No Power after installing Power Connector

[dubby25]

I just purchased a Nest Thermostat. I have a heating only controller with 2 wires. This works, but there are issues due to no C wire. I purchased the Power Connector and installed. However, after this was installed, there was no power to my thermostat at all. I checked all the wires 5 times, and they are correct. The C wire was hooked up to my COM port (I have a Taco SR503 board), but there is no power with or without this hooked up.

[Patrick_Caezza]

@dubby25 

The Taco valve controller is notorious for being a pain to use.

Your best option is to use an external 24Vac transformer and wire it like this.