It has come to my attention that Astron has a built-in design flaw that may cause problems for some repeater operators.
I discovered this when I replaced a suspect Pyramid power supply at my mountaintop 220 MHz repeater with an Astron SS-12 switching power supply. When I got back home, and in a very quiet environment, I was shocked to hear a very prominent 60 Hz hum on the 220 carrier. Since a switching power supply uses a switching frequency up in the 40 kHz range, I could not understand how there could be 60 Hz hum! The very next day, I took a known-good DuraComm switching power supply with me and returned to the repeater site and exchanged the two units. This time, I moved some distance away from the repeater building and tested with a handheld to ensure that the carrier was hum-free, and it was. I could not detect the hum on the first trip because electrical equipment next to the repeater is quite noisy. Once I got the Astron power supply on the bench, the cause of the hum was obvious: The negative output terminal was grounded internally! Although most large Astron power supplies such as the RS-20, RS-35, SS-25, and SS-30 have a black jumper wire between the negative terminal and the case, the SS-12 uses a trace on the PC board to make the connection. I then e-mailed Astron Tech Support and received a schematic of the unit, along with advice as to where the offending trace was located. A quick bit of work with a hobby knife cut the trace, and floated the negative output lead. Problem solved! Astron seems to be the only power supply brand that routinely grounds the negative lead; none of my units made by DuraComm, Samlex, Astec, or Pyramid have this connection. The hum was caused by a ground loop injecting 60 Hz into the DC source feeding the repeater. Since the radio, duplexer, and antenna feedline is always solidly grounded for surge protection, that means that the DC power source is grounded in more than one place- a very bad idea. I have modified all my Astron power supplies- both linear and switching- to remove any internal connections to ground at the negative DC output. I had a similar problem several months ago on my 6m repeater, which had a recurring problem with controller lockup. After I swapped the Astron RS-35M power supply and put in a DuraComm supply, the problems went away. As you might expect, the Astron RS-35M was causing a ground loop, but this time it didn't cause audible hum. It did, however, corrupt some of the data signals going to the controller. I strongly suggest that owners of an Astron power supply make a simple test with a V-O-M. With the output connections open and the power supply unplugged, measure the resistance between the grounding prong of the AC plug and the negative DC output terminal. If the reading is in the megohms, fine. If it is a short, you know what to do... 73, Eric Lemmon WB6FLY
