Still not had picture of bell box or voltage readings ?
When was the last time you triggered the alarm to see if the outside bell box was working correctly ?
Could be faulty bell box not sounding ? and has an issue causing tamper fault
I do have a multimeter yeah.
From the control panel Z3 is the furthest away, followed by Z4, then Z6 and then Z2.
I'm intrigued as to what are you thinking next?
THATS where I would start ! 1980s c type not even a polycarbonate box ,be prepared for it crumbling when you touch it !
Texecom 1e cheapest and same make as your panel
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