Drake step-start for AC4 power supplies

Extend the life of your Drake gear

What's a step-start?

The concept is quite simple. A step-start circuit inserts a low value resistor in series with the main supply. The resistor limits the incoming current for a few seconds while the power supply capacitors charge. This limits the instantaneous current the rectifier diodes (or rectifier tube) must withstand for the first several cycles while the capacitors take on their charge As a side benefit, the power switch doesn't have to switch full current at turn on. When the circuit times out, a power relay shorts the resistor and full main voltage is applied to the power supply's transformer.

If they have lasted this long why bother?

That's a valid point, and I have had several conversations with other like thinking hams. They argue if they supply hasn't been damaged in fifty years, why fix what isn't broke?

That's true, but with today's higher main voltage, mine sits at 127 V ac most of the time, the step-start is cheap insurance. Damaged to those old power switches are another factor to consider. Although that switch might have been fine all these years until one day it fries. Try and find a replacement power switch for a Heathkit SB-102.

How it works

Every step-start circuit relies on some way of delaying the application of full main voltage to the power supply's transformer. I had thought of using a timer IC such as the ever popular 555, but decided it was simply way to Rube Goldberg. So I too opted for a simple RC timer.

Some step-start circuits depend on a large, usually several thousand of microfarad, capacitor across the relay's coil. The relay is held off until the capacitor is charged. Once the capacitor is charged, the relay pulls in. Depending on the voltage charging the capacitor, it's possible the delay time could be shortened to the point there is very little, if any, delay. There are too many variables that make this design iffy at best.
The step-start comes fully assembled and tested at $35.00

Will work with any supply!

Although designed to fit inside the Drake AC4, this step-start will be at home in all kinds of power supplies from high voltage like the Heathkit HP23 to an Astron low voltage supply.
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