A little while ago I ran across a 1968 Power Designs 3650-S power supply and, of course, I had to add it to my collection. This is a single-channel linear power supply, 36V and 5A, with current limiting. Besides the front panel aesthetics, this supply stands out with its use of an auto transformer as a pre-regulator. "They just don't make 'em like this anymore."
This unit was stored in a garage and was in need of cleaning, so I took the opportunity to take some photos to share while I got a few decades of dust out. A future restoration will attack the stickers, residue, and oxidation.
![](http://lh3.googleusercontent.com/-ZekYx_OsXlI/VjPyJ2vg7rI/AAAAAAAAEYI/KdSDkK5R3Fo/s640-Ic42/IMG_20150920_165445.jpg)
It's got a little rust on the case, but not too bad. At least it wasn't rained on like some I've seen.
![](http://lh3.googleusercontent.com/-gyLek3dNjQI/VjPyLxB4QuI/AAAAAAAAEYI/qrcMjoLlOwA/s640-Ic42/IMG_20150920_165455.jpg)
![](http://lh3.googleusercontent.com/-EwePVE9cfN8/VjPyNsvfSHI/AAAAAAAAEYI/qpaMNM-qqn8/s640-Ic42/IMG_20150920_165504.jpg)
There's a metal plate on the back with specs, instructions, and remote programming curves.
![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif.pagespeed.ce._JElyJQqdB.gif)
![](http://lh3.googleusercontent.com/-dgd9il-gWiI/VjPyPvokUrI/AAAAAAAAEYI/5DDI4P90qMw/s720-Ic42/IMG_20150920_165517.jpg)
Now, let's open it up. Here's the view from the top with the cover off. Big ol' transformer for the main rails, cool green ceramic resistor, a smaller transformer for the bias circuitry, and the toroidal pre-regulator.
![](http://lh3.googleusercontent.com/-YowMhYDFE88/VjPyRjNHXvI/AAAAAAAAEYI/4IoZlUCUGNQ/s720-Ic42/IMG_20150920_170133.jpg)
And a closer look at the front half.
![](http://lh3.googleusercontent.com/-Bw9ZHeBCSH0/VjPyTf09fjI/AAAAAAAAEYI/8TQyCMnO13U/s720-Ic42/IMG_20150920_170143.jpg)
Back to the main transformer and the pass transistors mounted to two large heatsinks. 180W of passively cooled, quiet power!
![](http://lh3.googleusercontent.com/-HlvxHWcrE4M/VjPyVCnAXAI/AAAAAAAAEYI/IuhGxrtwbKs/s720-Ic42/IMG_20150920_170150.jpg)
The main board is on the right side of the chassis. Everything is point-to-point on turrets. Gotta love the big shiny trim pots.
![](http://lh3.googleusercontent.com/-ZhsmGPKaogs/VjPyWytCxJI/AAAAAAAAEYI/PX4HafRVxI8/s640-Ic42/IMG_20150920_170206.jpg)
Here's a quick look at the other side of the circuit board.
![](http://lh3.googleusercontent.com/-t9aP5lE4o90/VjPylF3xM4I/AAAAAAAAEYI/rtN83TVrbUU/s640-Ic42/IMG_20150920_170427.jpg)
Now to the left side. Get a load of that whopper of a capacitor down there! It looked like someone left a can of soup hiding in there.
![](http://lh3.googleusercontent.com/-0oX0pyK7xZU/VjPyZ2fdNBI/AAAAAAAAEYI/WM-I8Em3F8I/s640-Ic42/IMG_20150920_170244.jpg)
We've got to flip this puppy over to get a better look. Sprague/Power Designs 6000 µF, 60V. Only 85ºC, but it doesn't get hot in here.
![](http://lh3.googleusercontent.com/-EixwtCDoVgw/VjPyjmsZHPI/AAAAAAAAEYI/P2Uensu60Qo/s640-Ic42/IMG_20150920_170410.jpg)
From the underside, you can better see the auto transformer that's used to drop the main input voltage relative to the desired output voltage. On the auto transformer's shaft is the typical potentiometer to set the output regulator. So, when you turn the voltage knob, you're adjusting the input and output at the same time. Pretty cool.
![](http://lh3.googleusercontent.com/-8ikQDQlZQtE/VjPynKblfcI/AAAAAAAAEYI/Mwz648qGem0/s640-Ic42/IMG_20150920_170452.jpg)
Well, this concludes the tour of the new-to-me PD 3650-S. It'll now go into my queue for cleanup and performance verification. I'm also curious to measure and compare its current draw at various loads relative to other power supplies to see how much efficiency that pre-regulator adds.
If you like Power Designs power supplies, also check out my post on a
Power Designs TP340A repair and facelift.