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Old 3rd May 2019, 6:54 pm   #1
trh01uk
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Default A tale of two calibrators

Here is a side by side comparison of two versions of the military “Crystal Calibrator No.7” – the Mk.1* and the Mk.2. Both were designed to allow a user to put a receiver or transmitter on to a specified frequency, usually military and usually in the SW bands somewhere. As such they were critical bits of kit for military users out in the field with no quartz crystal for their sets.

Its not often that we get two iterations of an equipment that spans a change in technology.

There are photos of both units at: https://my.pcloud.com/publink/show?c...BUkmkS3jePkWsX

Essentially, the two units appear to be very similar in function. They both:

• Feature a 1Mc/s crystal controlled oscillator
• Offer outputs at 1Mc/s, 100kc/s and 10kc/s – plus harmonics of them up to probably 20Mc/s
• Offer audio modulation of the RF signals
• Run on external battery (or doubtless an AC mains) supply.

What is interesting is the contrast in valve types, and the type of construction methods and materials employed by the two incarnations of this instrument:

1. The first obvious difference is the case construction. The Mk.1* uses paxolin sheet, both externally and for the internal component "baseboard". Very 1930s construction style. Component tags are scattered across the internal baseboard to attach components to.

The Mk.2 is all metal. Component tagstrips now have to be insulated from the metal baseboard, which means layout is less flexible because there is no longer freedom to locate tags exactly where they need to be.

2. The metal box means the Mk.2 weighs about 0.33kg more than the earlier version, despite being smaller all round.

3. Valve types clearly change markedly. We go from the 1930s lineup of 5 x VR21 + 1 x P2, with B4 bases, to all B7G style miniature battery valves: CV785 (1T4) x 5 + CV782 (1R5) x 1. According to the valve museum, the first set were introduced in 1930, while the latter were 1940 (1T4) and 1945 (1R5) - thus putting the Mk.2 as strictly a post-war creation.


We might speculate why the update occurred - I have no data on either unit not even a user manual. Presumably the older VR21 & P2 valves went obsolete and become hard to find. Maybe the new B7G types were just cheaper.

Was one type easier to make than the other? I suppose metal can be punched for valve holders etc - I'm not sure about paxolin - presumably that's drilled only?

We might note that the technology they were designed to support - namely receivers and transmitters with free running VFOs that had to be tuned to a specific channel didn't alter very much over the period 1930 to 1950. It was still AM on roughly a 6kc/s wide channel - and a zero-beat every 10kc/s throughout the HF spectrum was good enough to reliably get such radios on to frequency.

Richard
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Old 3rd May 2019, 8:01 pm   #2
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Default Re: A tale of two calibrators

Interesting topic.
Dry batteries vs accumulators(?)
Graham
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Old 4th May 2019, 9:10 am   #3
trh01uk
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Default Re: A tale of two calibrators

Are you suggesting "dry batteries" vs "accumulators" for another side by side comparison? Or wondering whether they were used to power these calibrators?

I don't have any info on what power supplies were used, but we can guess with the older one wanting a 2v heater supply, the standard lead-acid accumulator would be used, but probably only when in a benign workshop environment. Given their military background, my guess is that they were shipped with dry batteries for use in the field.

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Old 4th May 2019, 9:56 am   #4
HamishBoxer
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Default Re: A tale of two calibrators

I have the NO 7 MK1 and there was an article sometime ago on it in RadioBygones.That is before Radiophile took it over.
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