Introduction: Restore a Classic Table Radio
I bought this nice little General Electric Moderne style radio at a swap meet. It was in excellent condition with no discoloration of the cabinet or scrapes. The knobs were in almost perfect condition also. It didn't work, so this Instructable mainly focuses on the electrical aspects of the restoration. All that needed to be done to the cabinet was to be cleaned with dish soap and water. A little scrubbing with an old toothbrush in the crevices and it was perfect.
Step 1: Disassembly
Remove the four screws on the back with a flat blade screwdriver. The small knob comes off with gentle prying. The large knob has an Allen screw which comes off with a 1.5 mm Allen wrench. After these have been removed, the chassis of the radio comes out of the plastic cabinet easily. Note that the purple fiberboard back is attached to the metal chassis so be careful when removing it with the chassis. Put the screws back in the holes and put the knobs back on the radio so that you do not lose them.
Step 2: Visual Inspection
This radio is what is called the "All American Five" because it was a 5 tube standard design used by many manufacturers from 1940 to the early 60's. This design eliminated the expensive and heavy transformer the used to supply the B+ (high voltage) and the 6 volt filament currents. This design featured all the tubes wired in series with three twelve volt tubes, one 50 volt and one 35 volt tube. This arrangement added up to 121 volts or the approximate line voltage. This radio was remarkably clean for it's age and was nice to work on. Note in the chart under the schematic, it tells you what voltages you would expect to see on each terminal relative to ground when the radio is on. This is a great troubleshooting aid which saves a lot of time. This is also a good time to visually inspect the line cord for fraying.
Step 3: Troubleshooting 1
The first thing I did before plugging the radio in was to check for a short across the plug. I turned the switch on and measured the resistance with an ohmmeter across the plug. It was 119 ohms which tells me it is OK to plug in. A short would of been under 1 ohm. It also tells me that the filaments are OK. Time to plug the radio in and see if the filaments light up.
I plugged in the radio and turned it on. After waiting a couple of minutes, I didn't see the filaments lighting up. Since I know that I have continuity through them, I look for cold solder joints. The most obvious place is where the new line cord was soldered. I can see that the solder joints are kind of white looking. A dead give away of a cold solder joint. I will re-solder both places and plug the radio in again.
I re-soldered the joints with a soldering gun and new solder and plugged it in. The filaments lit up but there was no sound.
I then wanted to verify if there is B+ voltage. With the radio on I checked the voltages from ground to the positive terminals of each filter capacitor. The filter capacitors are the large blue ones and you take the positive voltage from the terminal that is opposite the one that the arrow points to.
Step 4: Troubleshooting 2
As can be seen in the picture the filaments are lit but there is no sound. In a situation like this you can start from either end. I decided to start from the output and work my way back to determine what stage was bad. I assumed that the speaker and output transformer were good as they almost never go bad. Tubes, resistors and capacitors fail more often. If a tube has filaments that are not open, they usually either get weak or short between the electrodes. With the radio on, I touched a screwdriver to the input of the output tube (pin 5) and there was no hum. If this stage was working there would be a loud hum. Time to check the voltages on the output tube. I checked all the voltages to ground on the pins and the one that was way off was pin 8. It should measure 5.6 volts. I measured 40 volts, indicating that I should check the resistance of the Cathode resistor. I turned the radio off and what should of been 150 ohms was in the thousands of ohms. I didn't have a resistor of this value in that wattage so I replaced it with a 220 Ohm resistor. After replacing it, I turned the radio on and rechecked the voltage, it was close to the 5.6 volts. I turned it on and it worked great.
Step 5: Reassembly
Put the radio back together in reverse order and enjoy the rich sound of an old vacuum tube radio.