- the radio turns on
- the radio makes noise, and gets louder with the volume knob
- but turning the tuning knob does not move the needle or change the station
Step 1: Disassembly 1: Remove Knobs
We need to get the radio open.
On these old radios, the last thing to go on was the knobs.
Therefore, the first step for us is to remove the knobs.
These knobs pull straight off; they aren't glued or screwed on.
Pictured: the offending tuner knob. The inner knob is the AM/FM selector. The translucent ring is the tuner. You can see how it's toothed on the inside.
Step 2: Disassembly 2: Open Case
An old radio is like a car - it was hand-assembled, and requires the proper tools. In my case I needed 5/16" and 1/4" socket wrenches.
The back panel is wood and is screwed to the case with wood screws.
Step 3: Disassembly 3: Workings
The back panel on my radio pulls out; it has the antenna embedded in it, and attaches to the rest of the workings with a plug embedded in the radio chassis.
Wood tube radios will have a metal case inside protecting all the electronics. This is because the tubes run real hot and at high power, the metal case is to physically protect the fragile tubes from any jostling, and also to insulate the flammable wood from scorching from the heat of the tubes.
This metal chassis is bolted to the wood body in some way - on mine it used wood screws on the bottom. Remove what you have to and pull the metal chassis and works out of the radio body.
Step 4: About the Tuner 1: Capacitor
A quick note: the tuner of the radio is a capacitor. On old radios this capacitor is a big metal thing that looks like a radiator.
Capacitors work by holding an electrical field between parallel plates, the more plate area, the more capacitance. This type of old, tunable capacitor is called a "variable air condenser" because it's capacitance is variable, and the insulator in between the plates is just air.
It's a big stack of metal plates in a half circle, which rotates into another stack of metal plates. The more intersection between the two half circles, the greater the area they have parallel, and therefore the more capacitance.
Because of the many plates, even a little difference in area, a little turn, equals a big difference in capacitance.
Step 5: About the Tuner 2: the Pulley
I mention this big difference in capacitance because this presents the radio designer with a problem:
- the user needs precision and control to distinguish the tiny difference in frequency between stations - in other words, the entire radio dial should be covered by many turns of the tuner knob
- the capacitor can only turn less than 180 degrees to cover this entire range - a half turn
Thus there is a kind of gearing between the tuner knob and the tuning capacitor.
On older radios this is done mechanically with a pulley and string.
Step 6: About the Tuner 3: the String
We've established the pulley is attached directly to the tuning capacitor.
There are actually two strings on this same pulley:
- the string that moves the tuning indicator needle up and down
- the string that connects to the tuning knob
The string was originally mass-produced for this exact purpose. It doesn't stretch, and it's slightly tacky to the touch. This is important because it cannot slip across the tuner knob barrel when it's being turned.
As you can see in the picture the ends of the string have a convenient metal eyelet on them. Which is convenient if you happen to have a string that is exactly the right length... I unfortunately do not!
Step 7: Failed String Idea 1: Twine
My initial plan was to replace the tuner string with something I had around the house -
I tried using a cotton string, a twine, but neither gripped the tuner knob barrel.
I noticed the "real" string was slightly tacky, so in an attempt to make the string stickier, I waxed it with a tea light candle. This still didn't work.
I also tried some craft string used for jewelry. Nope!
Step 8: Failed String Idea 2: Rubber Band
Since the string wasn't tacky enough, I decided to use a rubber band. This already wasn't a good idea, because while the original string had lasted over 50 years, a rubber band will degrade over less than 5 years.
It turned out to be more of a bad idea than that - the rubber band did stick to the tuner barrel, but it was too stretchy. The knob would turn but the rubber band would just stretch and shrink rather than move the pulley attached to the condenser.
Step 9: Finally: Use the Real Thing
- black, non-stretch
- fiber glass core
- 0.028" diameter
- comes on a plastic spool
- modestly priced!
I bought this from Bob's Antique Radios and Electronics, and Bob, if that's who I spoke to, is pretty awesome. Which is fortunate, because as far as I can tell, he's the only person left alive who sells this stuff.
As awesome as the vintage radio string is, it still wasn't perfect - it was still a bit slippy. So I ended up spraying "plasti dip" on it, which is a rubber coating in a can, intended for use on metal tools to give them a nice grippable layer of rubber. It also comes in a dip version (liquid without the spray).
Alternately, I've heard some people use rubber cement.
The string doesn't have to be sticky, just not slippery on the wood tuning knob barrel.
Step 10: Replace the String - Tension
The springs keep the string taut against the pulley and the tuning knob barrel - without them, the string would be too slack to move with the barrel.
I had to experiment with tensions, but it ended up that the ends of the strings had to be just inside the pulley unstretched.
- tie knots in the ends of the string (bonus points: get that nice little crimped metal eyelet...)
- thread the string along the exact path it was before it broke. Pay attention to any crossings between pulley and knob!
- pass spring end through string knots
- using pliers PULL the other end to reach the point on the pulley that the spring mounts to
Step 11: Test Tension and Finish
- get the needle to one end of the radio dial by moving the pulley
- turn the tuning barrel with your fingers and make sure the pulley is moving with the string, with the barrel
- Only using the tuning barrel, get the needle all the way to one end of the radio dial
- Only using the tuning barrel, get the needle all the way to the other end of the radio dial
When you've got it, put a bit of glue on your knots to make sure they don't come undone.
Lastly, cut any excess string so it doesn't get caught on anything. Incidentally, this is where you could also adjust the tuning needle if it doesn't match the dial markings. As I mentioned earlier, it is connected to the other string on the pulley.
That's it! Reassemble your radio!