SCR Motor Speed Control made from salvaged parts

[Paul Jones]

I want to share some photos of a SCR motor speed control made from salvaged parts removed from surplus rack mounted electronics. In the early 1970’s my college roommate and I made several of these motor speed controls from surplus parts and aluminum used in rack mounted electronics we found in salvage yards. The one shown in the photos is still used in my home workshop (now controls my Unimat SL 1000 lathe (see http://www.homemadetools.net/forum/m...00-lathe-10111 )).

The salvaged printed circuit boards (PCB) and aluminum rack fronts and chassis were sold by the pound with no guarantee that anything would work. De-soldering the parts was easy using low heat soldering irons and braided copper wicks (we didn’t have the spring loaded vacuum pumps). The surplus electronics came from salvage yards in Seattle and the equipment was usually left in piles out in the rain or open-walled sheds. We always had a list of part numbers so finding best preserved parts took a lot of searching. There were always several junk yard dogs but kept caged by day and free to roam after closing. I found the salvage yards fascinating.

We saw an article in either “Radio Electronics” or “Popular Electronics” about making a 120VAC motor speed control using a 2N3669 SCR (Silicon Controlled Rectifier) rated for 16A at 200V. In the early 70’s SCR motor speed controllers were state-of-the-art so we had to have one. The most difficult salvaged component to find was the power resistor shown mounted on the right-hand side of the chassis in photo 3 (and better yet, this power resistor was adjustable). The aluminum box was cut from aluminum rack panels and chassis enclosures (hence some of the “extra” holes) and the stainless steel machine screws came from surplus rack components. We designed and etched our own printed circuit boards, and added an extra coat of solder to the copper foils. We thought it would be cool to have the PCB plug into an edge connector to make it easier to repair blown components (never needed). Even the blank PCB material was surplus.