Haste Makes Waste or
What is That Awful Smell?

by John J. Nagle, K4KJ
From OTB Vol. 24, No. 1 (June, 1983)

At Canandaigua this past year, you found the receiver you had been looking for ever since you started collecting - and at a price you could afford. You've set it on your work bench to take a good long look at it; it's actually in better shape than you thought it was . . . a real bargain.

But the big question is, Will it work? You can't resist the temptation to plug it in the wall and turn on the power switch and . . .

A loud sizzling noise comes out of the receiver, the lights in the workshop grow dim. Faster than you can reach to pull the plug out, the fuse blows and smoke comes pouring out of the power supply section followed by that awful smell of a burned out power transformer.

To add insult to injury, the XYL calls out from the kitchen, "Is everything all right, dear? The lights just went out!" No, everything is NOT all right. The set doesn't work now! The sad part is that it might have worked if you had taken a few precautions before plugging it in the wall.

There are three principal villains that kill old receivers: deteriorated insulation, moisture and high line voltage.

Remember that receivers of interest to antique radio collectors today are as much as 50 years old. Insulating compounds were not as good in the '30s as they are in the '80s, so some deterioration is inevitable. Couple this with the fact that the receiver of your dreams was probably stored in someone's attic or basement for 25 of the last 50 years which means that it is likely to have been exposed to periods of high humidity. And as this moisture has worked its way into the insulation, it has further weakened the insulation.

Compounding the problem is the fact that the average line voltage has gradually crept up over the years. In the early 1930s, typical line voltages were on the order of 110-113 volts By the late 1930s, 115-117 volts was generally found; this increased to about 117-120 in the early 1950s. My line voltage runs about 125 today. Thus weakened insulation is stressed by a higher voltage than it was originally designed to withstand: the result is trouble, if you are not careful.

The following ideas are suggested to help keep that old set playing as well as it did when it came out of the factory.

First, as a matter of safety, make sure you have a good, solid power ground (not RF ground) tied to the receiver before you plug it in. The cold water pipe or the common terminal in your distribution box is a good choice. Run AWG 10 flexible wire. The object is to have a much lower metallic resistance to ground than you are; this is especially important if you are working on a headphone set.

The next problem is to drive moisture out of the power transformer. The traditional method for doing this is to bake the transformer in the oven. This, however, involves disturbing the original wiring and solder joints, something a collector hates to do. There is another way of accomplishing the same thing: bake the transformer in place.

To do this, first put a good solid short-circuit on all secondary windings. The high voltage and rectifier heater windings can be shorted at the rectifier tube socket. The filament winding should be shorted as electrically close to the power transformer as practical. Next place a light bulb whose wattage is approximately equal to that of the receiver in series with the primary and apply line voltage. Make certain the 120 volt wiring of the receiver is in good condition before you do, of course.

The object of this is to cause rated current to flow in the windings without stressing the insulation. In this way the power dissipated in the windings will heat the transformer from the inside with a maximum voltage stress of only about 20 volts across the insulation on the 250 volt high voltage winding.After a few hours, the transformer should be warm to touch. The wattage of the light bulb can then be increased to about twice the rated wattage of the receiver. The transformer should be allowed to become comfortably warm to touch but not uncomfortably hot.

Assuming that the capacitors check out OK, look at the rectifier tube, probably a type 80. If it has an old-style envelope, replace it with a new-style tube. No sense in endangering an old tube if something breaks down.

Next remove the short circuits and, using an adjustable transformer, increase the primary line voltage from zero in 10-volt steps. Wait about 5 minutes at each step; stop at about 110 volts for early 1930s receivers. Hopefully, you will have a working receiver.

If you don't want to tie up an adjustable transformer for normal operation, use a 12-volt filament transformer instead. Connect the 12-volt secondary in series aiding with the primary. The 125 volt line is wired across this series combination. Approximately 113 volts will appear across the original transformer primary winding. The power handling ability of this combination is about 4 to 5 times the volt-ampere rating of the filament transformer.

One final comment about operating a 1930s receiver; most antenna systems today are designed for 50 ohm coax. Yesterday's receivers have a 300-500 ohm input impedance so that an impedance matching device may be helpful in improving sensitivity.

 BACK TO ARCHIVES