🚨Jah tubbys 4way pre repair

You should be able to measure a lower AC voltage on those wires. (Note that AC does *not* have polarity. So the earlier advice about looking for "+ and - 20v AC" is not quite right). The three wires are either end of the secondary winding, and a centre tap (halfway along the winding). So two of them will read ~24v AC, and the third will read ~12v AC (relative to either of the other two).

Then you will be looking for a rectifier, this is either 4 diodes on a circuit board or a solid block with 4 terminals on it, near to a pair (or quad) of quite large capacitors. (round metal cans).

Two of the terminals will be connected to the transformer output. At the other two, relative to ground (chassis ground) you should be able to measure *DC* voltage of about + and - 15 to 18 volts. You can also measure across the capacitor terminals if these are easier to find.

If any of those measurements come up as zero, or much higher voltages than written above, there is a fault with the power supply. 

Meters going crazy, and audio circuits making strange "feedback", can *sometimes* be caused by either the + or - power rail not being present. All the audio circuitry needs both that + and - 15 volts present to work properly, so it's the first thing to check ;)