Analog Stick problem

[Minwah]

I recently aquired a G-LOC analog stick, which I have now took apart, cleaned up, painted and replaced the pots for 100k ones.

Now, I wired the 2 pots and 2 buttons up to an old PC stick cable.  It works perfectly, until I hit the top button - then it seems to go out of calibration.  If I go into the Windows control test screen, and hit this button, the button itself works fine, but the crosshair flicks up/down/left/right randomly and to varying degrees.  After this it seems to be out of calibration.  The same occurs if I play a game - it works fine til I hit the top button.

The connections are fine and the cable is fine (I tried 2 different cables).  The cable from the buttons is a totally separate cable to the pots, so there is no chance of a short.

Anyone seen anything like this before?

It is not a major problem, since I will probably wire the buttons up to my I-PAC for my panel anyway.

Cheers

[grafixmonkey]

Any way you can check what currents are going through the pots and the button, with and without the button pressed?  If the button is somehow causing lots of current to draw from the gameport (it's gameport right not USB?) then that'd cause the voltages across the pots to drop, which would cause the voltages sent back to the gameport from the joystick to go funny when the button is pressed.

This could happen if the button had some resistors on it in the original design that somehow got bypassed or removed...

[Minwah]

Thanks for the reply, I'll try checking that (it is gameport, but through a Gameport > USB adaptor).

[grafixmonkey]

How exactly did you tap it?  there may also have been some voltage pull-up amplifiers in their design, connected to the pots, for driving the line back to the gameport.  100 Kohm sounds like a lot of resistance to be driving an input port without a line-driver component of some kind.  A tiny change in the current drawn by the gameport could cause a large change in voltage over the line.

Here's a pic of what I mean...