Aircraft had a Standby Power "OFF" light flashing intermittently on the overhead panel. Along with the light, a EICAS message of "Standby Bus Off" cycling on and off.
767's use two relays for standby power failure indication. K110 senses DC power and K138 is used for AC power. As long as power is present at each relay, they remain energized. If either power fails, the relaxed relay supplies a ground signal to both the light and EICAS systems.
Both relays are located in the forward most section of the P6 panel in the lower right section of the cockpit. (I've found that using a bungee cord to hold these types of panels open is a great help. If the aircraft is powered up, everything is hot, including the breakers on the fold out section itself. the further away from my hands..... the better.)
K110 and K138 are mounted next to each other. A clear "click" was heard every 10 seconds or so. I pulled K138 first, the sound remained. K110 was the culprit. The coil would go open when it got hot. It would close after it cooled.
767-300 ASM 24-33-01