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Commodore CBM PET 2001-32B Keyboard

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  Commodore CBM PET 2001-32B Keyboard by Bill Degnan - 11/03/2017 20:16
The Commodore CBM (PET) 2001-32B, after restoration. Click image for larger view.


After years sitting on top of a dryer in the laundry room, the entire motherboard became covered by lint dust. This photo shows some dust removed, revealing a pretty serviceable circuit board with little rust. Click image for larger view.


Keyboard Repair

The system boots but other than the "u" key and the "Enter" key the keyboard does not respond to keystrokes. I took apart the keyboard. The circuit board tests OK and the keyboard logic is OK (tried a different keyboard). The computer itself is fine. I have informally thus isolated the problem to be the black carbon tips that touch the circuit board upon a tap of a key from the keyboard.

Time has not been kind to the keyboard contacts. This photo shows the keyboard removed from the chassis. Click image for larger view.


Keyboard partially disassembled. I desoldered the caps-lock wires temporarily in order to remove the circuit board from the keyboard key housing. Click image for larger view.


Closeup view of the keyboard contacts. I have cleaned the contact tips with 91% isopropyl alcohol and that had little effect, only a few more keys respond. I speculate a glaze has formed on the tips and the conductivity has been lost.
Click image for larger view.


I have heard one should paint/brush a metallic paint onto these tips, but I am thinking there must be a way to restore the conductivity without adding paint, something that will lead to issues down the road. I have also seen a page stating the solution is to scribble on the tips with a pencil. Also not to me a good long-term solution.

Would it make sense to simply use a pencil *eraser* on the tips instead, or a very smooth sandpaper/file to buff them in order to clear off a thin layer/film that may have formed?

I searched the web, surprised to have not have found more written about this. Up to this point I have had spare keyboards and this issue never came up. Time to experiment!

UPDATE: Got a tip from the CBM-Hackers mailing list to try "Keypad-Fix", a product that one can buy on Amazon. It's made for restoring conductive tips found in TV remotes, basically the same thing as the CBM keyboard issue here. I ordered a kit.

My daughter used a Cyber Clean cleaning compound to clean the motherboard up nicely.
Click image for larger view.


OK..I got impatient and took some graphite powder to the keyboard's conductive pads and it worked perfectly. One application and every key snapped back to life.

Pictured above is the wire extension I installed onto the caps-lock switch connection points so it will be easier to work on the keyboard in the future, should the graphite eventually need to be redone. Click image for larger view.


And another Commodore PET 2001-32B is brought back to life. Click image for larger view.


More photos

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  CAP C16 Missing, why? by Bill Degnan - 11/17/2017 13:04
Got this message, posting for future reference:

VintageComputer.net Inquiry Contact Information Name: J Walter Hawkes
Email: --- Phone: --- ------------------------- Comments:
In regards to blog post titled Commodore CBM PET 2001-32B Keyboard :

Hey there Bill - I'm a member of the VCF list. Quick question: I'm noticing in your picture of the motherboard that C15, a tantalum cap near a power transistor, is missing... I have two 4016s I just picked up. One of them has that cap broken off. On the 4016 it's a 1uF 35W cap.

1. I'm guessing it's a coincidence, but I can't imagine anyone would remove it on purpose?
2. I'm about to order some of them and would happily send you one no problem no charge :)

I have some of that cyber clean here in the house somewhere...I may try that.

Thanks for posting and for sharing on the list. I'm diving into a PET for the first time...


Note the 4016 is not the same board as the 2001-32, but here are the approximately same locations on the motherboard, note how the 2001 board has a cut off cap on C25 (or is that C15? need to check the schematic). The question is whether this is a field repair recommendation from an old Tech Topics issued to repair shops or the cap broke off at some point. Either way, the machine works fine. I need to determine why it's missing before I replace it.

4016 mobo
2001-32B missing C25 (or is that C15?)

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