Magnavox VideoWriter keyboard (USB conversion with NKRO mod)

About one month ago a friend of mine managed to buy a Magnavox VideoWriter in NOS condition. He wanted to use it as a daily driver, so he asked me to perform a 'brute-force USB conversion' similar to snacksthecat's VideoWriter (see his post about convertion) with addition of NKRO.
Snacksthecat has already done the matrix tracing, but I had to found all the rows on the PCB to know where to cut and add diodes. I have taken a photo of the PCB and drew all of the traces for rows and columns, each in separate layer.

Here are the results:

The matrix has 10×8 size, which give us a possibility to use cheap Pro Micro which has just enough exposed pins (10+8=18). Overall condition of the keyboard is great (it's NOS!), I only need to remove that thick layer of 30-years old smelly flux from the PCB and replace the decomposed foam rubber pads under the Spacebar.

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There are four diodes in the matrix which provide partial deghosting. They must be shortened or replaced with bridges.

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Left and right Shift keys are connected in parallel. To separate them, right Shift can be moved to another row/column. The closest (by mean of physical distance) place is 3rd column and 10th row.

Adding diodes will require cutting each switch from it's row, and soldering a diode in between. Some row traces on the PCB go through a switch leg, so there will be need to cut both ends of a trace and solder a wire between diode's cathode and row.

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Only NEC D8049HC (MCU), SN74LS145 (BCD-DEC decoder, drives the rows) and resistor array (pulls up all column pins) need to be desoldered, but I have also removed several large capacitors to get more free space.

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I have used a pin header for columns and a ribbon cable for rows.

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Soldering all those diodes was a time consuming job. I have used Dremel with conical bur to cut the traces. 1N4148 diodes in SOD-123 package fitted nicely in most places and I had to solder about five DO-35 (through hole) diodes.

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After a few hours of testing I've noticed that three switches stopped to register: Space, X and CapsLock. They had failed just one after another. WTF!? They supposed to be NOS and I'm 100% sure that I didn't any harm to them while soldering. I've checked these switches with multimeter — no contact at all. I had no experience with Alps switches, but it turned out that cleaning them is not so difficult. I have opened the top with four flat bamboo sticks and removed the spring, stem and tactile leaf. Then I desoldered the switchplate and removed a contact pressing leaf from it. Switchplates were washed in ultrasonic bath with dishwasher detergent (about 1:10 solution) and then put in isopropyl for several minutes to get rid of any traces of water. After cleaning and drying they started to work again. I'm still not sure about the reason of their failure. Oxidation, dirt or something else?

Bottom line. I'm not a fan of tactile switches and all that I've tried previously were just pointless for me ("linears with sand"), but Alps Brown are completely different, they are just great. Too bad that they are so rare and expensive. Magnavox VideoWriter can be cool as daily driver, but it desperately needs one or two keys near the Spacebar for modifiers.

Links:
Last edited by Quartz64 on 21 Nov 2018, 18:41, edited 1 time in total.
Quartz64
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Unread post21 Nov 2018, 15:02

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Brilliant!!! What a great job of getting the VideoWriter converted!

I agree with you about SKCM Brown Alps. They provide an unparalleled tactile typing experience. However, I have found them a bit heavy for prolonged typing, although my experience with them was with a custom 60% that had a stainless steel plate. When I have typed on a VideoWriter or an IBM 5140 with Brown Alps, the switches did not feel as heavy as they did in the custom board.
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Unread post21 Nov 2018, 15:23

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This is a super cool project! I'd be interested to know how it is mapped for daily driving.
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Unread post22 Nov 2018, 12:27

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zrrion wrote:This is a super cool project! I'd be interested to know how it is mapped for daily driving.

If you look in the links at the bottom of the post there is a link to the github page that contains the keymap that has been programmed.
Anakey

Unread post22 Nov 2018, 12:52

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zrrion wrote:This is a super cool project! I'd be interested to know how it is mapped for daily driving.

Here is how it looks in KLE: http://www.keyboard-layout-editor.com/#/gists/681583f10ea28a50bdf4c8e27c70dac8
Quartz64
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Unread post22 Nov 2018, 14:41

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Wow! Awesome job with all those diodes. That had to be a pretty delicate procedure to cut the traces and solder them in without ruining anything.
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Unread post22 Nov 2018, 15:27

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