Grumpy,
The tube behind the dial in the Zenith is the tuning eye, visible on the dial. Look at the very top edge of the back of the cabinet for the Zenith model number. They frequently stamped it there, often with very faint lettering. If you look closely, you might be able to see it.
That Philco chassis is one of the 40-150 or 40-155 series (1940). The servicing info & schematics are available here (each link is a separate page):
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D150
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D150
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D150
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D150
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D155
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D155
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D155
http://www.nostalgiaair.org/Nostalgi...f&INF=40%2D155