PDP-8/I at the RICM

Ian King wrote of aluminum electrolytic capacitors: I respectfully disagree. If you want to ensure that the artifact will be operable for the maximum possible time with the minimum required maintenance (e.g., replacing the capacitor again), then you are correct that the capacitors should be replaced. However, this is not generally the primary objective of museums and similar entities with regard to the artifacts. For example, in the PDP-1 restoration, we went to great pains to reform the original capacitors, using a grossly over-engineered technique, and to document every aspect of this process (and of the entire restoration). It is true that the reformed capacitors can't be expected to have as much remaining operating life as a new capacitor, but that isn't the point. We attempted to keep as much of the original artifact as possible, and found that with careful reformation, nearly all of the capacitors still met the original specifications. Given the relatively small percentage of time that the machine is powered up, generally less than 2% of the time, the capacitor lifetime will be predominantly determined by the non-powered wearout mechanisms, which are quite slow. The machine is normally powered up for demonstrations for at least a few hours every two weeks, which is sufficient to prevent deterioration of the oxide layer which would eventually necessitate another round of reformation. We expect to check the power supply ripple on occasion to see how the capacitors are holding up, and certainly we will replace them if they fail, but we don't expect that to happen for some time. If at some point the demonstrations cease and the machine goes unpowered for a long interval, the capacitors will need to be rechecked thoroughly and possibly reformed before the machine can be powered again. Eric