TRACK DEV CAUSED BY A GND PWR FAILURE.

Narrative

WHILE DEPARTING RWY 34R AT SEATTLE ON THE SUMMA 5 DEP; I MADE A SLIGHT L TURN TO INTERCEPT THE 338 DEG RADIAL OF SEA. APPARENTLY MY HSI CARD DRIFTED APPROX 20 DEGS TO THE R. THE HSI NEEDLE THEN INDICATED L OF COURSE. A CORRECTION WAS MADE TO THE R TO A HDG OF 360 DEGS TO GET BACK ON THE 338 DEG RADIAL. I DID NOT REALIZE THAT THE COMPASS INFO ON MY HSI WAS IN ERROR. THE COMPASS SWITCH WAS IN THE SLAVED POS AND THE COMPASS SYS WORKED NORMALLY ON THE PREVIOUS LEG. DEP CTL ADVISED US AT APPROX 7 DME THAT WE WERE ABOUT 1 MI L OF COURSE AND TOLD US TO COME R TO INTERCEPT THE 338 DEG RADIAL 'FOR NOISE ABATEMENT.' THE CAPT REALIZED THAT MY HDG INFO WAS IN ERROR AND SO ADVISED ME. I WENT TO DIRECTIONAL GYROSCOPE MODE AND SLEWED THE COMPASS TO AGREE WITH THE CAPT'S. THE REMAINDER OF THE SID WAS FLOWN AS PUBLISHED. ONCE AT CRUISE; THE COMPASS WAS AGAIN TRIED IN THE SLAVED MODE AND IT WORKED NORMALLY FOR THE REMAINDER OF THE FLT. WHILE ON THE GND IN SEATTLE; APPROX 25 MINS PRIOR TO TKOF; THE GND PWR FAILED AND THE ACFT WAS WITHOUT PWR FOR APPROX 5 MINS.