Thanks for the interest in the thread. I believe the fellow the equipment belongs to would prefer that I simply=20 restore the supply to original condition. He had the problem and also had backup equipment to take the pressure=20 off so I could take my time. I prefer switchers for my own use. Early=20 switchers were noisy in amateur radio service and linear supplies are=20 still popular as a result. I felt the 2.2 ohm resistors in the base=20 circuit of the paralled pass transistors should be replaced due to the=20 extended use in overload conditions. I am waiting for an EBAY order to=20 arrive so I can take that step. I may have bought into a problem there=20 because I ordered 3 watt metal film units to replace 2 watt Wire=20 wound's. The failure of one of those kept the general failure from=20 potentially causing a big smoke event. The primary fuse is not enough=20 (IMHO) for full safety protection due to its normal load requirements. When I get the parts all back in place I plan to verify that the Pass=20 transistors are sharing the load across the operating range and tidy up=20 the internal wiring before returning it to the owner. I believe the=20 power supply will outlast the owner & I both. Apparently the 17 volts or=20 so that occurs with a shorted pass transistor is not considered enough=20 to damage connected equipment. A crowbar on this power supply would=20 likely have wiped out more components in the supply. A better idea would=20 be a circuit that disconnects the load when the source is out of spec. I have not approached the matter of what to use for a test load yet.=20 Probably water-cooled wire. On 8/9/2012 3:12 AM, Vasile Surducan wrote: > 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 w= rote: >> {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 John Ferrell W8CCW =93During times of universal deceit, Telling the TRUTH becomes a revolutionary act=94 George Orwell --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .