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Picture: Cossor 535

Cossor 535
Mains Radio

This Cossor radio of 1934/5 uses one of the then newly developed heptode frequency changing valves as a combined local oscillator and mixer. The provision of delayed AVC allows a neon "rising column of light" tuning indicator to be employed.

Unusually, the volume control is equipped with its own cord-driven cursor and scale.
Picture: Cossor 535 The valve line-up is:

41MPG Frequency changer
MVS/Pen IF amplifier (IF = 128 kHz)
DD4 Detector
42MP/Pen Output
442BU Mains rectifier

Band-pass tuning is used in the aerial circuit. The frequency changer functions as a multiplicative mixer. Oscillator tracking is by capacitative padders.

The variable mu frequency changer and IF valves are controlled by delayed AVC. This means that as the current through the IF valve changes with AVC action, the voltage on the anode changes as well because a varying voltage is dropped across the anode load resistor. These voltage variations are used to drive a neon tuning indicator, which shows a rising column of light as a station is
tuned in.

The detector is a double-diode valve, and this is coupled directly to a pentode output valve. The loudspeaker is the usual mains energised variety, the field coils being used for HT smoothing in the positive HT supply. A tone control (simple top cut) is located inside the speaker cone.

Provision is made for a high impedance extension speaker (the terminals are at HT potential).

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