Calibrator, Crystal No7 MkI*

 

 This is quite a rare item as most will have been scrapped for parts. In fact this example is missing its 1Mc/s crystal.

The case is made from five pieces of bakelite and the front panel from ebonite giving it the looks of an early 1930s equipment although it probably saw service in WW2.

 

 With valves removed for cleaning.

 

 Under the bakelite chassis

 
 The set of six valves (all triodes).. on the left an Osram P2 and five VR21 or 10E/7738. Note the size of the anode differs.

 

 Chassis reassembled but minus the crystal

 

 Circuit diagram screwed to rear of case

 
 The circuit shows a pair of relaxation oscillators running at 10Kc/s (V3/V4) and 100Kc/s (V1/V2). They're synchronised by the 1Mc/s crystal oscillator (V5). The oscillators modulate the grid of the output valve (V6) which can be switched to oscillate at an audio frequency by connecting an 0.01uF condenser across its grid and anode.
 
 The calibrator could be used for "confirming" the frequency of a receiver by using the "A and "E" terminal connected to the receiver aerial, or by listening on headphones to for a beat note from a nearby transmitter adjusted to the nearest 10Kc/s. Of course the calibrator wasn't designed as a general signal generator but for checking to see the exact operating frequency of an already fairly accurate receiver or transmitter. Skill and experience were required in its use. Other, similar equipments were used during WW2, for example the BC221.

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