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geekhack Community => Keyboards => Topic started by: Professor Fate on Fri, 07 June 2024, 16:47:28

Title: LK401: Some keys have died.
Post by: Professor Fate on Fri, 07 June 2024, 16:47:28

Hi folks,

I've run into a serious problem with the LK401 of my VT420: Some keys in the left upper corner (F1 thru F5, caret, number '1') lo longer emit any keystrokes after not having been used for a couple of years. As some of you may know, F3 gives access to the set-up menu of the terminal. I've opened the case of the keyboard and removed the keycaps in question, but can't find anything strange, not even on the PCB:

(https://lh3.googleusercontent.com/pw/AP1GczONGnrUglwLwNlZ4jvCZz6ClnYB9onFd3PHG2HCVBCWv_Ngbq-5troKaB0LwV0X9FnXfKdIdd_SFhjGx2tdpRlkTjrXfJnTmp8PJhNzq2Skjj4UgH0sqN7R3XtxwzDxLx-LjjBKvL14AebQCQFMOMpikw=w1231-h923-s-no-gm?authuser=0)
(https://lh3.googleusercontent.com/pw/AP1GczONHcp6YxpDIguT5WNzyOpTyu2-qyo6ATHadHJ7qpenGUrXUq81wm0hEWeHULK5a7WeXP9V45QCCwmCqWgf4O38coUiElNaK71er5ZlXNsvMo2XK9jMslGf3_GCxqaK1gKMrSL-qznA4zSzG9jKNrn76A=w1231-h923-s-no-gm?authuser=0)

See https://photos.app.goo.gl/NJFCjiokFc5zxukR6 (https://photos.app.goo.gl/NJFCjiokFc5zxukR6) for images in full resolution.

Now: How can I bring these keys back to life? In know the LK401 is a rubberdome board with conductive strips printed on plastic sheets, and I've already run into a similar issue with a Model M (which turnd out to be lethal), I really wanna scrap another keyboard. Any hints?

Title: Re: LK401: Some keys have died.
Post by: Professor Fate on Sun, 16 June 2024, 11:25:06

OK, I disassembled the board and found out that some of the conductive strips on the lower of the two plastic sheets are corroded:

(https://lh3.googleusercontent.com/pw/AP1GczMAyaTudnFitE8rzRake93p_R4ECUfMsA0xZERcQa6o1dcAnacq6ikFdtE5uI6NBA-nK4zDAvyplbuYfzBfyl28YobpnvlBxBAgSURSqCGmYX0IaX2Qe3oV95roa8GrArpiGbXsBp6weW-dtmwZmzYYtQ=w692-h923-s-no-gm?authuser=0)

So I applied some conductive varnish to the strips, reassembled and connected the board…and suddenly found out that the keys do work, but emit the wrong keycodes:

F1 → s
F2 → e
F3 → w
1 → a

What's wrong here? :mad: Did I apply too much varnish, or did it flow over an adjacent strip when reassembling the keyboard?