Power Designs 3650-S Teardown - Page 1
Forum section: electronics / testgear

author: bitseeker
author_rank: Super Contributor
timestamp: October 30, 2015, 11:28:06 pm
content: 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.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.There's a metal plate on the back with specs, instructions, and remote programming curves.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.And a closer look at the front half.Back to the main transformer and the pass transistors mounted to two large heatsinks. 180W of passively cooled, quiet power!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.Here's a quick look at the other side of the circuit board.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.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.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.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 aPower Designs TP340A repair and facelift.

author: eas
author_rank: Frequent Contributor
timestamp: October 30, 2015, 11:40:02 pm
content: That thing is a beast! How much does that sucker way?I've been waiting to see the inside of one of those. Thanks for posting your teardown photos.

author: bitseeker
author_rank: Super Contributor
timestamp: October 30, 2015, 11:46:04 pm
content: The form submitted prematurely. Just updated it to get the rest in.It is a beast; it weighs 33 lbs.

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 31, 2015, 12:26:52 am
content: Fantastic!!Never seen those green cucumber-looking resistors before!Quote from: bitseeker on October 30, 2015, 11:28:06 pmYes please

author: bitseeker
author_rank: Super Contributor
timestamp: October 31, 2015, 06:34:29 am
content: Yeah, I'll post pictures after it's cleaned up. The good news is that it's still functional. The guy I bought it from demoed it by connecting a 12V lamp. It'll be interesting to see how well it functions with a full load test.

author: Vgkid
author_rank: Super Contributor
timestamp: October 31, 2015, 08:46:16 am
content: Thanks for the teardown, a really unique unit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2015, 12:34:21 pm
content: Ahh, that's a monster. And look at those specs, well filtered and quite stable!I recently came into ownership of a pair of surge generators, which I'll have to take pictures of. They're not even all that old, 1983 or so vintage -- but nonetheless made with 60s-classic construction: tinned PCBs, no soldermask; point-to-point (though not as much as your example!); DTL logic chips; canned capacitors; and yes, even variacs, though much smaller (they seem to be using them almost as rheostats!).Tim

author: coromonadalix
author_rank: Super Contributor
timestamp: July 11, 2025, 02:50:06 pm
content: manual with schematics from Artisan