PDP-5 Rescued!

Chuck McManis <cmcmanis(a)mcmanis.com> wrote: Hi Chuck If I was worried about the capacitors, I'd disconnect them and use a bench supply with a nice current limiting resistor. I would think a step rate of 5% of working voltage every 1/2 hour would be safe. Put a volt meter across the resistor. Any significant large voltage drop would indicate leakage. These large caps are known to have some leakage. How much is OK, I don't know. That is something you'll have to make a judgment call on. I would think that for large caps like I see in the pictures, 1/32 watt is on the large size for DC power. Don't forget the smaller electrolytics in the circuits of the supply. Do a value check on these. I usually lift one lead and use an ohm meter. Comparable values should take the same time to charge. You might want to connect these to a setup like used on the large ones. If they respond as having low capacitance on the ohm meter, you'll most likely have to replace them, forming won't help. While you are fiddling with the supply, you might also add some new heat sink grease under power transistors. The silicon oil in this grease does eventually go away. It is easiest if the transistors are socketed. I don't recommend the slow power on for the entire unit. Many circuits will draw higher current until full up to voltage. If you want to do a slow power up and the supply is a linear, disconnect the load and just bring up the supply. If it is a switcher, you may damage it with a brown out. Most linears can handle it. Watch for any signs of smoke or popping sounds. These are almost always bad news. At one of the meetings at Stan's, one of the fellows, I don't remember who, had three tantalums blow. These were all power line input filters on boards. After we removed them, the unit worked fine. If we'd left the power on, these would have burned the boards. Dwight