This is an old field charger I designed and built back around 1985. It's a constant current charger design that is essentially two chargers in one enclosure...one for RX and one for TX. The input voltage needs to be a couple of volts ahead of the battery being charged, so I would often run mine on 24 volts at the field to quick charge my transmitters. My later pulse charger design could charge a 9.6vdc TX battery from a 12 vdc field box battery, but since I usually kept a spare field box battery and some clip leads with me, it was no issue to operate on 24 v if needed.
It uses a pair of LM317's wired in a current regulator configuration, with values calculated for about 600mah output. The output LEDs have a parallel resistance to allow full illumination at 600mah output. Once a NiCd or NiMH battery begins to approach saturation, the charge current deteriorates and the LED will fade and turn off around 250mah, indicating nearly full charge and ready for use. The photos below show a few pics and an old "toilet paper" drawing of the circuit. While not nearly as "whiz-bang" as all of the digital chargers of late, it's easily built with a few pieces from Radio Shack and some junk box parts. and will provide years of service.
Here you can see one of the two LM317's . They are installed to the heat sink with insulators and thermal heat sink compound. They can not be electrically connected to the heat sink.
My crude schematic. The 2 ohm 10 watt resistor for each charger circuit is a pair of 1 ohm 10 watt resistors in series. The 5 ohm 4 watt resistor in parallel with each LED is a pair of 10 ohm 2 watt resistors in parallel and attached to the front panel with silicone adhesive. These can be seen in the last photo. There is no difference in the circuitry for the RX or TX charger ports, since it's current regulated.