Restored Early 1973 Mu-Tron III Envelope Filter with 3-Prong Bi-Polar 9v Adapter

Very early release with that super low production number, back when Mu-Tron was still erasing the ID marks on the opamps (they are 1458 dual opamps). You can see there are two metal can and one early Raytheon plastic opamp. This worked when I got it, but not optimally and not much in the reverse sweep setting. The controls just didn't seem to do what they were supposed to do. When I opened it up I found that the Gain pot had been replaced, possibly due to what looks like old moisture damage (see pic showing corrosion on inside front of enclosure - there is no leaking battery damage). First I checked the caps and replaced them - all were drifted beyond spec, one badly. Then I looked at that replacement pot. The original Gain pot is 1meg with a reverse log taper - it turned out to be a 1meg linear taper pot and was additionally modified by adding a 1.5meg resistor across the which turned this into a +/- 550k linear pot. Not the best option there - I had previously sourced the correct value and taper pots, so put in a 1MegC pot with a knurled shaft. Then the trick was re-calibrating the limiting resistor on that old optocoupler. Took some trial and error, but finally got it right. Now it works perfectly, with warm, smooth filter sweeps, and the reverse sweep now has that unique sound (think the opening chord to U2's "She Moves in Mysterious Ways"). Other than the corrosion on the inside front, this is in great shape and looks excellent on the outside. I am including a new power adapter, made by Richard Lingenberg (one of the original designers of the Mu-Tron III). Allows you to use a standard 9v, neg center power adapter. Includes a cable for use with your power brick. I recommend minimum 250mA current output from you power source. BUT, do NOT have batteries installed when using the adapter. It will cause them to overheat and leak.