On Nov 3, 4:52 pm, Steve wrote:
I pulled an old Astron power supply out of mothballs today. It's a
13.8 volt / 20 amp supply (the RS-20A). I find that there's a
noticeable hum when I plug it in, and I also notice that a small
output voltage of one volt or so is measureable on it even when it's
turned off and unplugged.

What should this tell me about the condition of this supply? As it is
I'm nervous about using it.

Thanks,

Steve

Steve,
Most power supplies with big transformers can hum. I had an Astron
RS-20A for years, and it always hummed. If the humming is really loud
then something may be wrong.

I'm assuming that when the supply is powered up, it is putting out
close to 13.5 to 13.8v. You may want to measure the supply ripple to
see if it is abnormal.

Regarding the voltage present at the output:
Did you check for the same behavior on your other supply?
You didn't say how long after you shut the supply off that you
measured the 1 volt at the output terminals. It is most likely the
huge capacitor has not had time to discharge. A bleeder resistor
accross the capacitor would probably shorten the discharge time. The
way to check is to open the supply, power it up for a minute, then
power it off. Monitor the voltage accross the capacitor. Discharge the
capacitor through a resistor (around 200 to 500 ohms) for several
minutes, then disconnect the resistor, and see if the voltage floats
back up. Some voltage on the capacitor would not be abnormal.
If the power supply operates normally when powered up and the ac
ripple voltage is low, then a bleeder resistor across the cap would
solve your problem.
Gary