On Wednesday, January 10, 2018 at 11:17:06 AM UTC-6, James Harris wrote:
> To my surprise, I find I have a Pet which works - all apart from the 
> keyboard, that is. So I have a query about fixing it.
> 
> So far, I have removed the keyboard and found that the PCB contacts work 
> if shorted with something conductive. Therefore the problem is with the 
> little black pads that a keystroke presses against the PCB. I have tried 
> these things:
> 
> * cleaning with isopropyl alcohol
> * cleaning with contact cleaner
> * abrading with a pen eraser
> 
> The latter was not really effective as there is too much give in the 
> part of the key which supports the pad so I would say that the operation 
> failed to abrade the pad successfully.
> 
> Where I am now is that a few pads work but most do not. I have tried 
> putting two meter test prods on the pads to measure resistance and found 
> that some only become conducive with significant pressure. I guess that 
> the rubber of some of them has become too hard with age.
> 
> So what can I do to fix them? Any ideas?
> 
> 
> One idea I've had so far is to apply some conductive matter to the pads. 
> I have tried a product called Keypad Fix on a remote but found it dries 
> to be too inflexible and crumbles on use, albeit that that was on larger 
> contacts.
> 
> Another option is conductive paint such as
> 
> https://uk.rs-online.com/web/p/solder-pastes/8352699/
> 
> A further option is to cut tiny copper-film discs and stick them to the 
> existing pads.
> 
> Any of those good options? Other suggestions welcome.
> 
> 
> -- 
> James Harris

Hi, I work for a company that makes keypads and I have refurbished c64 and
VIC-20 (PET style) keyboards.

The contacts are usually (maybe always?) conductive rubber.  There are two ways
to make conductive rubber, impregnation of the base rubber with conductive
material or a surface conductive paint.  I am pretty sure the typical c64 uses
the impregnated 
conductive material.  I'd be surprised if the PET was different.  I don't
remember the PET VIC-20 being that different.

Cleaning with isopropyl alcohol is really all I've ever done to restore
commodore keyboards to like new condition.  I am surprised that didn't do the
trick for you.  Make sure you clean both the bottom of the conductive rubber
pad AND the contacts on the 
circuit board.  You can use some force. 
 You are cleaning off any build up of surface films etc. that affect the
contact resistance.  This is the same method used to clean the contacts of the
"newer" membrane keyboard contacts used in the sx-64 and I think the Amiga (I'm
strictly 8 bit, so I 
don't know Amiga that well...).

I haven't researched it, but I would avoid using contact cleaners.  i would
think those are designed to clean plated surfaces and might attack rubber in a
bad way.

Abrading with an eraser could work to clean the surface films off but I don't
think it is necessary.

If the rubber contacts were made with surface conductive paint (really cheap
way to go by the way), that ink can were off over time.  Abrasion cleaning
would be bad for these.  If the rubber is any color than black, you can see the
wear patterns for this.
  Pretty sure though, that the commodore parts will be black...

If you really think you want to try conductive ink again, this looks like the
right stuff.  Usually some conductive material is mixed with a silicone paint
so that it flexes with the rubber.  Don't try anything that doesn't claim to
stretch with the 
rubber.  The surface should be cleaned before application.

https://www.amazon.com/CaiKot-Conductive-Silver-Coating-surfaces/dp/B003D8G8SY/
ref=pd_sbs_201_1?_encoding=UTF8&pd_rd_i=B003D8G8SY&pd_rd_r=FAXQRX7Y9CCEFSA99NT1
&pd_rd_w=3RbXX&pd_rd_wg=P5wSL&psc=1&refRID=FAXQRX7Y9CCEFSA99NT1

But, please try the alcohol cleaning again first.  It really should work.  You
could also move some contact around to see if the issues follow the contacts or
there is some issue with the PCB/cabling.

Good luck!

Jim

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