[cctalk] DEC H7821 / H7822 power supplies (was VAX and OpenVMS)

There are five specific 1800uF 25V (usually brown) capacitors that routinely leak in H7821 and H7822 power supplies. They must be replaced with good quality components suitably rated for use at the outputs of switch mode power supplies. Unfortunately replacing the capacitors is the easy bit. The more difficult bit is cleaning the mess from under the other components and identifying which other components have been damaged. Some of the resistors in series with temperature sensing thermistors have really oddball values and their values may vary from one power supply to another. If the bands have corroded off them, it can be a real guessing game trying to figure out what resistance values to replace them with so that the power supply doesn't trip all the time and so that the fans run when they are supposed to run and/or at the right speed. (The fans can also seize up all on their own so check that too.) Dismantling the power supply for access to the solder side of the board is a pain in the ass but you start to figure out how to do it without damaging too much stuff after the third or fourth time doing it. (At least it is not as bad as the H7816 in the DEC 3000/600. This one has a heatsink connected to the live side of the mains for no particular reason that I can see other than to make it difficult to do testing on it when it is running.) Keeping the Microvax 3100 in its normal orientation helps reduce the amount of leakage. Storing them on their front, rear, side or upside down makes it worse. The machines can continue to work for some time with capacitors leaking unseen inside them. The first thing to be noticed may be a strange smell... The same power supplies are also used in DEC SCSI disk enclosures that use the same case as the Microvax 3100 & Vaxstation 3100. They probably also appear in the Decstation 5000, Infoservers and other similar stuff. Regards, Peter Coghlan.