Voltage from a plugpack measured with no load

What is the current rating for the plug pack?

Load the plug pack with say 50%, 75% of the rated current output and see what the voltage supplied is.

If you dont have power resistors for a load try using automotive globes.

That was quick! Thank you. The plugpack is labeled 9Volt DC 1 Amp. It is from an old Casio music keyboard. I will hunt around for a suitable globe.

Perhaps more importantly, what load did you want to run off the plug pack? How much will it draw?

I don't know. The keyboard must be twenty years old, and I was asked to look at it by the organist at the church. The keyboard had died and at first sight it looked as if the plugpack was dead. A closer look revealed that the coaxial plug that plugs into the keyboard had pulled out of the two-pin connector on the end of the cable from the plugpack. It wasn't easy to see because the join was covered in heat-shrink. It was hard to see which way the co-ax plug had been connected to the cable, and I assumed that the centre connector was the positive and the outer was the negative. That is when I used the DMM, to check the polarity. And that is when I became a bit worried about the high voltage reading. I'm still looking for a globe. I guess the correct answer to your question(s) is I will be running whatever load was being required of it before.

I'd be confident the plug pack is ok and wouldn't worry about testing with a globe.

Centre positive is common but not always the case, I'd suggest opening up the keyboard and working out what the polarity should be, perhaps there is a protection diode off the power connector that would indicate polarity - otherwise look for filter capacitors or a voltage regulator.

Perhaps look for dry joints / cracking where the power connector is soldered to the PCB.

I would check the input for possible voltage and resistance , it might have electros in there or a diode which should offer a clue as to polarity

All is well. With a taillight globe the voltage read about 9V. I was wrong about the polarity. Centre pin is negative and the outer connector is positive. Thanks again for your help.

Lot of problems I've noticed with DC plugpacks are dried out electrolitic capacitors. Affected plugpacks show reasonable valuevoltage under no load but at normal operation produce excessive ripple that affect powered circuit. I observed a lot of that with modems, routers and other stuff with microprocessors.

I guess the best test to find out if the plugpack or the keyboard is faulty is to get another 9-12 V known good power supply and power the keyboard from that.

Tom

T.T. Inscribed thus:

Oddly enough I have one of these in my Daughters old toy box. It is marked as you say 9V 1A. The manufacturer is "Niko" who ever they are ! Off load it measures 14.52 volts, into a 15 ohm resistor load it drops nicely to 9.15 volts.

HTH.