When your antique radio hums, it's never a happy tune. Speaker hum is almost always the result of electrolytic capacitors gone bad---one of the most common problems in old tube radios. Because capacitors (also called condensers) are inexpensive and have a short shelf life, it's simpler to replace them all---a process old-time repairmen call "recapping" a radio---than to test and replace individual capacitors. The most practical way to do this is to mount replacements under the chassis. In no time, you'll enjoy the rich bass tones of your old radio without the monotonous 120-hertz hum.
- Skill level:
Things you need
- 1,000-ohm resistor
- Schematic diagram for your radio
- Soldering iron
- Wire cutters
- Needle-nosed pliers
- Lineman's pliers
- Replacement condensers
Unplug the radio. Pull off the knobs, use a screwdriver to remove the screws that secure the chassis to the cabinet and slide out the chassis.
Turn the chassis on its back to expose its underside. Determine whether the capacitors are housed in an aluminium (or sometimes cardboard) canister above the chassis or mounted beneath the chassis; capacitors under the chassis look waxy and resemble a roll of coins.
Discharge the capacitor using the resistor. For underside capacitors, touch the resistor leads to the capacitor tails. For capacitors in an above-chassis can, hold one resistor lead against the chassis and touch the other lead to each tab that connects the canister to the chassis.
Determine each capacitor's voltage and capacitance values, printed on the sides of under-the-chassis capacitors or on the above-chassis can. You'll need a schematic diagram if you cannot read the values.
Disable an above-chassis canister by touching a hot soldering iron to its chassis tabs and desoldering the connections. Leave the can in place to maintain the original appearance.
Remove under-the-chassis capacitors one at a time so you do not forget which replacement goes where. Using wire cutters, snip the tails extending from one capacitor, leaving a half-inch tail attached to each of its chassis terminals. Touch a hot soldering iron to each terminal to loosen the solder, then use the needle-nosed pliers to pull the half-inch tails off the terminals.
Use wire cutters to trim the tails of the replacement capacitor to a length slightly longer than needed to span the two terminals. Place the ends of each lead through the terminals, crimp in place with the lineman's pliers, and solder the connections. Repeat steps 2 and 3 for each capacitor under the chassis.
Replace above-chassis capacitors with new condensers mounted under the chassis. Loop replacement capacitors' (+) tails through the holes in the chassis tabs that correspond with each above-chassis capacitor being replaced, then connect the replacements' (--) tails to the common ground terminal. Use the lineman's pliers to crimp all connections in place, then solder them securely.
Place the knobs back on the shafts, plug in the chassis, and turn on the radio. If it plays correctly, unplug the radio, put the chassis back into its cabinet and secure it. If it still hums, check that the polarity and values of replacement capacitors are correct.
Tips and warnings
- Replace a capacitor with another of the same voltage rating.
- Replace a capacitor with another with equal or higher microfarad (mfd or µF) values. It is acceptable to replace, for instance, a capacitor that has a resistance value of 30 mfd with one marked 35 mfd. Using a lower replacement value will make your radio hum.
- Above-chassis canisters are marked with shapes to identify the capacitors inside. The shapes on the canister's chassis tabs are the (+) terminals for each corresponding capacitor.
- Always unplug a radio before working on it. You can be electrocuted if you touch a "hot" AC/DC radio chassis.
- Capacitors store enough voltage to give you a nasty or even fatal shock, even with the radio unplugged. Always discharge a capacitor before replacing it.
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