Repair notes: A1200 Stuck right mouse button, audio clicks

An A1200 came in for repair, previously recapped but problems with the right mouse button stuck on. When pressed, you get a click through the speakers but it remains “pressed”.

Booting Amiga Test Kit shows not just the right button, but also the middle button stuck, and the same on the joystick ports – buttons 2 and 3 stuck. The left button and first fire button work perfectly however, as do the directions on both mouse and joystick. Changing the ports does not change the symptoms.

A look at the schematics gives us our first clue – Left mouse button and primary fire go to a CIA chip, and these work. Middle and Right mouse buttons, and secondary and tertiary fire buttons go to Paula via four signals – POTX and POTY, with 0 and 1 variations. Below is a colour coded diagram, with the faulty signals marked in red – all going to Paula so we turn our attention to Paula as the common factor.

POTX0, POTX1, POTY0 and POTY1 (we’ll refer to these as the POT signals for ease of reading) route almost straight to Paula from the mouse and joystick ports. They are decoupled to ground and have a ferrite bead to keep the interference police happy, but the circuit is pretty simple. Internally, Paula pulls these signals up so they’re normally high, and when you press a mouse button or a fire button you short it to ground and the signal goes close to 0v.

These four POT signals go to pins 35, 36, 38 and 39 of Paula. If we stick a logic probe on these we should see them go low when a button is pressed – and we do. So the signals are getting to Paula, but being ignored for some reason.

The clicking is interesting when the right button is pressed. Why would this button issue interact with the audio output? This type of situation can sometimes be seen in other circuits where a ground or earth is missing, and the “path of least resistance” ends up being through another part of the circuit rather than straight to ground. An example often see is car lights, where the brake light flickers dimly when the indicators are used. You’ll usually find the earth point has rotted away due to water ingress and the indicators are earthing through other lights. But anyway, this isn’t a car repair blog….

Checking Pin 37 of Paula, which is Audio Ground, shows an interesting development. It’s sitting at around 2.8v, in fact it’s very similar to AVRef which is used on the audio circuit. It should be zero volts as the schematics show it’s tied to ground via a ferrite bead near the audio jacks. If we look under C334 we see the via that takes it from the top of the board to the bottom…. and it’s totally rotted through. Audio ground isn’t connected to anything, it has a bunch of components tied to it but the whole lot is floating – as is anything connected to it.

Below is a representation using AmigaPCB – the blue trace was correctly tied to ground, and then where it swaps to red is the via under C334 that was rotted so nothing in red was actually grounded.

A quick fix with some wire soldered top and bottom to restore continuity, Audio Ground is back to 0v, and all buttons work. It’s quite interesting that sound was still working perfectly through all this, and that the POT signals use Audio Ground and not the other grounds that Paula connects to.

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