even if it would work. Opening her up I found that, indeed, the belt was stuck. But for this unit, the motor was stuck as well. A bit of a turn made it come loose but it does not run smoothly. After assembly it does turn and loading seems to be ok but the motor is noisy. Rewinding is a struggle and saving gives an IO error. I guess it will need a bit more love. There are traces of corrosion in the drive, looks to be caused by moisture.


Looking at the behaviour of the Mitutoyo MOSU board it seems that switching off using the soft on/off switch corrupts memory. Not only does it need a reset to recover, in most cases it also needs re-initialization.

 

[GeoffB17]

I'll keep an eye on this thread, thanks.

I've had my HX since I bought it new in 1983. Over the years, I have used it on-and-off, but mostly it has been kept charged. Apart from some specific problems, like a few problematic keys, it is still working OK. I have not replaced any electolytics, although I understand that some people do this. Maybe keeping the device generally charged helps here? Also re the LCD display, I hear that these can fail, and new caps does not help. Again, if the device is left uncharged for a LONG time (years ?) this can cause problems, I suspect.

Geoff

 

[dutchacorn]

Both keyboards now work fine with a known-good MOSU board. So the Epson video issues seem to originate from the Epson MOSU board.

I've had another HX-20 where I had to turn the contrast up fully to see anything. That was fixed by replacing the caps. If the caps don't fix it I'll dive into the display logic on the MOSU board.

The cassette drive works fine after lubricating all cogs and axis, also now saving files without an IO error.

 

[GeoffB17]

Well, that's useful to know. I've got a 'spare' HX, not working. One of the problems is the LCD failure. The person I got it from had tried to replace the LCD unit, but the repl made no difference. So, the problem was within the MOSU board all along? I've heard of the problem with the contrast, and I accept that caps replacement would help, but it's not something I'd undertake just for the sake of it.

I've got 2 MCUs, but not working. One day, I need to try to get at least one working! I'm OK with the .WAV route for now.

Geoff

 

[dutchacorn]

Found two more faults with the Mitutoyo MOSU:

• The keyboard does not function fully: Z, X, shift keys, function keys, - to \ keys don't work
• The printer does not work

The keyboard issue is not caused by the keyboard itself. The keyboard works fine on another MOSU board and a known-good keyboard has the same issues on the Mitutoyo board. I scraped the keyboard connectors on the MOSU board with a thin knife and that seemed to solve the issue, but an hour later the fault returned.

 

[GeoffB17]

The faulty keys on my machine are places such as to suggest that one line on each mfc is not making proper contact. Your missing ones sounds like something different. In my case, I wonder if it's something inside the connector, rather than the actual contacts on the cable? But how to get in there, i've tried with some rough card soaked with IPA.

Your printer - what does it do. If I try to use my printer, it starts, noise for a second or two, then I get the CHARGE BATTERY message and the machine stops. Break out, machine operates fine again. I suspect printer mechanism bunged up due to long un-used, and when it tries to print, draws too much current? Needs a little oil, but where?

Geoff

 

[dutchacorn]

For the keyboard; it's actually the Z X [shift] [ ] \ [cursor] keys that don't work. And if I do press them repeatedly the computer hangs up.

The printer does not show any signs of life other than when I switch it on and try an LPRINT command the computer starts to beep repeatedly.

CHANGE BATTERY is an indication that it draws too much current, indeed. I got the same thing with my rusty cassette drive.

 

[dutchacorn]

Update: the Mitutoyo keyboard issue is fixed after reflowing the joints for the keyboard connectors on the MOSU board. They were very sparsely soldered and looked a bit gray. All keys have worked since.

 

[dutchacorn]

Both printers were not working. As Geoff suggested, they were both simply stuck following a long time of idleness, a few squirts of silicone spray got them back to life.

Steps:

• Loosen the 2 small screws on the front side of the printer assembly. This frees the clips on the front side of the metal cover (they are behind plastic tabs). No need to completely remove the screws.
• Remove the metal cover
• Put some silicone spray in the moving parts (the turning white cylinder, the black wheel / axis on the left of the printer)

I found that the paper feed button does not immediately work, an LPRINT statement works to loosen it up.

 

[dutchacorn]

All capacitors now replaced. The caps in the Epson were in bad shape, many of them had leaked. It took a bit of effort to remove most of them, the solder was affected by the leaking caps (and/or battery) and would not flow.

The caps replacement has not improved the functioning, the Epson has now stopped working alltogether although it did come to life a few times right after the replacement. And I could see a very light shimmer of the characters on the display when it did (may have been there before).

The Mitutoyo still has power switch issues. It sometimes powers off normally but often just hangs or corrupts the system and does not go into low power mode. There are quite a few components involved in the power off sequence so that will probably take some time to investigate.