I have a bunch of old electronic hardware here. That includes old Macs back to the 128 and much older things like Hewlett-Packard radio frequency generators that run on vacuum tubes.
The biggest single problem I have is aluminum electrolytic capacitors and it reflects on attitudes of modern engineers and consumers. Those capacitors are made from two sheets of aluminum foil which are rolled up into a cylinder with paper spacers. The roll is placed into a, usually metal, container and then soaked in a weak acid; sulfuric is common. After sealing, with an over pressure escape provision, the insulator between the two aluminum sheets is formed by applying a voltage that causes the aluminum on the positive plate to oxidize forming an insulating coating of, Al2O3, aluminum oxide, That's called anodizing and is very much like they way aluminum drinking cups are prepared. The remaining acid and porous paper are left in place and electrically speaking, are nothing more than part of the conducting aluminum. Over time, without any voltage applied, the oxide returns to the solution and the capacitor shorts itself out. Applying voltage with the correct polarity can reform the plates but it's a pain because you need to start with a low voltage and slowly raise it. That's tough to do without removing the caps from an assembly like a circuit board. Applying full voltage to a shorted capacitor can heat it up and cause acid to leak. From the capacitor's point of view it's better to leave the voltage on at all times but that's not a green way to think these days. Those HP oscillators don't have capacitor problems even though they're much older than the Macs. But the capacitors are as big as my fist and even dwarf the old vacuum tubes. They are built in cans that attach to the frame with nuts and bolts. In short, they were built to last with attention to spacing between the aluminum sheets, choice of paper, sealing procedures, and thermal considerations for compatibility with vacuum tubes. Newer designs just can't tolerate those big caps and there is pressure to miniaturize everything including those tight rolls of aluminum. The paper and the aluminum get thinner, the pressure seals become radially scored plastic or thin metal, and the metal cans are replaced by plastic cups. Manufacturers actually specify lifetimes of 2000 hours at rated temperature of 85 C! 2040 hours is one man year in a typical accounting model. Those tiny electrolytics are a disaster. You can't store them without applying a forming voltage once in a while. They leak acid when they get warm. They're just too small for the technology even though they're still the tallest items on the circuit board. But they're cheap when compared to other technology such as tantalum sponge. The moral of my story: Don't expect an old computer which has been sitting on a shelf for a decade or two to work without attention to the capacitors and perhaps to the circuit boards which have been exposed to the leaking acid. If an old machine starts up but has small problems it makes sense to leave it running for a few days to reform capacitors. The worst that can happen is that a cap will explode. At least you'll know which one it was and a water-only dishwasher pass will remove the spilt acid if used right away. -- /* LIFE IS A SEXUALLY TRANSMITTED DISEASE WITH 100% MORTALITY */ -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
