Macintosh Plus keyboard M0110A C partially faulty

[Mu0n]

I have two of those, the original one from my childhood, and a platinum style one that I bought used from around 2005 or so. The latter works flawlessly while the former has some problematic keys.

Here's a direct screenshot (thanks RGB2HDMI) of the full alphabet and numbers test (top: working one, bottom: faulty one)

As you can see, the letters K and U as well as the number 7 don't work. Are they part of the same line in the keyboard matrix? Where can I find the schematics of this keyboard before I open this up?

 

[Mu0n]

I tried to spray some contact cleaner and press the keys a bunch of times

7 and U: goes to lower 10's Ohms when pressed
K: seems intermittent to 1000 ish Ohm, sometimes underneath.

I'll give it a massive overall clean before I continue.

 

[Paolo B]

It happened to me with a few keyboards. Desolder / disassemble / clean / check / solder the switch back is my way to go.

 

[Mu0n]

After doing a surface clean and getting as much gunk out without desoldering every key in order to access the pcb from the top side, I got out the '7' (non numpad) out by desoldering it and it seems to act totally normal when I test out the resistance across the pins (I'll post a video on the next reply)

I'm thinking there may be a problem somewhere on the board, because there are shared traces between some of the problematic keys, but I dunno. There are 3 such "strings" if it's that.

Faulty keys as of now:

7 K dot, numpad 4 and numpad 2 don't work. A is intermittent. N can get stuck and ghost repeat.

 

[Mu0n]

Normal behavior of a keyswitch when it's out of the board. It didn't work when it was in the board, doesn't work now that I've soldered it back.

 

[Mu0n]

Hmm, what's this? continuity blockage?

ENHANCE!

Gotcha (this will solve 7, K, dot)!

 

[warmech]

How on earth does that even happen, lol?

 

[Mu0n]

Maybe some weight applied to it at one point during its life since 1986?

 

[warmech]

Good eyes! That thing does look like it got torqued a bit - that trace leading into it looks like it just popped apart due to stress. I've seen cold solder joints, but not a snapped trace before. Glad you found the issue!

 

[Mu0n]

all these 4 keys will work when the crack is resolved.

 

[eric]

Thanks for posting this debug thread, always like seeing the process and what people try.

 

[Mu0n]

All that remains is to fix comma and numpad 0. These all work:

 

[Mu0n]

For the other keys that weren't part of that sequence of traces, I had to tweak the conductive part that gets pushed to establish continuity when you press the key. I found this video which greatly help:

The part of a non-working key looks like this when it's removed:

This is how I looked after I squished the "shoulders together and brought out the "coattails" out - keep in mind that this new shape WASN'T enough to fix the issue:

I had to go way more all out to get it working:

As for reassembly, I recommend waiting before inserting back the C and V keys if you want to deal with the space bar and its spring. It's easier to bring the bar from that side to do all the required steps.