Hi Peter, Great information. Thank you very much! If I get a 3100 I will make sure to check things out before I power it on. Thanks for pointing out the heatsink connected to mains. That could be a painful discovery otherwise!
- Peter On Sun, Nov 30, 2025 at 5:52 PM Peter Coghlan via cctalk < [email protected]> wrote: > > > > I have no problem replacing caps in PSUs so if that's the only problem > I'm > > good. > > > > 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. >
