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Re: Building an OSI 542 Rev C Keyboard

Posted: Sun Mar 31, 2019 3:53 am
by Jeff
I have been busy.
E4A22242-2DC8-4B96-ACEF-A87795A4512D.jpeg
E4A22242-2DC8-4B96-ACEF-A87795A4512D.jpeg (1.87 MiB) Viewed 11075 times
I have completed the keyboard.

Re: Building an OSI 542 Rev C Keyboard

Posted: Sun Mar 31, 2019 4:10 am
by Jeff
gekaufman wrote: ↑Thu Nov 08, 2018 7:54 pm Finished up the keyboard last night - Hopefully will have time to test this weekend, and anxiously awaiting the Keycaps.

- Gary
Gary, post some pics of your finished keyboard 😃

Re: Building an OSI 542 Rev C Keyboard

Posted: Sun Mar 31, 2019 10:42 pm
by Klyball
Does it work?

Re: Building an OSI 542 Rev C Keyboard

Posted: Mon Apr 01, 2019 12:58 am
by Jeff
Not yet tested. ðŸĪŠ

Re: Building an OSI 542 Rev C Keyboard

Posted: Sun Aug 25, 2019 4:03 pm
by Jeff
I finally got around to testing the completed keyboard and it works (Mostly)!
Keyboard.jpg
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What does not work is the BREAK/RESET circuit. While it DOES automatically reset the system upon power up, it does NOT STOP resetting the system! I put my scope on the Reset wire that goes to the 502 and confirmed the pulse.
Reset Pulse.jpg
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I am not familiar with how the circuit works, so I am asking the online experts here what to check next.

/Jeff

Re: Building an OSI 542 Rev C Keyboard

Posted: Sun Aug 25, 2019 6:45 pm
by Jeff
I found the problem with the Break key circuit. There is an incorrect trace on the PCB.

I solved it by lifting pin 8 of U1H and then connecting +5V to the U1H pin 8 OF THE SOCKET, not the lifted pin.

I can see how this happened. When Grant was laying out the PCB based on photos I sent him, I sent him a diagram incorrectly showed the trace going to pin 8. I incorrectly assumed that the power would go to opposite corners as is traditional.

Here is the photo I provided to Grant a few years back... I added the orange line today to show the trace under the socket.
Original.png
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The 74LS75 breaks from tradition of placing the power and ground on opposite corners of the chip.
+5V is designated on Pin 5 and Gnd is on Pin 12.
74LS75.jpg
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Here is a Note File that describes the fix.
PCB IMPORTANT NOTE.pdf
(309.37 KiB) Downloaded 777 times
And now my keyboard is working perfectly!
/Jeff

EDIT: Originally, I thought Grant made the mistake on the incorrect trace. I came across a photo of a diagram that I sent to him with the mistake clearly made by me. I have edited the above to reflect this new info.

Re: Building an OSI 542 Rev C Keyboard

Posted: Sun Aug 25, 2019 6:46 pm
by Jeff
One final note... the MX keys feel awesome!

/J

Re: Building an OSI 542 Rev C Keyboard

Posted: Sat Feb 01, 2020 9:38 pm
by gekaufman
Jeff -

Thanks for posting the fix. I ran into the same problem with a continuous reset and found that a 4.7K resistor across the .001uf cap let the circuit work - but clearly this was not the proper repair.

- Gary

Re: Building an OSI 542 Rev C Keyboard

Posted: Wed May 20, 2020 4:14 pm
by Jeff
Here is my Parts List. I use the spreadsheet to track my build progress.
542C Keyboard Parts List.xlsx.zip
(11.31 KiB) Downloaded 594 times
/Jeff

Re: Building an OSI 542 Rev C Keyboard

Posted: Thu Jun 11, 2020 7:31 pm
by keith_r25
Thanks for the assembled keyboard Jeff!
I was using the Parts Layout.pdf at the beginning of this post for parts placement and D1-D4 are shown reversed. I should have used your Keyboard Part List.pdf pic as they are show correct.
Thanks again,
Keith