Building an OSI 542 Rev C Keyboard

Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post by Jeff »

I have been busy.
E4A22242-2DC8-4B96-ACEF-A87795A4512D.jpeg
E4A22242-2DC8-4B96-ACEF-A87795A4512D.jpeg (1.87 MiB) Viewed 11069 times
I have completed the keyboard.
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Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post 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 😃
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Klyball
Posts: 230
Joined: Tue Dec 09, 2014 12:53 am

Re: Building an OSI 542 Rev C Keyboard

Post by Klyball »

Does it work?
Replica 600 Rev D:8K,CEGMON
Replica 610 Rev B: 24k,MPI B-51 with Custom Data separator D-13
510 on the bench/replica 582 backplane/replica 470a /replica 555/original 570B/2 x Shugart 851
Ongoing : 630 ,620 ,510,542c,custom 590,SA1200,592,594,596,598
Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post by Jeff »

Not yet tested. 🤪
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Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post 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
Reset Pulse.jpg (1.08 MiB) Viewed 10312 times
I am not familiar with how the circuit works, so I am asking the online experts here what to check next.

/Jeff
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Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post 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.
Last edited by Jeff on Sat Sep 14, 2019 5:33 pm, edited 1 time in total.
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Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post by Jeff »

One final note... the MX keys feel awesome!

/J
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gekaufman
Posts: 72
Joined: Mon Nov 23, 2015 12:57 pm

Re: Building an OSI 542 Rev C Keyboard

Post 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
Jeff
Posts: 370
Joined: Fri Mar 07, 2014 4:44 am
Location: British Columbia, Canada

Re: Building an OSI 542 Rev C Keyboard

Post 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
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keith_r25
Posts: 4
Joined: Thu Apr 09, 2020 2:21 am

Re: Building an OSI 542 Rev C Keyboard

Post 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
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