Fixing SMK J-M0404 Keyswitches

I got chatting with someone on one of the Discord channels I haunt as to whether one of my clone Apple IIs would be for sale, specifically the Apple IIe clone. I refer to this one as my “Orange” as rather than apple keys, it has much more “orange” looking keys

That well known brand… ©2021

As part of my “Sure, I’ll look into it for you”, I tested the unit and noticed that the keyboard was bad. But not completely bad. The number 8 key on the number row worked, and the minus key on the numeric keypad was intermittent.

This suggested the problem wasn’t the usual culprits of an encoder chip or a cable, but in actuality, it was that all but one of the keys had failed.

(Sings) Oooh Seventy four keys on the Orange keyboard, Seventy four keys on the board, you desolder one with your desolder gun, there’s Seventy three keys on the Orange keyboard. ©2021

I found this hard to believe but easy to test, so I desoldered the 7 key and the 8 key (So I had a control) and tested them. Sure enough, the 7 key didn’t work and the 8 did.

Oh goody!

75 keys later I had desoldered the rest of the keyboard, and every single one seemed to be faulty. A poke around and asking on Discord, I had the model. SMK J-M0404. Commonly used in all sorts of computers of the era, and thus made of unobtanium. I was going to have to work out how they worked and see if I could fix them myself.

Keyswitch partially separated. ©2021

I initially tried a squirt of DeOxit, but it didn’t help, even letting it soak.

I carefully pried apart the keycap (Managing to break off the clip. D’oh!) and had a look inside. It was an intriguing design, clearly built to a price point.

Carefully prying apart ©2021
Plunger and Spring in Keyswitch housing base. ©2021

The switch consists of a plunger, a spring and two metal contacts. The two metal contacts consisted of a “U” shaped piece and “E” shaped piece. In the switch, the center prong of the “E” piece pushed against the bottom of the “U” and made contact. The plunger pushed the prong away from the other contact and was held there by the main spring.

“E” contact at the top, “U” contact at the bottom. ©2021

What seemed to be the problem was that the edge of both places where they made contact was quite tarnished and wasn’t making contact. Further to that the center prong had lost its spring and was no longer making good pressure against the “U”.

After carefully removing the contacts,I polished then with an ink eraser, paying particular attention to the areas that would make contact. I also cleaned off any goo on the sides. These switches had seen better days.

Once clean, I carefully bent the prong of the “E” contact out a little, using a pair of pliers. I then reassembled the contacts in the housing, making sure to keep the prong depressed until it was behind the “U” contact. I did this with a small jeweller’s screwdriver and my thumbnail. By the end, my thumbnail was significantly degraded. Good thing they grow quickly.

Bending the prong ©2021
The trick is to do this AND press down that contact, but then you’d just have a photo of my hand. ©2021

I could now test continuity between the contacts with my multimeter. Assuming the contacts passed, I then stretched the spring before reassembling the switch, followed by another test.

Testing before reassembly ©2021

This worked well for most of the switches, with the only complication being that the “U” contact sometimes would not want to come out. Initially I was removing them by reaching into the switch body with a pair of needle nosed pliers and seizing the base of the “U” contact, then pulling them out. This worked most of the time. Sometimes it needed a lot more force. Eventually I worked out, having snapped one “U” contact in half, that if it was stubborn, soaking in DeOxit, plus rocking the piece side to side, by tugging on the exposed ends would remove the contacts with a lot less force.

The process was then repeated for all the switches. Once all were done, I retested all keyswitches (5 failed and needed another pass of cleaning) and once I was satisfied, I resoldered all keyswitches.

Testing a keyswitch ©2021

I was then able to plug the keyboard back into the Orange and test it. Thankfully it passed all tests and is now back in my collection. Unfortunately the original interested party had changed their mind. Considering how this keyboard feels, this may be a good thing. The key caps are cheap ABS and feel rough. The refurbished keyswitches don’t help. I won’t get much for this when I do sell it, but it will probably make a great, cheap introduction system for a new hobbyist.

Published by ilike8bits

I collect old computers and consoles

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