It was a standard 8A generic 3AG glass fuse. It didn't blow explosively; it just got soft and opened. When I put a new fuse in, it blew instantly with a burst of molten metal coating the insides of the glass.
Fuse quality should not be of concern at this point. The fuse shouldn't blow with about 400 watts of load on the supply's output. Also, the crowbar firing shouldn't blow the fuse that quickly, unless almost everything after the diodes has shorted out. If one diode shorts, it'll likely do damage to the other one on the next half cycle. It blew out with the 25A load. The only way to tell for sure is to unwire the two diode bridges and measure them with a meter. But since this is exactly the same failure my first one had, I'm pretty sure it failed in the same way. I'm not pulling the diodes until Astron does something about the "under warranty" condition. I did disconnect the positive output wire at the capacitor and when I measure across the cap and circuit board, I get a rising resistance of several hundred ohms. When I measure across the red and black leads (disconnected from the cap and regulator board) I get 0.1 ohms in both directions. I would expect to see some resistance from the diodes in at least one direction, but no soap. Someone else mentioned that 25A is the limit for continuous use. Yup, I'm not disagreeing. However it should be capable of that load for hours at a time. Bob M. ====== --- skipp025 <[EMAIL PROTECTED]> wrote: > 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. ____________________________________________________________________________________ Sucker-punch spam with award-winning protection. Try the free Yahoo! Mail Beta. http://advision.webevents.yahoo.com/mailbeta/features_spam.html
