I have spent hours searching the web for information on this 20+ year old zone control panel, which supports 3 zones of Heat and A/C.
The thermostat I'm replacing (on Zone 3) connects to T4/T5/T6 as W, R, Y respectively (the wire colors do not match because it's sharing a cable with Zone 2, which go the matching colors). I need a C (Common) wire to supply power to the thermostat, and found 24VAC across M2 and T5 (M1/M2 are the power leads to the zone damper, and seem to be connected internally to "1" and "2", the 24VAC supply.
However, when I connect the C terminal on the thermostat to the M2 connection, there is a click from some of the zone relays and the transformer hums as if it's overloaded. I removed power after a few seconds and disconnected the wire (now hanging off to the right of the picture).
I've disconnected the wire from the C terminal at the thermostat, and measure 24VAC across it and the Rh terminal on the 'stat, so I don't believe the issue is in the wiring.
The thermostat is a Filtrete 3M50; I've read that it draws less than .1 Amp, so I don't think this should be enough to put the system over some maximum current draw.
I'm looking for ideas on how to troubleshoot this.