So I'm playing with an old AN/APR-4 radar search receiver. Kinda
works right off the bat, if weakly.

Looking at the schematic, there's a thermostat and heater. Not too
surprising, maybe they want to keep the oscillator coils warm.

Then I look under the chassis, and the thermostat is clamped between
the two main power supply electrolytics. The heater is a resistance
wire, wrapped around one of the 20-20/450 electrolytics.

WTH? Why would anybody go to the trouble of keeping an electrolytic
capacitor warm?

Very puzzling.

Ancient_Hacker wrote:
So I'm playing with an old AN/APR-4 radar search receiver. Kinda
works right off the bat, if weakly.

Looking at the schematic, there's a thermostat and heater. Not too
surprising, maybe they want to keep the oscillator coils warm.

Then I look under the chassis, and the thermostat is clamped between
the two main power supply electrolytics. The heater is a resistance
wire, wrapped around one of the 20-20/450 electrolytics.

WTH? Why would anybody go to the trouble of keeping an electrolytic
capacitor warm?

Because electrolytic capacitors suffer an extreme capacitance reduction
when they get cold. At -10C, they essentially cease being capacitors.

-Chuck Harris