Some of the regular readers may recall I had an RS35M (bought new in 2002) blow its diodes in July 2006. These were two 35A, 50V bridges made by Diotec. Astron only uses the positive half and wires them in parallel. I replaced these with Vishay GBPC35 rectifiers, tested the supply, and put it aside.
A friend loaned me his old RS35M which got the repeater back on the air in 90 minutes. I bought a brand new RS35M supply in July 2006 and finally got around to installing it in October 2006. The other day the same thing happened. The repeater had been quiet all morning, someone called me on it, and mid-way through the CW ID (after transmitting for about 10 seconds), it just went off the air. I grabbed the repaired RS35M, went to the site, and put it into the repeater, pulling the other one back down to my shack. The repeater was back on the air after 3 hours (hey, it was cold outside and I didn't feel like driving up there right away). Back on the bench, I tried a new 8A fuse; it blew immediately. I measured the resistance across the diodes; I read 0 ohms, but without disconnecting them from the transformer, this is not an accurate reading. I'd surmise that one or both bridges have a shorted diode in them. Seems like the same failure as the 2002 supply had. This one, however, is still under warranty. I'm not sure it will be economically feasible to ship it to Astron where they'll put in exactly the same diodes, in the same configuration, where they'll just blow again. Until I hear back from them, I'm not going in there to disconnect the diodes to actually measure them. I've purchased some new 50A 1000V bridges which I will put into these supplies from now on. No more "diodes in parallel". Incase you ask, the load on the supply is 25 amps at 14.0 volts, it has plenty of forced air cooling on it, the environment is 70F, and the repeater is low usage: a couple of hours per day with a sustained usage at drive time of about 45 minutes on a busy day. I was taught that running semiconductors (diodes and transistors) in parallel without some kind of load balancing components is a bad thing. I'd rather have a single pair of diodes in these supplies instead of the pairs of bridges. Anyone else care to comment pro or con? Needless-to-say, my next power supply will be a different brand. Bob M. ____________________________________________________________________________________ Do you Yahoo!? Everyone is raving about the all-new Yahoo! Mail beta. http://new.mail.yahoo.com
