If you don't know anything about electronics, don't do anything. Give it to someone who knows what he's doing. Old radios (and televisions) can easily develop internal short-circuits that may damage the set; it could also be a shock hazard, and being shocked by these things is no fun at all.
If you do know basic electronics, even if you don't know anything about tubes, and you know how to work safely, then there are some things you can do to test your radio.
The main thing you're worried about on old radios is that there's an internal short, which will draw excessive current and will burn something up. Tubes are replacable (many are cheap, some are expensive, but any tube you're likely to encounter is still available). Resistors and capacitors are replacable. But some components, like transformers and coils, are much tougher to find. Anything you can do to avoid burning out components is worthwhile. Short circuits are also shock-hazards, which is an important safety issue.
The first thing to do is inspect the condition of the radio. Look to see if anything is missing. If tubes are missing, they will have to be replaced. Some types of radio will work (though not properly) without all of the tubes Some types of radio will only work if all their tubes are present and functional. Also look at the wires—look for broken wires, bare spots in the insulations. These should be fixed before you apply power.
Look for broken connections and things that are floating free that were probably anchored somewhere. These will need to be fixed. Look for scorch marks, burns, and other signs that of electrical arcs, fire, or excessive heat. If a resistor or capacitor looks like it's been burned or scorched, it ought to be replaced.
Blow out as much dust and loose debris as you can without damaging anything (e.g. breaking a fine wire).
If you believe the radio is complete and ready for power, there a few ways to go.
If you have a variable AC power supply (typically called a variac), use this to power your set. Start with the power set relatively low (maybe 80V), turn it on, and wait for the set to power up. Tubes take some time to warm up, even at low voltage. You probably won't hear anything, but you want to be alert for electrical arcs and smoke. If you see or smell smoke, or you can hear it arcing somewhere, turn it off immediately. If not, after a minute or so, bring the voltage up to 90V. Wait a minute or so. Then up to 100V, and so on up to around 115V. Consider that operating voltage.
If you don't have a variac, there are a couple of different ways to go. The most common way is to make up a light-bulb tester. Steve Johnson at Radiolaguy has a good description on how to do it. So does Phil's Old Radios. If you use this method, be sure you use the type of light bulb that uses the full rated wattage on it. A lot of new CFL and LED bulbs are rated at, say, 60W equivalent, but in reality they pull far less and this method won't work properly.
Another test, though not as good as the former methods: if your radio has a power transformer, identify and remove the power rectifier tube. If you aren't sure which one it is, you should get the schematic diagram for the radio (it's a good idea to have it anyway) and identify it from that. Typical rectifiers tubes are # 80, 5Y3, and other types with the letter W, X, Y or Z in the middle. Whatever it is, make sure you pull the power rectifier tube out and set it aside.
Now you can power the set. The rectifier tube provides high voltage to the set, so without it, the only power the set gets is the tube filament voltage--typically 6.3V or possibly 12.6V. The radio definitely won't play, but the remaining tubes should light and warm up. Again, you're looking for smoke or other signs of electrical shorts.
If your set passes this test, you can try it again with the rectifier tube back in the circuit and try again with regular line power. This time the radio may play, but again your first concern is evidence of short circuits.
Once you've determined the radio isn't pulling excessive current and won't burn something out, then you can see if it'll play. You need to give the tubes time to warm up. The the audio volume all the way up. If you hear a steady buzzing or humming noise, that's called AC hum. AC hum is constant and will NOT get louder or softer depending on the volume control. I faint AC hum is accepable, but ideally you want none. Tubes do not like AC hum, so if the hum becomes noticable under ordinary use, you need to stop or you'll shorten the life of your tubes. A lot of hum is bad no matter what.
If you have AC hum, you will need to have the filter capacitors replaced. Actual repair is beyond the scope of this FAQ. But AC hum is a very common malady in antique radios. Filter capacitors, when they're new, are designed to last 20, 30 years, maybe more. So when these radios were new, AC hum wasn't a common problem. But now these radios are old and they have geriatric problems, and AC hum is a common problem.
Please provide as much information as possible. Look the radio over for anything that may give a manufacturer's name, because just narrowing down to the proper make helps a lot. Many manufacturers pasted paper labels on the bottom of the cabinet or inside, and that should tell the make/model.
Sometimes there's a diagram with a tube listing: that helps. RadioMuseum has a searchable database, if you list the tubes used, it will tell you which radios had that tube compliment. Other websites on the internet may do the same.
The main way a lot of us identify radios, however, is by brute-force internet image searching. If you're looking for a GE clock radio, do an internet image search on "GE clock radio" and start scrolling through photos. It's tedious and time consuming and it is not guaranteed, but it can and does work.
That depends on your own personal goals and value of "worth." If you want to repair or restore a radio and sell it for a profit, then it depends on how good a job you do and how patient you are in finding a buyer for it. The latter is usually harder than the former, unless you have a good conduit for likely buyers. Otherwise, it can be difficult to sell old radios for much money. Many people will admire an antique radio but if you ask them if they want to buy it, they'll say no. They have no place to put it, it doesn't fit their decor, or they don't want to pay a lot of money for what is likely just another piece of bric-a-braq on a bookshelf or tabletop. And most people aren't going to go to the trouble of rigging their XM oldies channel into the radio so they can hear Vaughn Monroe in glorious AM lo-fi. It's a novelty that's fun for about 10 minutes.
If you want to repair/restore it for yourself, then it's a matter of how much time, effort and money you're willing to spend on it. One problem with old radios (and televisions) is that you almost never know at the beginning what the problems are. Sometimes the radio can be fixed with a new filter capacitor and it plays great. Some will need new capacitors, some new resistors, new wires, new tubes, a new speaker cone—and then you find out that an IF transformer is broken and needs to be replaced (which is a harder-to-find component). Or worse, the cabinet is fine but you drop and break the dial glass, or the family cat knocks the cabinet off the bench and breaks it.
