I suspect the rectifier in your NG power supply may aged and developed higher than nominal internal resistance. Be careful when attempting to replace it with a modern silicon diode bridge, its much lower resistance may jeopardize the mains transformer by excessive in-rush current upon power-up. A suitable resistor, or better yet, a NTC thermistor in series should be used.
I have a supply that has been blowing fuses. I isolated the transformer to just the fuse holder and power switch and still: poof. On the 210v tap (blue and yellow wires) I measure 2.7k ohms and the 20v tap (green wires) reads 5 ohms. I assume that means it's shorted?...