Q My friend and I picked up an ancient General Electric “Prog” line FM mobile transceiver that we are going to convert to 2 meters and use as a base rig. Although the radio apparently operates on 12 V, it is full of vacuum tubes. I assume that it must convert the 12 V to higher voltages in some fashion, but that would involve changing the 12 V dc to ac before it could be stepped up by a transformer. We don’t have a schematic diagram yet, so I am mystified about how the transceiver manages this trick. Can you enlighten us?
A The GE rigs, and quite a few other mobile radios of that era, relied on ingenious devices called vibrators. A vibrator is essentially an electromechanical switch. The switch opens and closes many times per second, making and breaking the 12-V dc line. By doing so, the vibrator creates pulsating dc with a voltage that rises and falls rapidly. This isn’t ac, but it is close enough to be fed to the primary winding of a transformer. The transformer responds to the pulsating dc just as it does with ac. As the dc voltage rises and falls, alternately expanding and collapsing electromagnetic fields form around the primary and induce higher voltages in the secondary windings, depending on the winding ratio, of course. Just pass the pulsating secondary voltage through a rectifier and filter and you have (drum roll, please) high-voltage dc for your vacuum tubes.
Vibrators tended to wear out (they also made quite a racket!), so they were designed to be easily replaceable. Look for a metal cylinder a few inches high and about an inch across. You’ll probably find that you can easily remove the vibrator from its chassis socket. The trick is finding a replacement if you need one. Fortunately, you can find so-called “solid state vibrators” (the switching is done with transistors) at Antique Electronic Supply on the Web at: http://www.tubesandmore.com/.
From QST March 2000
