Best way to vary output of 24V transformer?

Hello all,

I have a power supply board minus the transformer (2.25" x 4"
footprint) robbed from a broken down Panasonic copy machine. I know
the voltage output on it is no more than 24V. The input is fused at
1.6 amps. It has 1 input and 2 outputs. One of the outputs is simply
rectified and unfiltered. The other output is well filtered. I want
to vary the output of a 24V transformer to find the output voltage of
the board starting with a low voltage and working up to the voltage
the regulator circuit is set for. The board appears to use a
darlington configuration for the voltage regulation and a variable
pot resistor for fine adjustment of the voltage.

My guess is that a reastat (spelling?) is the answer but I'm not sure
just how to go about hooking this up to handle both sides of the
transformer output or what value it should be.

OR

Would I simply be able to use a light dimmer on the 110V side of the
transformer to vary the output voltage? I've never tried this and
would have to purchase the dimmer switch.

Does any one have an answer?

Looks like I could use it for powering relays or a dc fan if the
output voltage is right.

Thanks,

Donald Brock