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jimblargh

VIC 20 - dead even number keys, -, and clr/home

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Hi everyone!

 

Just got my first commodore VIC 20 today! Nice clean looking unit. I'm already enjoying it, but I was surprised to find that a few keys are not working. I took off the keys and I can't get them to work even when I manually press the 'posts' that the keys interact with. Very clean, so that's not the issue.

 

My next step would be to take apart the computer to see if the contacts on the board need cleaning, but then I noticed a pattern: The keys that don't work go every other key on the top row, with the exception of the left arrow key. Even numbers along with '-' and clr/home (mostly sure on the clr/home. Truth told I don't know what it's supposed to do exactly, but it's not doing anything atm)

 

Does anybody with a little more experience than me have any ideas what might cause every other key on the top row to go dead? 

 

Thanks for the help!!

 

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My first thought is a partly bad 6522 VIA chip, since these read the keyboard in columns and rows. There is a POKE to enable joystick in one of the directions (left? right? can't remember) that disables part of the keyboard instead.

 

The two VIA chips are equivalent, but handle different things. If you're lucky, the two chips are socketed and by carefully lifting them out of the sockets you can swap them around and see if it changes the keyboard function. Note that it might introduce other errors instead, depending on which pins on the chip are used for other purposes.

 

One might think those chips either work or are completely damaged, but more than once I've encounted that both VIA and CIA chips (in the C64) work partly, some of its inputs or functions misbehaving while others still work. That is what makes it tricky.

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Thank you very much for the reply carlsson!  Your reply got me to googling a few things about the 6522 VIA chips and I found several references to similar keyboard issues with the VIA chip when a Sega Genesis controller is plugged in. While I was not using a Genesis controller, I was using a Hyperkin Ranger - a modern controller designed for the Atari 2600. Sure enough, when I plugged in my Suncom Slik Stik my keyboard issues were gone! 

 

I don't know how the Hyperkin Ranger controller is different from other atari 2600 controllers but apparently it is. Lesson learned - don't use a Hyperkin Ranger on a Vic! 

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Interesting. I suppose the Hyperkin controller somehow implements a secondary fire button by pulling some line that normally is not pulled, whether it uses one of the paddle inputs or pin 7 that is 5V on the VIC-20.

 

Well, it was an easy fix.

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