What? All of the joystick/mouse *direction* inputs are read by U450 (Denise).
There are physically only 4 inputs to Denise, but 8 direction inputs from the two ports, so a multiplexer is used to sample between eight lines into four.
Download one of the many joystick testing programs from Aminet such as
this one.
If the "stuck direction" problem occurs on both joystick ports, or more than one direction input, the problem is more likely to be between the multiplexer U401 and Denise, or the Denise input(s). Perhaps a bad socket contact.
If the problem is isolated to one direction input pin only, then you may have a problem with the multiplexer input (unlikely) or something between the multiplexer and the joystick port. Quite often the EMI filters tend to get cracked, but they usually tend to go open circuit so cause no direction movement. In your case, it sounds like there is probably a short to ground on the "down" input.
With the computer powered on but no joystick plugged in, you should be able to measure the same voltage on all direction inputs on the connector, i.e. pins 1 to 4.
Since each direction input is the same separate circuit repeated four times, you have three other references to compare against.
Schematic of the joystick I/O
here (bottom left corner) which may explain things a little better.
