A4k Left Mouse Button issue

[Castellen]

The capacitors which generally leak are the 22µF and 4.7µF ones, which aren't involved in AC decoupling of the 5V supply.  The positions they're in are unlikely to damage the left mouse button line due to localised corrosion.

CIA U350 does read the left mouse button, but an intermittent CIA failure like this is not common.

Measure between the left button pin (CN975 pin 6) and U350 pin 10.  Should be about 47 ohms if it's working normally.  If it's open circuit, then I'd start looking for a problem with a fractured pin in CN975 (which is fairly common), or a crack in resistor ER976.

To check for a cracked connector lead, heat the bottom side soldered connection of the pin.  If the pin moves very easily, or slides out when you pull it gently with tweesers, it's obvoiusly broken.  Easy enough to fault find/repair.

[Castellen]

Not sure, but my A4k has a problem with the right mouse button jamming on, even without a mouse in the port.

Had a job come in with this exact fault the other day.  The problem had started with leaking capacitors in the audio stage, the corrosion around op-amp U402 caused enough damage to the device so that the reference voltage of 2.3V was no longer present on the DAC output pins (U400 pins 33 and 34).  It was either at 0V or something much higher than it should have been, I forget now.  This caused the ADC channels to malfunction, hence the right mouse button would fail to operate and jam on.

I've written details of how to repair it all here.