Hello Alan,

> You said you had looked at various capacitors and diodes, but have you
> checked the ones on the secondary side?

The problem (spotted by Bob Blick) was on IC supply filtering. A crappy cap 
was dried up.

> The reason that the good supply behaves this way is that it requires a
> minimum current draw to be able to regulate the output voltage. With no 
> load
> the voltage rises to a value that causes the control loop to cut out the
> drive, and this is the cause of the "squegging" that you observe.

I also opened the second PS. While identical to the other in outer shape, 
inside this one is different and I think it actually needs a load to 
operate: there's a tag on a side saying "Led on: ok, Flashing: abnormal 
condition" which I assume stands for "We know it could flash" :-D

Many thanks to you too!

Cheers!

Alessandro 

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