Original Atlas power supply (220-CS or 200-PS) has zener diode for 13V (1N5350) and together with 1N4005 diode, provides 13.7V on the output.
Zener diodes change their parameter with increasing temperature, so it can happen that they “slip” when the temperature increases, and thus the voltage at the output of the rectifier increases. The higher the value that the zener diode has (eg 12V), the more the temperature change affects it.
Instead of a zener diode, it is better to use a TL431 circuit, which provides far better accuracy of the set voltage, regardless of the temperature.
TL431 circuit on the Atlas power supply…
On reference lead (R), TL431 needs 2.5V (Vref).
For 13.8V on the output of power supply, R1 and R2 need to be 68K i 15K…
Vout = (1 + 68K/15K) * 2.5 = 13.83V
Note: use metal film resistors for reference lead (68K and 15K). They are more accurate.
1.2K 1W resistor can be carbon. This resistor is minimally heated when installed outside the device (during testing), but is heated when installed in place, due to the surrounding components being heated. The TL431 heats up a bit.
