external 24 volt power for 4th generation thermostat

[seawitchm36]

One of my 3 Nest 4th generation learning thermometers keeps giving me n410 messages.   I measured only 22VAC is making it through the common wire.  Thermostat works but not Wi-Fi.    I have White (heat), Yellow (cooling), Green (fan), Red (power) and Blue (common?) wires coming to the thermostat.  The wiring is 40 years old, and tough to replace, so I'm told I should attach an outside power supply.  What are the proper connections to the Nest base for an external 24VAC power supply?

[Patrick_Caezza]

@seawitchm36 

I suggest checking the C wire and ensuring it is correctly installed with clean, shiny copper fully seated into the C terminal. If the copper is tarnished in any way, you will experience voltage problems; it's best to ensure that all wires have shiny copper.

The N410 error usually points to a C wire problem