You often don't know, going in, what you'll end up when you're done. Some very modest looking radios from 3rd tier manufacturers turn out to be fantastic players; some beautiful looking high-end radios will end just doing a just-adequate job no matter what you do. Is this particular radio worth repairing/restoring? My answer is that unless it looks it was in a fire or a flood or is missing a lot of parts, I would say yes, it's worth an attempt to repair or restore it. Otherwise it should become someone else's project or become a parts donor.
The number 1 source, by far, is Nostalgia Air for commercial radios. For boat anchors (i.e. Hallicrafters, Hammarlund, and other big radio receivers), try BAMA, the Boat Anchor Manual Archive. If you're willing to purchase an account, Radiomuseum has schematics for many radios as well.
An internet search will turn up more, but this will get you started. I've purchased from each of these vendors and have been happy with the transaction.
There are a number of sellers on the internet who sell tubes. While some very popular types (e.g. 12AX7) were recently manufactured, the majority of tubes are either NOS (new old stock, meaning they are old but have never been used) or used. Prices for tubes vary widely depending on the seller, the relative scarcity of the tube, whether it is new, NOS or used, shipping costs, and guarantees. It is highly recommended that you purchase tubes from a seller who will guarantee that the tube works and will replace or refund bad ones.
A few on-line sellers listed alphabetically. There are many more but this should get you started:
The old tried-and-true method is tube-replacement; swap out the old tube with a new one and see if it does what you expect.
Most tests require a tube tester, but you can test a tube's filament with just an ohmmeter. It's a simple continuity test: find out which two pins are your tube's filament (or heater) pins, set your meter for ohms, and check continuity across them. If you get continuity, the filament is intact (at least it is when it's cold. Sometimes a filament will break when it heats up, and then make contact again when it's cold. It's unusual but it does happen enough to mention here). If you don't get continuity, then your filament is blown.
There are a large number of full-line electronics retailers, but these appear to be the most popular:
In addition, a few retailers sell common parts that are more directed for people who repair radios:
There are a number of good books which deal with learning radio repair. They all have their advantages and drawbacks, so a lot of it is finding the book that you personally find most readable.
Modern books (written in the last 30 years or so) have the advantage of knowing that tube radios are obsolete, and a lot of things the "old" books take for granted (like being able to buy a new tube at your local drug store) is no longer the case. The drawback is that they don't go into the wealth of detail that the old books often do. Modern books are all beginner's books. Older books are usually more intermediate or advanced.
Modern books I recommend:
Old books I recommend:
In the days before printed-circuit boards, a typical radio had a chassis made of a sheet-metal box. Holes were drilled or punched into the panels to mount tube sockets, tuner capacitors, IF cans, switches, and other components. Inside the box, all the wiring was done point-to-point. That means that a capacitor connected Pin 3 of a tube to the input lug of an IF transformer, then the capacitor was (usually) physically soldered to Pin 3 and the input lug on the IF transformer.
Most radios have a lot of components that all connect to electrical ground. You can solder all these to various places and run wires to connect them together (called a bus), but the easiest way by far is to make the sheet-metal box "ground" and connect anything that needs to be grounded to the box. The box is called a chassis, so this is called chassis-ground. A very large number of radios from the pre-PC board era are chassis-ground.
The problem is how the radio gets AC power. On a battery (DC) set the batteries are connected one way and that's it. But an AC set puts the "hot" wire at the head of the tube filament string, and typically put the "neutral" wire on chassis ground. If your AC cord is plugged in so that the neutral wire is physically connected to chassis ground—fine and dandy. But if you flip the plug 180°, then the chassis becomes "hot." If you touch the chassis in this state, you become a path between it and ground, and you can get shocked.
A hot chassis will still work properly, so you won't know it's hot unless you get shocked or you test it with a volt meter. Since metal-chassis radios are typically mounted into wood or plastic cabinets, and have wood or plastic knobs, the user wouldn't be exposed to a shock hazard unless one of the knobs comes off and he touches the bare metal shaft; or gets into the back of the set and touches something metal like the chassis. Since it's fairly easy to do either or both of these things, it's a good idea to prevent a hot chassis from occuring at all.
The fix for this is a polarized plug, which can only go into the socket one way. You physically solder the AC cable into your radio so that the neutral line is on chassis ground, and there's no easy way to plug the set in incorrectly and get a hot chassis. So yes, it's a good idea to use a polarized plug on these kinds of sets.
The All-American Five is a modern name for radio with no power transformer and five American-made tubes. It's a relatively simple, low-cost circuit that became extremely popular for low-cost portable and small table-radios. Their heyday was post World War II until solid-state replaced the tubes.
When they were being manufactured, the common name for this kind of set was transformerless or AC/DC sets. There are a lot of variations in the AA5: you can take the same basic set and add a sixth tube up front as an RF amplifier, and you get a radio with better performance but only a small boost in cost. Likewise, at the beginning of the solid-state era, it was common to replace the rectifier tube with a selenium rectifier and a dropping resistor. This cut weight, space and some power consumption, but otherwise the radio was the same-old thing.
Because AA5s are relatively simple and sold in huge numbers, they make good project radios today, especially for beginners, because they have relatively few components, are easy to understand, use common, easy-to-find tubes, and replacement parts can often be found (at least on popular sets). Six and 12-volt tube varieties tend to be forgiving to work on. (One-volt tube types should be avoided by beginners because it's easy to make a mistake and blow out the filament string on all of the tubes; but otherwise the principles are all the same.)