Watts on the bench #9 – VK Powermaster upgrade

VK Powermaster with ammeter and Anderson connector
Cutting the hole for the meter

I built my VK Powermaster back in 1985 and, apart from a blown rectifier a couple of years ago, it has served me well. powering a variety of radios.

It’s an old style linear DC regulator. No fancy switch mode stuff in this baby. Just a big transformer, four 2N3055 pass transistors and a handful of other components to tie it all together.

Since I made it I have wanted to add a voltage meter and a current meter to it to monitor what was going on. Today I did half of that.

I added a current meter and, since I already had the cover off, I also added a high current Anderson connector. I have similar connectors on my portable batteries and one in the car.

The whole task took about 6 hours from start to finish including cutting the hole for the meter and modifying the circuit board to connect the meter between the regulator and the voltage sensor circuit to nullify any voltage drop across the meter.

New connections inside

I needed to remove the transformer to get access to the nuts on the back of the meter and I found that the transformer was only secured with two bolts instead of four so I fixed that too.

Once it was all back together I keyed up the FT757GX at 100 Watts into my dummy load and measured the output voltage. It dropped from 13.8 to 13.6 volts when delivering 20 Amps. That’s better than 1.5 % regulation and I’m happy with that.

I don’t know if I will ever add a volt meter to it. It’s taken 25 years to get an ammeter in it.


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