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Finally following up on the cause of this failure...

It turned out to be a faulty C853 on the main board. I put a DMM across it in circuit and got about 300 ohms, even when I reversed the leads. That seemed suspicious to me so I lifted one of the leads. Still measured 300ish ohms. Did a quick capacitance check and instead of 0.47uf, I got 0.02nf. Not a happy capacitor.

To test, I bodged in a 0.1uf cap, and now the cursors work perfectly. So I'll need to get a replacement 0.47uf capacitor. Since I don't fancy removing the A1 board, I was planning to get some SMD caps and connecting the stubs of the original capacitor leads to the SMD pads.

I can't think of another incident where a (non abused) ceramic capacitor has failed. Electrolytics and tantalums, sure, but ceramics? There are 4 of these caps in a row, all related to the cursor signals. I'm tempted to just replace all four. Anyone know if these particular caps have a common failure story?

Scott