BBC Type C and Presto Disc Recorders

Research department began work on the design of a disc cutter for mobile use as early as 1938. The result, called Type C, was highly successful. Two units were sent to France in 1940. Later, another unit covered the campaign in Eritrea and Ethiopia and later still, four units worked with the Forces in North Africa, where they coped well with extreme heat and sand.

right:
the power supply unit

left:
the amplifier unit

The Type C was in three units. The amplifier, originally a DR/5, weighing 44 lbs, used the OBA/8 circuit for its voltage amplification stage and an LSM/4 loudspeaker amplifier circuit for its power output. The output was later redesigned to use AL60 valves and re-designated DR/5A. The power supply, SUP/2A weighed 46lbs and contained a small motor generator to produce 400 volt HT from two 6 volt Nife batteries. It also had provision for the adjustment of the recorder turntable speed by means of a rheostat in series with the turntable motor field winding.

The recording machine itself could use 10,12 or 13 inch discs with a speed of 78 rpm. A built-in oscillator and neon lamp were used as a stroboscope for adjustment of turntable speed. A strobe setting for 80 rpm was provided so that off-load adjustment could be made, the load imposed by the cutter reducing the turntable speed by about 2% so that once recording was started and the strobe re-set to 78 rpm, very little further speed adjustment was required. The speed constancy was, however, considered barely good enough for the recording of music. No radius compensation was provided. Groove pitch was adjustable between 100 and 125 grooves per inch.

The BBC type A cutter head used a steel cutter and was designed especially for mobile recorder use. It had high sensitivity at the expense of frequency response, which was limited to a range of approximately 60 Hz to 4500 Hz. The cutter arm was balanced. A small lamp was provided at the end of the cutter arm to facilitate the changing of cutters in poor light conditions.



The turntable was driven by a 12 volt DC motor. The cutter arm traversed the disc by means of a lead screw driven by belt from the turntable spindle through a rubber-faced clutch.

A reproducing arm with an EMI type 12 pick-up was provided which could be plugged into a socket on the recorder in place of the swarf brush. This allowed recordings to be sent by line when insufficient time did not permit records to be taken to studio centres. When not in use, the arm was stored in the lid.

The amplifier could also be used for OB's if necessary. Although normally operated from a stationary vehicle, Type C recorders were used to follow processions, in aircraft, in trains, on ships and even in a submarine.

Additional equipment included: a three channel mixer, right, a loudspeaker, 4017 and 4021 microphones, microphone hand-grips and stand, two 100' drums of microphone cable, a talk-back unit, test records, interconnecting leads, spare parts and a tool kit.

Six sets of Type C equipment were ordered in November 1939, and six more in December 1940. By 1944 fifty sets had been made

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The Type C machine shown above can be seen at The Radio Museum, Watchet, Somerset. Thanks to Neil Wilson for supplying these photographs and notes.

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Type C equipment in the field.

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In September 1941 a set of Presto portable equipment was ordered. It was a mains operated twin turntable design which could cut 16 inch discs at 78 or 33-1/3rpm, though the latter was never used. A further six units were ordered in November 1942 and saw some service in the field. The Type C was preferred, though, as it was more robust and reliable.