superhet design: r.f. amplifier (12SK7), converter (12K8), two i.f. stages (two 12SK7's(4), or 12SK7/12SF7(5)), diode detector/BFO (12SR7), and one audio stage (12A6). Transmitters use four tubes: 1626 oscillator, two 1625 finals, and 1629 magic-eye tuning. The latter allows a check of the dial calibration by giving a visual indication, viewable by raising a small cover, when the oscillator's frequency matches that of an internal crystal. (4)ARA and SCR-274-N. (5)AN/ARC-5. Compatibility between systems Audio frequency receiver output and modulator sidetone impedance for the ARA/ATA and the AN/ARC-5 is 300 to 600 ohms. In the SCR-274-N "-A" version, the receiver and modulator impedance is 4000 ohms, while "-B" and later version units have a 250 ohm tap on the AF transformers which can be connected. ARA/ATA units and equivalent SCR-274-N units are interchangeable between systems, aside from audio impedance differences. However, AN/ARC-5 units generally are not interchangeable with the units of the earlier systems. In contrast to ARA and SCR-274-N receivers, all AN/ARC-5 receivers have automatic volume control and a modified tube complement. The AN/ARC-5 navigation receivers have terminals and a switch to connect a DU-series direction finding loop to the receiver, and have a special audio line for an MX-19/ARC-5 adapter to allow the receiver to serve as an LF/MF localizer for the Navy's short-lived AN/ARN-9 Air-Track (related to ZA, ZAX) instrument landing system. These two capabilities were rarely if ever utilized. Otherwise, equivalent receivers of all three systems can interchange as a unit. Few transmitter components of the AN/ARC-5 are interchangeable with ATA or SCR-274-N equivalent units. Mechanically, the transmitter rear power connector is slightly different, so inserting the wrong transmitter in a rack can damage either the rack or the transmitter power connector. Electrically, AN/ARC-5 transmitters use high-level final amplifier plate modulation, and the output tank circuit is shunt high voltage fed. The two earlier systems use less effective screen modulation, and the output circuit is series high voltage fed. The only electrical components of the AN/ARC-5 transmitter system that are interchangeable with the earlier systems are the dynamotor and the antenna relay. Unlike earlier systems, none of the AN/ARC-5 control boxes have audio jacks for the microphone, headphone, or key. A separate jack box is used instead. AN/ARC-5 transmitter control boxes contain no Morse key. Homing adaptors and VHF The broadcast band receiver in all of these command sets is intended to host a homing adapter for the