Isolating the various loads is one way, but that can be somewhat difficult when a printed circuit is involved. Just don't forget that a blowing fuse is a secondary failure that's always caused by something. Don't ever let anyone talk you into the "weak fuse" theory.
The typical fuse blowers are shorted output transistors, shorted filter/bypass capacitors and shorted power supply rectifiers. The lists doesn't end there, of course, but those items probably account for 90% of the short causes.
Look for darkened/burned resistors that might be in series with overloads.
The crazy way to troubleshoot (I've done it after giving up all hope) is to replace the standard 1/4 x 1.25" cartridge fuse with a 1/4-20, 1.25" long bolt. Then the cause of the blowing fuse will often rear its ugly head in the form of smoke, fire, destroyed circuit boards, etc. I do not recommend this method.
This is where a schematic of the equipment is mighty handy. Then you could discover loads that can be disconnected by pulling a low value resistor or a jumper.
The Variac is used to bring the line voltage up slowly while monitoring the current through the problem fuse. Let the line voltage hover just under the fuse's blowing threshold and start checking voltages, looking for low voltages. Alternativly, you can measure cirucit currents, which is a bit more difficult unless you have a DC current probe and access for the probe in the equipment.
Dean