I have fixed many Astron, in fact i have a RS-50A on the bench right now. A couple of things to check, First got to
http://www.repeater-builder.com and download the Astron Schematic. I will warn you that the schematic is for reference and you may find part in the unit that do not exist on the schematic.
The half wave bridge on the board normally 1n5393 but they can be most anything. They are 1N5404 on the 50A supply. Check the two diodes first. If they are bad replace both and the 2200uf electrolytic capacitor. See if this stops the fuse blowing. If you want you can disconnect the transformer from the regulator board (all four taps) and make sure it is not the full wave bridge, but I have rarely seen this bridge out;
If the supply does not blow the fuse, but there is no voltage out, check pin 10 out of the regulator and see if there is 14v on the pin. Next go to the emitter of the Tip29 (this may be another transistor, but normally a Tip29) and see if you have voltage. If you do not there is a 1.8ohm resistor inline with the emitter that will open. Treat it like a fuse and replace it and you should have voltage at the output. If you have voltage, but it is not stable replace the .01uf disk cap from ground to pin 13 of the 723 regulator.
If it is still blowing fuses check the pass transistors.
Good luck,
Paul Mandel