It will have either an A89### or A90### picture tube. If a89, then it is 89 cm 2.54 = 35inches viewable picture. id A90, then 35.4 inches viewable (considered to be 36v). The 39" will be a measurement to the outside of the glass on opposite diagonal corners.
might be a board problem, or a signal connection. But sure would not rule out a bad blue gun in the tube, in that case, it's a boat anchor.
Good luck
Rick