I bought this HP 721A power supply from an eBay seller for about $40 total. The old timer (dated 12-15-59) needed some TLC – the line cord jacket was frayed and falling off so I replaced it. It’s all solid state, germanium.
The case needs to be cleaned up, but it’s got some signs of its previous owner, the U.S. Navy – see photo. It put a 100 mA load on it and it regulates solidly, meaning that the capacitors look like they’re all okay. It only puts out a maximum of 225 mA at 30VDC. Someone earlier replaced the voltage control with a ten-turn pot. About the only deficiency is the neon pilot light is very dim, but that’s of nearly no consequence.
Notice on the top center of the PCB there is C8, a wet tantalum cup capacitor. I’ve never found one of these gone bad. This one is over 60 years old, along with the other electrolytic capacitors and they’re all going strong, however I’ve had to replace some of the capacitors in the 2 other 721A power supplies I’ve got. 👍
Yeah, the neon lamp is the problem. I checked the 100k, 10% resistor – it’s 107k. The neon light is sandwiched between the chassis and front panel, and I’d have to pull the front panel off to get to it. So I’m not going to mess with it.
But what I am going to do is add one of these DVMs, which will serve as a pilot light – see the photo. The little wiggle stick meter is not accurate – too small. But it’s good for monitoring the current, so I leave it on milliamps. The bright digits are visible from a distance. 👍👍
