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800_Rocks

1200XL Joystick0 Troubleshooting Help

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My newly acquired 1200XL has a bad joystick 0.  Using the little program below, when a known/good joystick is attached, it is reading '13' instead of '15'.  What are the toubleshooting steps to diagnose and fix such an issue?

Other details... Joystick 1 works great.  Joystick 0 works somewhat with some games.  Example:  with Centipede I can move left/right and fire but can't move up/down.  with Donkey Kong, it works great.  with PacMan it does not work at all as PacMan immediately goes South and gets stuck on the first wall it hits.

Thanks!!

 

On 2/8/2021 at 3:27 PM, gozar said:

Start up basic and run this code:

10 ? STICK(0)
20 GOTO 10

Now, move the joystick to the different directions. These are the values you should get for the different directions:

     10  14  6
       \  | /
        \ |/
     11--15--7
        / |\
       /  | \
      9  13  5

 

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Yep, agree with the above. Confirm no damaged pins, reflow solder on the joystick port, swap the PIA if necessary. 

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I replied in your other thread.  Here is my reply, copied from it:

 

That looks like your joystick thinks that it is always pointing down.  Does it do this with no joystick plugged in?

 

The down pin on the joystick port is pin 2, which is connected to pin 3 on the PIA chip.  The PIA is seeing this pin as grounded.  If you pull out the PIA chip, and test pin 3 of the socket without the PIA plugged in, is it connected to ground?  If so, there is some short to ground somewhere on your board.  If pin 3 of the PIA socket is not grounded, then the issue is with the PIA chip itself, as it thinks it is grounded.  One test you could do is to bend up pin 3 of the PIA before you put the chip back into the socket.  That way, it is completely disconnected to anything.  Then, if you run your joystick test, does it still show 13?  If so, then it is definitely a bad PIA.  If it shows 15, then you could use a wire to ground the pin that is sticking out and then it should show 13.  If so, then your PIA chip works properly (at least for this pin).

 

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5 minutes ago, 800_Rocks said:

1200XL FSM.PDF 924.29 kB · 2 downloads 1200XL Schematics.pdf 182.8 kB · 3 downloads

 

Thanks for the response and advice.  I found this somewhat better scan of the same Field Service Manual and this great modern version of the Schematics!

FYI, those are the “Sobola schematics,” done decades after the fact. They do have errors so don’t trust them 100%. The reality is, the 800XL is fundamentally very similar aside from the two-chip OS ROM and lack of internal BASIC. For most else - sound, video, SIO, controller ports, etc - you can refer to 800XL schematics in something like Sam’s ComputerFacts for the analogous circuit and compare with your own 1200XL using your multimeter in continuity mode to verify stuff. 

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4 hours ago, 800_Rocks said:

and this great modern version of the Schematics!

As @DrVenkman has already mentioned the Sobola schematics have some errors, I posted a revised schematic of just the 1200XL video circuit a few years ago.

One fairly obvious error I corrected is the VIDEO RF and COMP LUM labels for the outputs are reversed.

 

Edited by BillC
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Posted (edited)
On 2/18/2021 at 11:06 AM, StickJock said:

That looks like your joystick thinks that it is always pointing down.  Does it do this with no joystick plugged in?

Update -  Fixed it!!   😃 😃 😃  To StickJock's prior question, 'yes' it does this even when no joystick is plugged-in (it is as if Joystick 0 always has the stick in the down position... always)

First I used a multi-meter to check the continuity of Joystick0 'Down' (pin2) all the way to PIA (pin3).  I compared what I found to other Joy0 and Joy1 pins.  Found no differences so assume it is not a solder joint, cap or resistor issue.
Second I replaced PIA with a spare pulled from a NOS 800 mainboard (all chip markings matched on both chips other than the date codes).  All testing of Joy0 now works as expected!!  I can now play PacMan!!

 

Questions:  How common this where one or more pins on PIA goes bad?  Are there modern PIA replacements or  do you always need to pull from another unit?


I got this 1200XL for $50 + 6 hours of drive time.  When I got it no keys worked and it had this Joy0 issue (like the stick was always stuck in 'Down').  After I performed 'surgery' on the keyboard mylar (removed, cleaned, used conductive pen on contacts to board) and replaced PIA... I now have a perfectly working 1200 XL that looks great too!!
 

Thanks again for all the prior responses.

Edited by 800_Rocks
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1 hour ago, 800_Rocks said:

Questions:  How common this where one or more pins on PIA goes bad?  Are there modern PIA replacements or  do you always need to pull from another unit?

 

Not particularly common but not unheard of.  There is a modern chip in the same family that is drop-in compatible but vintage originals aren't particularly expensive or hard to find. 

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