Wall adaptor problem

[erio fraga]

I have a multi-voltage wall adaptor ( input : 110  / 220 VCA; outputs: 1,5; 3; 4,5; 6; 7,5; 9 & 12 VCC ; 18w, 1.000 mA ) which construction is very simple: a muti-tap transformer, a voltage selector switch, the 4 rectifier diodes a filter cap and a LED with a resistor current limiter ( I know, this is a very poor supply treatment, it can and should be improved to be used in pedal effects; my son was using this adaptor in his disk-man ).
Well, what is weird is that independent of the voltage set in the switch, there is always 20 VCC ( without charge ) as output.
I checked the PCP, no shorts. The voltage values before the rectification circuit are OK ( ie, the AC voltages mesured at each tap of the transformer and at each position of the switch ), but after the rectification there is always 20 VCC .
Does anyone have any idea what could be the reason from this ?

[niftydog]

This is because you have no load attached to the power supply.

With no load, the capacitor charges up to the maximum (due to lack of  regulation) and sits there because it has no discharge path.

Connect a resistor (around 1000 ohms) accross the terminals and measure it again.

It's a great trick to play on ignorant people. "Hey, this wall wart is broken, but I'll buy it at a discounted price!!"