bad transformer?
 

During the winter, I noticed a loud hum coming from my furnace at all times.
The other day, I tried to turn on the AC and got no response.
Checked the furnace room, and found the noise is gone. No response at all from the thermostat, either heat or AC. Checked power, and there is power at the breaker.
Should I start by replacing the 24V transformer?

Randy

Megatron was a bad transformer...
But on a serious note, I would check the output voltage on the transformer. What kind of furnace do you have?

Only replace components when they test bad.
Could be the thermostat that is bad but, you would have to test that too.
You will need a volt/ohm meter to perform most tests.

The furnace is a Janitrol AC30-10, SN 9412014 74
I know my way around a VOM meter, so tell me what's next.

Randy

At the transformer check both the input and output side, should be 120V on input and 24V on the output. If that checks ok then check for 24V at the thermostat.

The thermostat is easier to check first.
The most voltage I could find was 3V AC.

Randy

Is there a component that will go out between the transformer input and the 120V?

Randy

OK, I'm getting 13V AC at the transformer output.
Can we assume the transformer is bad?

Randy

That would be my guess. Only thing that should be ahead of the transformer would be a switch or breaker.

13v indicates the contactor on the a/c is partially shorted if the stat was calling for cooling.