The Astron RS-50A unit I repaired that came from a set of 3 that was used to power a church organ.

When I originally got this beast, I didn’t have an HF rig or real interest in HF. At some point later in 2020 I got interested and got the Kenwood TS-850S, and I found out what inrush current was due to inconsistently tripping the downstairs breaker (depended on WHERE on the AC cycle I powered on the power supply is my best guess). I learned about larger capacitors like the ones in these units, linear transformers, and started diving into schematics and circuit design.

The circuit this power supply was in was, unfortunately, the only one I had access to for this purpose so I had to make it work. I learned of two methods to make this work: a soft start circuit, or a inrush current limiter. I asked in the local club email list and there was a lot of push for the soft start. So, I went to do my own research and see what I came up with.

You see, I decided to research both options and I contacted Ametherm about my dilemma and some questions about their offerings and that I was looking at a couple of options based on the excellent article on Repeater-Builder by WA1MIK. I wasn’t expecting much, but instead I got a detailed reply with calculations based on the power supply and recommended a specific part number for my purpose: Ametherm MS32-10015. They were even so kind as to include the formula they used to calculate the values needed to select the proper part!

I ordered it and installed it directly at the tap of the transformer by breaking the connection between the hot cable from the power switch, creating a physical hook for the new thermistor using its legs and soldered it liberally between the two things I disconnected. I am happy to report that the issue with inrush current has been completely mitigated and is no longer an issue even after moving out of that place I lived in at the time.

The email is referenced in the Repeater Builder article.