When I removed the 512 board, I noticed a bit of battery leakage in the vicinity of PL8 (despite having removed the battery years ago, before there was any visible leakage):
This is going to be removed as a matter of course (I'll reflow the joints at the same time), but could it be contributing to the current issue or is it a distraction?
Looking at the Service Manual and the decode matrix for the keyboard, it would seem C1, C3, C5, C7, C9 and C11 are all absent, ie all the odd columns. The columns are read by SA0-SA3 as a 4-bit number, so I'm guessing the first bit is at fault? This would be SA0, I presume?
So, out with the logic probe. SA1-3 are usually high, but start oscillating when a key is pressed, consistently with the keyboard matrix being scanned. SA0 is stuck permanently low, no matter what.
So it would seem SA0 is at fault, being permanently tied low so only even columns on the keyboard matrix are read.
I'm struggling to work out how SA0 is generated from the docs at my disposal - I can see where it goes, but can't work out how it's generated. Where should I look next?
Edit: I've just found in the Service Manual where it says the System 6522 generates the Slow Data Bus. Since SA0-3 all read 1.73k to ground, I'm assuming there isn't an external short and it must be the 6522 itself that's at fault?
This is going to be removed as a matter of course (I'll reflow the joints at the same time), but could it be contributing to the current issue or is it a distraction?
Looking at the Service Manual and the decode matrix for the keyboard, it would seem C1, C3, C5, C7, C9 and C11 are all absent, ie all the odd columns. The columns are read by SA0-SA3 as a 4-bit number, so I'm guessing the first bit is at fault? This would be SA0, I presume?
So, out with the logic probe. SA1-3 are usually high, but start oscillating when a key is pressed, consistently with the keyboard matrix being scanned. SA0 is stuck permanently low, no matter what.
So it would seem SA0 is at fault, being permanently tied low so only even columns on the keyboard matrix are read.
I'm struggling to work out how SA0 is generated from the docs at my disposal - I can see where it goes, but can't work out how it's generated. Where should I look next?
Edit: I've just found in the Service Manual where it says the System 6522 generates the Slow Data Bus. Since SA0-3 all read 1.73k to ground, I'm assuming there isn't an external short and it must be the 6522 itself that's at fault?
Statistics: Posted by Boydie — Fri Nov 29, 2024 11:03 am