deskthority

An NEC APC-H25 keyboard date stamped September 1986, very late production considering the original APC computer's release in 1982, by 1987 NEC would release the APC IV putting this unit nearly 3 generations after it's original design.




Switches are NEC branded linears with grey top housings and black sliders.
Mount is nearly identical to, and is two way compatible with, SMK vintage linears but seems slightly larger or tighter toleranced.


Disassembly is also very similar to SMK vintage, two side tabs hold the bottom and top housings together. Unlike SMKs however there are a second set of locking tabs on the top housing which engage with the mounting plate.


Clearly inspired by SMKs switches, even though all the parts are different.
Key travel is a full 4mm, definitely an improvement over SMK.
Weighting is fairly light, around 60 grams bottoming out.
The key feel is pretty good, decently smooth and nicely weighted with a deep clacky sound.

The stabilization of the larger keys is done with dummy switches, but these don't suffer from the binding issues which SMKs do.
In fact these are almost perfect, even better stabilized than some wire ones do.

Special switches have paint applied to the top of the slider to indicate locking switches or different spring weighting. These gold painted ones are slightly lighter such that the combined weighting isn't overbearing.


The board itself has a different internal designation, TC-856 vs TC-806 and Revision 3A4 vs 3A, as well as an updated PCB with additional components and circuitry.





This particular unit has also had a modification in the form of a custom controller board which has been Velcro-ed to the bottom of the case. I suspect this is to convert the the keyboard for use with some proprietary computer.



Overall this seems like a fantastic keyboard, definitely looking forward to converting it for use on modern computers.

Really intriguing function row and arrow cluster.

Had to log on just to say that I love the look of that beauty. I'd say a very usable layout as well.

Have you made any progress converting it? I've got an almost identical unit, would love to get this working over USB.

Currently waiting on some teensy boards to arrive, but I believe the conversion should be a fairly simple controller replacement.
I've already done an initial tracing to find where the rows and columns lead to on the main IC.

Columns are input through the multiplexer on the left, and the rows go through a bank of diodes and resistors before leading into the controller.

This same setup is also used in my NEC PC-8801 keyboard just without the diode bank, maybe NEC had a thing for multiplexers.

Disassembly of the latching switches



The white cam track piece simply slides out.
The wire also simply slides out of a hole through the slider.



The space for the cam is present in the normal switches as well, clearly some sort of modular design.
Out of curiosity I tried inserting an Alps click leaf into the slot, and it fits and functions perfectly!

From this I suspect clicky and tactile versions of this switch must have also existed.

These rubber donuts were installed on the latching switches along the bottom row.
I suspect these were to help prevent accidental presses by increasing the force required, but now after 30+ years the rubber has completely hardened making them unusable.

Conversion finished!


I decided to repurpose the original debug button as a reset switch.
The switch pin needed to be bent sideways along the pcb in order to avoid it getting grounded immediately on plug-in.

The only other necessary modification was to short both enable pins of the multiplexer together, as originally they were pulsed on and off.


I've also attached the QMK code I wrote for this conversion in case anyone is interested.

Those rubber donuts look like fairly standard rubber grommets (which would be easy to replace, but probably felt like the hasty modification even before the rubber hardened).

Very cool to see boards from this period! Every layout was unique and every computer was expensive, so many unique switches were designed and manufactured.