Tearing Down And Improving A Professional Power Supply

[OZ2CPU] has an HP power supply that is about 30 years old. It looks brand new, though, and has three outputs and includes tracking for the adjustable positive and negative supply. After a quick demo of the unit’s features, he tears it all down so we can see inside. You can catch the video below.

Some similar supplies offer a 10-turn adjustment knob, but this one doesn’t. Inside is a beefy transformer and quite a few through-hole components. There was room to change the main adjusted pot to a 10-turn unit, so he made the mod.

Testing was mostly good, but there was some oscillation in certain situations. To tackle that, he worked through it with another unit in the second video below. The new unit had the same behavior. He probed around and found that one capacitor leaked electrolyte, which didn’t fix the problem.

Analyzing the schematic was more productive. A feedback transistor had excessive gain at high frequency. This appears to be a design problem, and adding a resistor into the feedback loop makes it work as expected.

It is hard to go wrong with a name-brand power supply, but, as this shows, they are not always perfect. You can, of course, roll your own. You can make them very fancy if you like.