Modifying a UPS transformer for powering vintage radio equipment

January 30, 2020
Kihwal Lee, K9SUL

The nominal AC mains voltage has been rising in the US. After WW2, it was raised from 110V to 117V. Then again in 1969, it became 120V. Some countries went from 110V to 220V, but that's unlikely to happen here. This still is a nominal value which allows +/- 5% or error. So some places are still getting 117V. In my QTH, it ranges from 123V to 124V, depending on the time of day.

Running a radio designed for 117V AC off of 124V can put stress on the equipment. E.g. the heater voltages can go up to 6.8V, which is out of the recommended range of 6.0V - 6.6V. The power transformers can saturate and get extremely hot. Prolonged exposure to such conditions can make the old insulation to fail. The high internal voltages can also stress voltage divider networks, generating more heat, change the component values and then cause other failures.

A friend of mine gave me a big heavy transformer he salvaged from a broken UPS. The output was 15V, which was too high to be used as a bucking transformer for dropping 6V. I've decided to partially unwind the secondary to lower the voltage. was a bifilar winding with two gauge 12 wires. That's good for at least 20A.

I left the core as is and only cut out the cover.

It was revealed that the secondary was a bifilar winding with two gauge 12 wires. That's good for at least 20A. It had three layers, so I figured winwinding two layers will get it down to about where I want it to be. It was measured to be 11V after one layer was removed. So I knew I was on track.

When there were one layer plus two turns left, it was measured to be 5.95V. I stopped there and connected it to the primary internally.

Put the cover back and wired up. AC input was 123.5V.

The output was measured to be 117.6V. Perfect!

It is safe to assume that the secondary can do 6V 20A. Since the overall load is down to almost 1/3, the primary is not going to be overloaded or the core be saturated, even if the 20A figure is a bit of overestimation. So this bucking transformer will be able to handle about 2,400kVA. Of course, we will never push it that hard. A regular power outlet is rated for 15A and the circuit breaker is at 20A.