Whow, LM723, nice! I still remember the day when 30 years ago when I've did my first linear power supply with 723. Do you have a linear or a switching supply? I can only guess is a switching one if indeed you're loading with 70A and 2N3773 were the original transistors. But if you're talking about 2N3055 as replacement for those is unlikely will switch again... So, my opinion is check what typolgy has your LM723 power supply first. best, Vasile On Wed, Jul 25, 2012 at 5:32 PM, John Ferrell wro= te: > {from the Yuasa Battery thread...} > > Great timing! > I have a 70 Amp(?), 12 volt power supply on the bench now. It uses a 723 > regulator. I don't normally volunteer for repair work but this one got > my attention. The failure mode was that a Pass transistor (2N3773) > shorted and the full bulk voltage (18v) appeared across the output. The > owner (a fellow Radio Amateur) says no damage was done to the attached > equipment but I find that hard to believe. > > Apparently, an earlier maintenance action replaced 1 of 4 2N3055's in > the Pass array with a 2N3773. The specs of the 2 power transistors are > not even close. The supply worked OK for a light load because the 2N3773 > was carrying the full load and the 2N3055's were just coasting along. > The 2N3773 failed by shorting and dumped the bulk to the output with > only the primary fuse to protect the attached equipment. Whatever the > regulator was intended to do did not happen because it was effectively > bypassed by the failure mode. > > The Base current limiter is a 2 ohm resistor which stopped the cascading > failure AFTER it shorted the driver transistor which is also a 2N3055. > After I determine this is an appropriate part for the driver and verify > the 723 assembly is working properly I can restore normal operation by > replacing the failed parts. > > Although this failure was brought on by substituting a more robust > component for the original part, it could have failed the same way with > the proper part. If you are a fix-it fellow it is not unusual to fall > into the habit of replacing failed parts with new parts with higher > ratings. Few repair operations receive (or should receive) the attention > that I have given this instance. > > Should my main power supply fail in this manner it could get very > expensive. I count at least $3000 worth of equipment that would be > placed in harm's way. > > At the moment I am thinking that an external crowbar circuit (SCR + > ZENER + RESISTOR) might be a really good idea... > >>> Looks to be an LM723 unless I'm very much mistaken.... >> Wow! >> 723! >> Haven't seen one of those mentioned for 'some while". >> The original universal regulator IC :-) >> I probably still have some in the deep dungeon. >> 69 cents/1 DIP at Digikey. >> TO100-10 still available but costs rather more. >> >> Fig 19 in data sheet bears a 'suspiciously strong' resemblance to their = fig 14. >> >> http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/= DATASHEET/CD00001008.pdf >> >> >> >> Russell > > -- > John Ferrell W8CCW > =93During times of universal deceit, > Telling the TRUTH becomes a revolutionary act=94 > George Orwell > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .