Bob, Were you using the standard generic 3AG glass fuse..? Was it a fast or slow blow..? If you still have the dead fuse... who is the mfgr?
It takes quite a bit to kill the diodes or a bridge. Replacing the mentioned with a higher capacity type is probably a good idea but you should also toss the generic fuse and get a high spec quality brand proper-value replacement. Something is probably causing the crowbar to fire... the fuse doesn't open fast enough. Are both diodes shorted or just one side? cheers, s. > "Bob M." <[EMAIL PROTECTED]> wrote: > > 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 >
