PDP-8/I at the RICM

On 1/18/12 6:21 PM, "Eric Smith" <eric at brouhaha.com> wrote: Instead of all of that, why not just replace the capacitors? We've restored a 2065 that runs 24x7, and that's where we learned that you can cross your fingers and pray, or recognize that even when they're sitting on a shelf aluminum electrolytic capacitors (AlECs) degrade with time. I understand the concern about cosmetics - modern AlECs may be significantly different in dimensions, and in situations where they are visible it can impact the appearance of the artifact. On our PDP-7, we elected not to replace the filter capacitors, which are the size of large soup cans. Instead, we added small power supplies in a non-destructive manner (no holes were drilled) and added cabling to provide their output to the machine's circuits. The visual impact is small and the amendment can be completely reversed. It's a real problem that modern AlECs, while superior in construction and density, are commonly far smaller than original components. But replacing existing components with contemporaneous "new old stock" doesn't solve the problem, because AlECs degrade simply sitting on the shelf, according to research. We're going to have to get creative. I've heard of folks who hollow out original AlECs and embed new, higher-density components. We supplemented the existing system with modern switching supplies (which also has the advantage of far higher efficiency). The tension between access and preservation continues, with no simple or obvious answers. -- Ian