Troubleshooting a Power Designs 3650S Power Supply - Page 1
Forum section: electronics / testgear

author: OsciX
author_rank: Contributor
timestamp: July 11, 2025, 01:04:34 am
content: Hello all,I have recently picked up a Power Designs 3650S power supply, but it needs a few repairs. When I initially got it, I tested the voltage and current limits full scale. The voltage worked perfectly 0-36V.I shorted it across an ammeter to measure current, and it supplied power and limited current as expected.However, the first true symptom appeared shortly after. When I set a given current limit at a low voltage, and then increased the output voltage, the output current dropped. I could successfully supply 5A at ~5V, but when I increased to 36V, it would only output around 2A. I was hoping to run it at its limit across a load (36V @ 5A) for performance verification, but every time the same symptom happened. After about 10 minutes of fiddling with it, I heard an odd popping sound.Now, the output voltage is radically different. At minimum setting, it shows:18 V between DC+ and DC-15 V between DC+ and Ground0 V between DC- and GroundAt maximum:57 V between DC+ and DC-47 V between DC+ and Ground0 V between DC- and GroundThe output voltage will never decrease below 18 V; the current limit LED never illuminates;the supply no longer sources any current.Notably, the meter on the front still shows the true voltage between the outputs (18-60V).I found the service manual at KO4BB.com, with schematic; however, I'm not sure where to start.How can I fix such a beautiful unit? I would hate to throw it away.

author: floobydust
author_rank: Super Contributor
timestamp: July 11, 2025, 04:15:45 am
content: Uh, you took a circa 1966 power supply and expected it to work at full load?Of course it failed. The electrolytic capacitors are surely dried out, and you will have failed transistors now. Take it apart, see what went pop. Careful, caps in those days had no safety vent.If you don't have the patience to repair it, then sell it on eBay or somewhere it can get TLC it deserves.

author: OsciX
author_rank: Contributor
timestamp: July 11, 2025, 02:23:08 pm
content: In fairness, my workplace threw it away because it was "too heavy". They last had it calibrated two years ago.I'll take it apart and see what I can find, but I was hoping for enough schematic guidance to make an educated guess.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 11, 2025, 02:45:50 pm
content: you have this thread ?https://www.eevblog.com/forum/testgear/power-designs-3650-s-teardown/seems pretty basic, transistorized to say the least ?manual and schematics from Artisan

author: OsciX
author_rank: Contributor
timestamp: July 11, 2025, 02:58:34 pm
content: Yes, I've seen that thread! Those pictures seem very helpful in terms of teardown. The schematic is completely transistorized, so it should be an interesting repair. I was hoping to deduce which components likely went bad from the behavior.

author: edavid
author_rank: Super Contributor
timestamp: July 11, 2025, 03:18:44 pm
content: Most likely it was just the main filter capacitor. Depending on how loud the pop was, it may have vented, or it may have spewed its guts. It was probably dried out enough that there won't be much mess.Don't operate the power supply any more until you've checked this out.

author: OsciX
author_rank: Contributor
timestamp: July 11, 2025, 03:27:53 pm
content: It wasn't overly loud, definitely not an explosion. When you say the main filter capacitor, do you mean C3 (8000uF)?Is that consistent with the meters/terminals receiving 18-57 V, but no current output / current limit light?After the failure, I only operated it long enough to get a quick diagnostic -- obviously, I'd like to fix this before doing anything else.

author: edavid
author_rank: Super Contributor
timestamp: July 11, 2025, 03:31:24 pm
content: Quote from: OsciX on July 11, 2025, 03:27:53 pmDon't overthink it, just open it up and look.