MTX keyboard fast troubleshooting

About original Memotech hardware.
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race8086
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Joined: 08 Dec 2013 14:45
Location: Spain

MTX keyboard fast troubleshooting

Post by race8086 »

A passive key matrix it’s attached to J1 connector of MTX motherboard.
Implementation on reading keyboard it’s performed by IN,OUT Z80 instructions.
Surfing on the internet it’s hard to find keyboard layout, so I performed my own investigation :geek: .
Results are shown in the following picture:
MTX_keyboradweb.jpg
MTX_keyboradweb.jpg (67.98 KiB) Viewed 6074 times
There is a discrete logic involved in the process, IC related are 2A, 3A and 2E, also IC provide the Enable signals for both IN5 and OUT signals, these are 7C and 7D
here is the isolated section:
keyb_schem.jpg
keyb_schem.jpg (90.83 KiB) Viewed 6074 times
Some things to take into account:

- If a single key doesn’t read, maybe a mechanic problem.
- Also consider some cut in cable that joins keyboard with motherboard.
- Over connector J1 you can test individual keystrokes, closing pins according keyborad matrix above attached.
- More unusual, but possible, the keyboard board itself, can have any cut o cold soldering.
- Always test the resistance set R0 to R7 and diode set D0 to D7
- If an entire row, alternates key doesn’t read, probably IC 2A and 3A are involved.
- In any case always consider 2E that acts a level buffer to help decode keys pressed and stored by 3A.

My case

My MTX, lacks in keys P I Y R and W. The rest of keys worked well, even shifted :? .
After testing mechanic parts, cable the error was located on IC 2E, this is the explanation: one flip-flop acting as buffer on line D2 was out of order :arrow:
image.jpg
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Think in retro!
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Dave
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Re: MTX keyboard fast troubleshooting

Post by Dave »

Hi Ramon,

a veyr interesting item - thanks for posting

regards
Dave
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Dave
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Joined: 11 Aug 2012 18:16
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Re: MTX keyboard fast troubleshooting

Post by Dave »

Hi All,

I have done a little investigation on the MTX keyboard operation and have made a few notes here :-

http://primrosebank.net/computers/mtx/c ... yboard.htm

I would appreciate some feedback on it, particularly if I have made any errors on the page.

I have also put a note on fault finding keyboard errors, including details of Ramon's problems on this page

http://primrosebank.net/computers/mtx/r ... yboard.htm

regards
Dave
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1024MAK
Posts: 757
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: MTX keyboard fast troubleshooting

Post by 1024MAK »

When I had trouble with one of my eBay bought MTXs, I made my own diagram.
When I get some time, I will scan it and post it.

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
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