On Mon, 11 Aug 1997 23:55:38 +1200 Russell McMahon writes: >The power supply expired quietly after about 3.5 years and was swapped >for >a new one. At the price (about $NZ40 including a case (approx $US23?) >its >hardly worth repairing them. Repair is largely an academic pursuit unless it is a unit with an unusual physical configuration. Even then, the board from a new unit can often be fitted into the unusual case. The biggest failure item I have seen is >the >mains filter electrolytics which generally go out with a bang and >split >longitudinally and blow their contents around the inside of the power >supply. They are supposed to "vent" quietly by blowing out either a thin place in the rubber plug at the bottom or a scored thin spot in the top of the can. If the pressure rose rapidly enough to make the side blow out it is probably because the other capacitor became shorted and suddently exposed the one that exploded to the full line voltage. Or something else pulled the midpoint of the two capacitors out of balance. As this country is universally 230 VAC and the supplies are >usually >switchable 110 or 230 V ours are run in a different mode from most US >ones. >On 110 AC the input electrolytics are run as a voltage doubler but on >230VAC they are simply placed in series across the rectified mains. I >would >have thought that the voltage doubling mode would be harder on >electrolytics and lead to more failures. The series connection has the serious problem that the voltage distribution between the two capacitors is not well controlled. In the 120 V configuration it is impossible for either capacitor to charge to more than the peak line voltage. In the 240V mode the center point of the capacitors is either open or connected to a blocking capacitor to one end of the power transformer. This blocking capacitor is necessary in the 120 V mode to prevent DC from flowing in the transformer primary if the positive and negative supply rails or the driver duty cycle isn't balanced. In 240V operation it may work to remove it and let the almost balanced duty cycle of the drive transistors force the center point of the capacitors to nearly the halfway point. But, without the blocking capacitor, a (single) shorted power transistor would make the opposite filter capacitor explode. Typically there are large 100K resistors in parallel with each capacitor but I think these are mostly for safety reasons rather than voltage balance. They will leak the charge out of the capacitors in case some fool ignores the dire warning on the label and takes the supply apart. (The fool would have to be smart enough to disconnect the line cord of course). In my experience switching >transistor failures are less common. Even if a transistor does fail I >think >it would be unusual for it to fail in such a way as to damage >connected >equipment. of course, sudden loss of DC power can lead to damage or >data >loss depending on configuration. In the US with 120V operation, filter capacitor failure is uncommon. Lightning or surges can take out the fuse, input diodes and/or the power transistors, usually in that order as the severity increases. A lot of lightning will blow them right off the board (Parts unit!). Most failures though are minor. Many have no problem other than a bad switch, bad connection, or worn-out fan. IBM spec'd Panasonic ball-bearing fans which I have never seen wear out. Two other trouble spots are the startup resistors (usually 330K) on the power transistors and the output capacitors. A particularly nasty failure mode results from the diode that taps power for the control and driver circuit failing shorted. This places a capacitor where there shouldn't be one and causes the +12V output to soar to about +26V, while the 5V line stays right at 5V. Very bad for hard drives. There is no overvoltage protection in any of these units, even the major brand ones. The only protective circuit per se is a current sensor in series with the input of the power transformer. This is usually included in the very cheap units as well. It appears that they are all built to the same application note somewhere.