Atari 800xl Shift Key Not Responding

[Sinjinhawke]

I recently took my 800xl to someone with soldering tools and skills to socket a couple of my chips.  I have one of those 800xl computers where most but not all the chips are in sockets.   I had him remove and socket the MMU, OS ROM and PIA.   I didn't have a socket for the BASIC so left that as is.   Since returning it seems to work mostly ok except the SHIFT key no longer functions.  The keyboard mylar seems to be in great condition still.  I tried removing and wiping the contacts with alcohol to no avail.

 

I notice on the PIA solder connections that there appears to be some flow between two pins.   Could this be the source?

 

Edited November 1, 2020 by Sinjinhawke
MMU typo

[xrbrevin]

check the pins within the keyboard ribbon slot - sometimes they get pushed back which causes poor contact

[Sinjinhawke]

The pins in the ribbon slot look ok.  

[xrbrevin]

Here is the keyboard schematic, It shows only 1 instance of 'Shift' so the keys must be wired in parallel with one another.

If only 1 shift key is faulty then i would check conductivity of that plunger, if both are faulty then check pins 8 and 19 of the keyboard cable

 

[Rybags]

PIA should have no bearing on SHIFT, it's handled by Pokey.  Yep, 2 keys but no way to differentiate between them.

[Sinjinhawke]

I tried cleaning the mylar and replacing the keyboard.  None on the springs were missing.  I notice the Control and Break key also do not seem to be working.

They are all in the same column on the chart above.  Is this a possible mylar break?

[djglish]

I had a similar problem with my 800XL keyboard. Two lines of contacts weren't working.  I bought a replacement mylar and it didn't help.

While fighting with it I only tightened a couple of the screws and low and behold those lines started working.  It turned out that I was tightening all of the 18, or so, screws too much.  Now it works fine, with the old mylar, no less.

[Sinjinhawke]

I currently have only replaced some of the screws without over tightening.  I also tried an eraser on the end contacts of the mylar (was a lot of corrosion I guess).  Still non functioning of the SHIFT/CTRL/BRK keys.

 

Looking at the mylar some of the traces are quite dark (almost black).  I could try using the eraser on them but they are at the worst where the mylar folds.  Maybe I should just order a replacement mylar.

[xrbrevin]

some people have repaired them using conductive silver paint

[Sinjinhawke]

On 11/3/2020 at 3:18 AM, xrbrevin said:

some people have repaired them using conductive silver paint

I’m going to have to try something like that.  
 

I took my Atari 600xl keyboard and slapped it into my 800xl.  Works and fits perfectly.   Removing the mylar from the 600xl pulled some of the contacts out of the keyboard connecter so will need to order a replacements from tt_hh.  
 

I did order a replacement mylar and it works ok when I just insert the mylar on its own but when I attach the mylar to the actual keyboard it is not responsive.   Not quite sure why that is.