Bingo problem solved!

Having another look around I noticed that I had jumpered the 5V-GND lines
with a resistor to provide min. load for voltage regulation.  I had
forgotten about that and didn't see it in my initial look.  The resistor had
come loose and was no longer making contact.

The unit is much smaller than a PC power supply but the idea is the same.

Thanks for all the suggestions.

Gordon Williams

----- Original Message ----- 
From: "M.L." <m@lkeng.net>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, November 09, 2009 9:25 PM
Subject: Re: [EE] Power Supply Health Question


> On Mon, Nov 9, 2009 at 8:48 PM, Gordon Williams <gwilliams@ncf.ca> wrote:
> > Hi,
> >
> > I've got a 120V switching power supply that produces 12 and 5 Vdc
outputs
> > that I've been using for the last couple of years to power a digital
piano
> > after the original quit. It is salvaged out of an old piece of equipment
> > somewhere. The 12 V goes to the piano and the 5V goes to an LED to tell
me
> > the main power supply is on. It stays on all the time and the piano has
> > it's own power switch.
> >
> > I noticed the other day that the LED was pulsing and the supply was
making a
> > clicking sound at the same time of the pulses, at about 3 Hz. The 12 V
> > supply output was fluctuating wildly with no load. When I loaded the
supply
> > with 200 mA current everything settled down with 11.5 V output and the
LED
> > stopped pulsing. I don't remember that the supply was doing this
pulsating
> > under no-load originally when I started using it 2-3 years ago. It's got
me
> > concerned that something is not working right.
> >
> > Could this be a sign that the power supply is starting to break down? Is
> > the clicking sound from the transformer?
> >
> > Suggestions?
> >
> > Gordon Williams
> >
>
> Some old switching power supplies needed a minimum load current in
> order to regulate. Your supply might have a power resistor that has
> burned out from being continuously on and running at 100C for 2-3
> years, or you might have bad electrolytic caps, both, or neither.
>
>
> --
> Martin K.
>
> -- 
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