Valve equipment carries very high voltages commonly above 250v this can be lethal,in fact some valve amplifiers commonly have voltages of 500v or more on their anodes so beware!!! join the group valveradio.co.uk on facebook. just type in valveradio.co.uk on the search tab on facebook to find me.
THE INFORMATION GIVEN ON THIS SITE IS GIVEN IN GOOD FAITH AND IS BASED ON 30+ YEARS OF REPAIR EXPERIENCE,HOWEVER I DO NOT PROFESS TO KNOW EVERYTHING AND IF YOU FEEL I HAVE MADE AN ERROR,PLEASE EMAIL ME USING THE CONTACT PAGE AND I WILL BE PLEASED TO CORRECT IT.PLEASE READ ON FOR YOUR OWN SAFETY. A lot of the early radios were built pretty much without safety in mind. The chassis was accessible sometimes through the ventilation holes in the back. Some of the ac/dc sets that were around were lethal. Usually one side of the mains went straight to chassis and the other via a switch then to a dropper resistor,the problem occured when you had a reversible mains plug i.e a 5 amp 2 pin plug,depending on which way round you inserted the plug in the socket you could sometimes find the live side of the mains went straight to chassis. If you happened to have the chassis outside the cabinet say for a service and put one hand on that and another on your nicely earthed anglepoise lamp.....well we wont go into that!!!suffice to say it could mean a trip to the undertaker!!
So you have just bought a nice old 1949 radio from a car boot sale/antique fair etc. what do you do? I bet your tempted to plug it in straight away into the mains supply. Dont!!!!!!many radios bought this way have been stored in lofts/sheds/garages etc and have been unused for many years. Damp can damage the windings on mains transformers eating away at the lacquer insulation and causing shorted windings. Its also a problem with i.f. transformers too causing green spot which is really copper corrosion and can and often does cause open circuits in the windings. Also check the mains lead as well because in older sets these were made of rubber and tend to become brittle with age and could short out.
Another good reason why you should not plug the set in straight away is the electrolytic smoothing capacitors or reservoir capacitors as they are known,typically 4 to 50 microfarads tend to age when they are not being used and seem to chemically break down and become low resistance. When you plug the mains straight in they can overheat rapidly and explode(i have personal experience of this)and can cause one hell of a mess and give you quite a fright!!! Worst case scenario of course is you could lose an eye. I have heard stories from other engineers that they can literally embed themselves in the ceiling....nasty. So hopefully you will have taken heed of my advice and not plugged in. Good.
First get the chassis out of the cabinet and give it a good visual going over. By using your eyes it can tell you a lot even without getting your multimeter out. Whats the wiring like inside? Is it rubber? Has it cracked or perished? If so it will need replacing or will short out. Also does anything look like its been getting hot? Also check if the radio has had any previous work done on it. Pay particular attention here to the mains transformer,look for waxy blobs in the cabinet near where the transformer sits,it could be the transformer has shorted turns but more likely to be overrun by leaking capacitors within the set. The waxy type of capacitors here are a problem because they tend to leak badly,and are much more likely to be the source of the waxy blobs within the cabinet. They can have quite large voltages on them of say 250v or so and if one leaks it can act as a resistor and cause a voltage drop and heat up causing the aforementioned melting wax. My advice is change all these even if they look ok. Also CHECK TO MAKE SURE THE VOLTAGE SELECTOR PLUG(IF PRESENT) IS SET TO THE CORRECT MAINS VOLTAGE.
At this point it is prudent to get your multimeter out and check the resistance between live and neutral(remember to switch the set "on")if its a mains transformer set it should read anywhere between 20 ohms and 50 ohms give or take a bit for a healthy set. For an ac/dc set this will be higher typically around 450 ohms. Incedently with ac/dc sets remember that the valve heaters are usually all in series so that if one fails you will have an open circuit,this sometimes applies to dial bulbs as well. I am assuming now everything is in order and now its that crucial time to power up the radio.
It is best now to use a variac and bring the voltage up slowly in the case of an old set. Newer sets tend to not be problematical with the reservior capacitors but there are always exceptions to every rule. I will assume now that you dont have access to a variac and are now going to plug the set into the full mains supply. NOTE if you see a waxy capacitor or in fact any capacitor across the mains supply CHANGE IT NOW. use on rated at 1000vdc or higher.

one good reason to change the mains filter capacitor before switch on... this one was getting hot and had obviously blown.

OK plug in. Just as a precaution it is best now to place a neon screwdriver on the chassis,if it lights,the chassis is live please reverse the mains connections. The valves should obviously light up,a keen sense of smell is useful here as are your ears. After a half minute or so just check the smoothing capacitor can is not getting hot..If it is pull the plug out immediatly. If it is not you should now be hearing something from the loudspeaker,keep listening for any sizzling noises if in doubt switch off,after 5 minutes of being on and no burn ups or explosions i think you can safely assume that the set is now ok to fault find on if needed. Check with your multimeter the ht voltage on the smoothing capacitor(s)this should be anywhere between 200 and 400v depending on the radio design assuming its a mains set.