Macintosh Plus mouse button not registering clicks

[Wiggs1979]

I purchased a Macintosh Plus which was in a little bit of a state and wasn't functioning - I do enjoy the challenge. A fair bit of corrosion on the back and all of the ports. It's been a little bit of a journey to resurrect it and it's getting close, but I'm stumped as to why the mouse button isn't registering clicks.

The machine has had the analog board recapped, the optoisolator was found to be faulty, so that was replaced too. Voltages are all good and stable. The logic board was showing a corrupted checkerboard pattern, this turned out to be an issue with the 6522 VIA which has been socketed and replaced. The SCSI and mouse ports were badly corroded so they have been replaced. I've fitted a terminal power diode to power a BlueSCSI from the machine. I've socketed and replaced the filter that the mouse signals pass through. The mouse itself works on another Macintosh Plus.

The machine boots the OS fine from the BlueSCSI, I just can't do anything with it. The mouse cursor moves, but it will not click. I have checked the signal on pin 13 of the VIA with a logic probe and I can see it go from high to low when the button is pressed. I've tried multiple VIA ICs just in case there's a fault and also checked the VIA is working in a BBC model B computer.

So I'm wondering if something unusual is happening on the upper data lines, but everything else appears to be working. Does anyone have any ideas where to go from here?

 

[demik]

Hmmmm...

The VIA cannot generate an interrupt on click and you checked the connectivity and the VIA so it isn't a VIA issue.

Random idea: the mouse click is probably polled @ VSYNC. Do you have a pulse on pin 40 (CA1) ? Does the IRQ line kinda follow on pin 21 ?
If yes follow the IRQ to TSG pin 9, then it should beon pin 19 (/IPL0) and to the processor

 

[Wiggs1979]

Many thanks for the idea, very much appreciated. I have been trying to understand how the VIA works in a bit more detail. With the logic probe, I can see a pulse on all of those pins, that roughly looks to be the same frequency. It doesn't start on the IRQ line until after the memory test has completed, when it's waiting for a boot disk.

The NMI or programmers switch wasn't working, but I've now replaced that and the reset switch, which lost a small trace in the process. Now I can boot into the OS from the BlueSCSI and when I hit the NMI switch I cannot type anything into debugger window. I'm not fully ruling out more corrosion on the keyboard connector, but I have cleaned it with contact cleaner and a light brushing with a fibreglass pencil. Does this sound like I might have a fault with the TSG? I've seen the code is online and I have some GALs I could program.