The power supply is the place to start, and electrolytic capacitors in the p-s are Job Number One. Here is basic advice about replacing capacitors if you haven't done that before:
http://antiqueradio.org/recap.htm
You have the schematic, right? It not only shows how things are connected, but usually has a voltage chart that's very helpful in diagnosing problems like this. Without a schematic, you're shooting in the dark.
Phil Nelson