My second Amiga A1200 motherboard is exhibiting a fault, whereby the mouse cursor can only be moved left to right, no vertical movement is registered.
This blog post is a record of the steps I took to (hopefully!) fix it.
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Replace the surface mount capacitors, as the capacitors are in a bad way and there is some fairly extensive leakage/ corrosion. This does not change the situation but needed to be done anyway.
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Identify the components / traces involved using http://amigapcb.org/ and also what pins the amiga mouse uses from https://old.pinouts.ru/InputCables/AmigaMouseJoy_pinout.shtml
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Seems we are interested in pins 1 and 3 for vertical mouse
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Test the capacitance / resistance of the components on the path to U34 (a 74LS166 SN74LS166ADR) - these check out as being reasonable. Can’t really measure capacitance in circuit
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Plug a joystick into the mouse port and using Keir Fraser’s Amiga Test Kit https://github.com/keirf/Amiga-Stuff, test out which of the directions of the joystick doesn’t register, as this will be one of the two lines used for the vertical movement. The left direction is permanently on, which is pin 3 in the joystick connector. Look more closely at the components between that an the shift register. Specifically E357C and E357R on the underside of the PCB, the signal goes through a via underneath U34 and then attaches to pin 3 of U34. The resistance between pin 3 of U34 and the joystick port pin is approximately 68ohms, the expected value of E357R so this does not feel far off. This is generally pointing at U34 the shift register being at fault.