Charles Checkout the Astron page on the repeater builder website. There is a lot of info on general linear supplies using the LM723 regulator chip. You should be able to change the zener reference diode, and tune it down to 13.8v and deliver the same current.
***Danger, notice here *** You will NOT have the 13.8v overvoltage - crobar protection without changing that circuit also. You will have whatever it had at 28v. In my experience, regulator transistors seldom fail open, they often short, delivering the full transformer output into the crowbar circuit, Which if it fails to crowbar, frys your precious ham rigs. This supply will probably have 35 volts or more at the caps, even under load. If you dump 35v into your 13.8 rig, it will not survive!!! It might burst into flames!! If only the output voltage is lowered, the regulator transistors are going to dropping more voltage, and will get MUCH HOTTER!!! You will be dissipating 3 times more heat then it would have at 28v. Fans!! Several of them! My theory, ( without seeing this supply ) 120v input to transformer, transformer output 35v, bridge, Caps, 35VDC, regulated down to 28v. The transistors are dropping 7v. 7v @ 15a = 105 watts of heat. If you just tune the voltage reg down to 13.8, you will still have that 35vdc from the rectifier. The transistors are now dropping 21.2v 21.2 @15a = 318 watts of heat. That might be too much dissipation per transistor, and cause them to fail fast. You might gain an advantage by changing the 125 volt input to the 220v option. This will lower the voltage on the transformer secondary. You should still get around 17.5v or more at the secondary/rectifier-caps, which will reduce the heat generated in the regulator transistors. I use a junker automotive foglamp (free from junked truck) for load testing of unknown powersupplies. It pulls about 5 amps, and is tolerant of over voltage conditions and even runs on 12v AC. 73 Ed N3SDO
