Before powering on

Above all, do not plug in before ....

The Voltage Selector

Set to 220V or 240V. If the device is designed only for 110V, use a step-down autotransformer or, even better, an isolation transformer. Indeed, many devices were so-called "universal current" models. They could operate on both 110V DC and 110V AC. Their configuration is such that the metal chassis may be connected to a live phase of the mains. The risk of electrocution is not negligible. Unlike the autotransformer, the transformer completely isolates the chassis from the mains.

The Power Cord

should be checked first. If it is damaged (risk of short circuit): replace it.

Reminder on Mains Plug Wires

Let’s recall some principles about electrical wires from the mains. Modern installations provide three wires with different colors. There don’t seem to be official standards, but rather common practices used by electricians. The wall socket wires are:

Live (Phase): This is the "active" point of the 235V mains. This wire is Red, Black, or Brown. If accidentally touched, an electric shock may occur as the current flows from the finger to the feet and the ground (Earth). Be cautious, as this can be fatal. The live wire can be detected using a small tool (usually a screwdriver - see fig.).
Neutral: This is the "passive" point of the mains. This wire is Blue. If touched, normally, no shock is felt. The phase detector does not light up when in contact.
Earth (Ground): This is a safety wire. It is Yellow/Green. If touched, no shock is felt. The phase detector does not light up when in contact. If a device plugged into such a socket has an insulation fault and its metal chassis receives part of the live phase, current will flow between the live wire and the Earth wire. The current will follow this path rather than the user's body. If the current is low (a few mA), it will protect the user. If it exceeds x mA, the circuit breaker will trip.

Capacitors

are the components that age the worst. In particular, the electrolytic capacitors. They are used for power supply filtering and decoupling. After several decades, they often dry out and risk overheating or even exploding. They must be replaced or, if their technology allows, reformed ... systematically. The capacitance values of these components could be 8 µF, 50 µF, etc., which are no longer available in modern catalogs. Since old electrolytic capacitors were at best 20% precise, they can be replaced with the closest standard values:
Example: 10 µF for 8, 47 µF for 50, etc.

The capacitors that cause issues when restarting an old radio set are:

The filter electrolytic capacitors — mentioned above, with an example of 22µF/450V marked in red fig.1
The audio frequency decoupling electrolytic capacitor. It is usually a single capacitor placed in parallel with the bias resistor between the cathode and ground of the final tube driving the speaker —22µF/50V in fig.2.
The paper coupling capacitors. They transmit audio signals from the anode A of an upstream tube (L1) to the grid G of a downstream tube (L2) —22nF/450V in fig.1. If a capacitor leaks, it changes the bias point of the downstream tube, causing distortion and/or overheating.

The Fuse

protects the device in case of a short circuit or excessive current consumption. For radios, the first scenario determines the fuse rating. For example, a radio consuming 200mA after one minute may have an inrush current of 5-10 times that value. The manufacturer chooses a 1A slow-blow fuse. If uncertain, use a T1000mA slow-blow fuse first.

Avoid using a makeshift copper wire!

Sources and References

[ 1] Installation Earth Wiring Guide