I will second @erik_squires recommendation about using a variac.
A couple extra thoughts:
They are probably OK after 20 years since it has been boxed, but prolly will not last long.
Do expect it to sound weird when the voltage is low, there will be a voltage threshold things have to cross before they start to function.
You can get a very cheap variac on Amazon for under $60.00.
It is very important to de-rate random Chinese stuff like this by 60% to 80%. These items are often fused incorrectly and will short the hot leg to the frame when the fuse blows or when they melt.
So a 5A variac is really only good for 1A to 2A. They are basically a huge variable resistor, so they less voltage to let through them, the hotter they get. If you let full voltage through, they will not get hot at all.
A 5A version safe enough to test a 5A fused component provided you do not let it idle for too long at partial power. Make sure you unplug it before touching the metal frame if the fuse blows. I have the 20A version and have let it idle for quite a while on a 4A circuit with it only getting very warm, which is exactly what you would expect by de-rating it 80%.