Macintosh Plus mouse doesn't move

[lastfreeusername]

Hi 68kMLA! I am completely stuck with my Macintosh Plus, and I don't know what to do. The situation: Macintosh starts, boots with chime, and OS initiates. Mouse click is working, but the mouse doesn't move. What I did:
1. I checked connectivity inside the mouse, all photoresistors, and diodes. Everything is fine.
2. I ordered a new M0100 mouse and tested it - the same status.
3. I checked the connectivity of the port and up to the VIA chip. Everything seems fine and connectivity for X1, X2, Y1, Y2 is fine. The same for resistors.
I am really stuck now, cause I don't believe that chip itself is an issue. The computer is in a very good condition – the only thing is that one memory socket clip is broken, but it can't affect, since the POST chime is here.
Could you give me some ideas where I can look for the issue?

 

[cheesestraws]

You say you checked to the VIA - did you also check X1 and Y1 going to the SCC? Responsibility for dealing with the quadrature signals from the mouse is split between the VIA and SCC. I think it's the SCC that generates the interrupt that is actually used to detect mouse movements, so it might be worth checking the wiring back from the SCC as well.

 

[lastfreeusername]

Thank you! Yes, I also checked wiring to the SCC, and those are also fine, can get a beep out of both.

 

[cheesestraws]

OK - forgive me, I am working from memory here, not at home. Next thing I might check would be that the INT output of the SCC is going where it ought to.

Do you have access to an oscilloscope or similar?

 

[lastfreeusername]

I have access to the proper lab, yes, oscilloscope should be there. Could you please guide me what exactly should I expect to have?