1. Check all resistors attached to the HT line, including the screen resistor to the detector valve, and replace if more than 10% out of tolerance.
2. There are about ten film capacitors. If they are of the red Hunts type, replace them all on sight; otherwise measure and replace when out of tolerance.
3. Valves in these usually last forever: you are unlikely to have to replace them.
4. Carry out the audio-frequency alignment in the manual if you have appropriate equipment, changing the notch frequency to 9KHz as per international agreement in about 1978.