Update: Testing Voltages and Frequencies


Here’s a quick update on the restoration of the 1940 Hallicrafters radio.  Last weekend I replaced the 4 remaining original high-voltage capacitors, the last of the original components to replace.  I was getting close to the initial power-up testing.  Since these radios use high voltages and a lot of crisscrossing bare wires, I carefully inspected the components and all connections with my magnifying visor to ensure that there were no crossed connections that could create a short circuit.

When restoring old electronics, it’s best to gradually increase voltage instead of simply switching it on.  If a component is connected improperly, or if a short circuit exists, it could overhead and create a fire quickly.  If an electrolytic capacitor is connected backward, it can easily explode.  To avoid these problems, professional restorers use a device called a Variac, or variable transformer, to gradually turn up voltage.  Since this may be my only electronics restoration project, I didn’t want to invest in a Variac, so I built the “poor-man’s Variac” – a Dim Bulb Tester.

A Dim Bulb Tester simply places an incandescent bulb in the path of the circuit.  This reduces the current to the device.  If a short circuit exists in the radio, the bulb would burn brightly, and the tester could be quickly switched off before damage occurs.  Testing begins with low wattage light bulbs, gradually increasing the wattage of the bulb for the testing until the radio is at full voltage.  You can see my Dim Bulb Tester in the picture below, to my right.

So far, no explosions or fires.  I’m still in the process of validating the various output voltages from the transformer.  Tomorrow I should be ready to place the first tube into the radio, called a rectifier tube.  This tube converts the AC to DC.  If the DC voltage from this tube is within spec, more tubes will be added.  I’m still likely a few weeks from connecting an antenna wire to test for sound, but I’m in no hurry.