HRO Receiver Tuning Mechanism Type PW

• alternative name: National Toy Co.
• Brand: Thrillbox

The classic HRO receiver tuning mechanism, which is based on a Sperry Gyroscope design, comprises a 'micrometer' dial and 'PW' (parallel worm) gearbox. (Parallel to the front panel that is; referring to the orientation of the output drive shaft). The dial incorporates harmonic drive of an eccentric number plate with a 20:1 reduction ratio. Although embossed with the National Co name, the cast unpainted number and index dials are believed to have been supplied to National by the former Doehler Die Casting Co, and there are many color variations.

There are 500 dial divisions in 10 turns of the dial for a tuning capacitor rotation of 180 degrees, giving a resolution equivalent to a linear scale 145 inches (368 cm) long. The gearbox has a spring-preloaded worm gear driving a spring-loaded split main gear to eliminate backlash. Calibration charts are required to relate the dial reading to frequency

The HRO using the PW dial was being developed during 1934, but the start of production was delayed by technical problems. In the December 1934 issue of QST it was announced that the receivers would be available in January 1935, but this date was missed, too. National did start building them in that month, but didn't ship the sets to customers while they worked on the problems. Finally the first receivers were shipped around March 1935. 

During WWII the cast metal gearbox was replaced by a stamped sheet metal one, and this was retained for the HRO-5 and later receivers. From 1950 the HRO-50 had a linear direct reading slide-rule dial but retained the PW dial for logging.

Here you find 446 models, 285 with images and 189 with schematics for wireless sets etc. In French: TSF for Télégraphie sans fil.
All listed radios etc. from National Company; Cambridge & Malden (MA)