How To Stop A Killer
Pictured above is a schematic of the Hallicrafters S-38C power supply as it appears in Sams Photofact. As supplied from the factory, all S-38 models came with a non-poloraized plug, meaning that you could plug the radio into any outlet, two ways. Notice above at the left, the on/off switch on the volume control, appears just below the plug and in an off setting. Suppose now that you insert the plug into the wall outlet and just happen to end up with the ground side of the outlet on the lower pin of the plug above, which would mean that the upper pin of the plug is connected to the hot wire of the outlet. Now follow that hot wire to the right, past C17, past R23 and to pin 2 of the 35Z5 rectifier. The circuit continues out pin 7 down to the other four tube filaments shown below. First into the 50L6 then the 12SG7, 12SA7, 12SQ7 and back to the chassis B-. Even though the switch is open as shown above, the full line voltage, about 120 volts, can be found all the way to the end of the above filament string. If anything such as bumping another radio or simply touching the radio in the wrong spot while in contact with another radio, an antenna or any sort of ground, the circuit would be completed and if the groung was good enough, the radio could play, even though the switch is in the off position! If you were the object completing this circuit, you either got a nasty surprise shock as I did many times, or worse, a ride in a hearse. This was the biggest hazard of all the so called AC?DC radios made in the 1950's and 1960's.
As shown above, the switch is in the ground leg of the circuit where manufacturers said it created less hum problems with the 120 volts on the switch. So our solution to the problem is in two parts. 1. Add a polarized plug so that the ground is always connected to the chassis B-, and 2. Put the switch back in the hot line so when you shut the radio off there's really no power in the circuit. On the S-38C, even the bolts in the back two feet of the case are directly connected into the hot chassis! Touch those screws and another radio at the same time and likely you won't do it a second time.
So, what needs to be done to change that switch and make things right? Click here for the next page.