System is 11 years old
I have a Honeywell Chronotherm IV Plus T8601D2019 Tstat. (A/C only)
Condenser is a Trane XL1200. 
Unit is a Trane XV80
5 wires to Tstat:
Blue = C
Red = R
White = W
Yellow = Y
Green = G

Unit has performed solid up to this point. 

Yesterday I changed the filter element (which I do religiously every 3 months). Same brand and model I have used for about 5 years. Unit ran on-and-off for 10 hours (light usage) then stopped. I noticed the screen was blank on Tstat. Checked indoor unit and it was humming, but flashing light out.  Checked unit panel and found 5A ATO fuse blown on board. Replaced and Tstat powered.  Soon as it sent a message to start AC (snowflake) it clicked and blew fuse.  Checked for obvious loose wires, checked breaker, checked fuses on condenser. Nothing found.  Installed new fuse and repeat. Blew again the same way. 

With that clicking sound, My thought was a short.  Not sure whether the click comes from the main unit or the Tstat. 

Suggestions on how to efficiently proceed would be appreciated. 

Thanks

Richard