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Old 23rd Apr 2019, 12:17 am   #1
Chris55000
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Default Use of Magic Eye Indicators by the BBC?

Hi!

More out of curiosity than anything, I was looking thro' the vast archive of equipment Technical instructions on the BBCinfo engineering pages, to see exactly how the BBC Drawing Office's circuits, which followed BSI principles to the letter, represented a "magic eye" indicator, and I can't find an obvious example!

Did the BBC have a policy of always preferring meters, etc., in their equipment?

Whilst I realise the "magic eye" was, first and foremost, a non–essential "sale gimmick" as far as radio was concerned, and had use in tape recorders as a level monitoring device, I'm surprised that the BBC, in designing hundreds of pieces of valved devices from the end of W.W. 2 until the early/mid 1960s when transistorised equipment took the place of valves, never thought up an application for these indicators!

Comments?

Chris Williams
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Old 23rd Apr 2019, 12:53 am   #2
Synchrodyne
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Default Re: Use of Magic Eye Indicators by the BBC?

The BBC RC1/1 General Purpose Receiver, which was a modified Chapam S6, had a magic eye tuning indicator.

The schematic might have been redrawn by the BBC, if only to include its own "add-on" features. I don't have the Chapman original for the S6, but that for the S6BS looks stylistically a little different.


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Old 23rd Apr 2019, 4:50 am   #3
TonyDuell
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Default Re: Use of Magic Eye Indicators by the BBC?

The Ficord 1A tape recorder, which was used in a modified form by the BBC, has a DM70 for the level indicator.

Technical Instruction R11 (which I must have found on the web somewhere) includes a circuit diagram of the BBC version (and claims that it is a BBC diagram). The indicator is shown as a directly heated triode with the anode as an open rectangle (think of the symbol for the +ve plate of an electrolytic capacitor).
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Old 23rd Apr 2019, 10:10 am   #4
Chris55000
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Default Re: Use of Magic Eye Indicators by the BBC?

Hi!

That RC1/1 diagram was exactly the sort of example I was looking for the RC1/1 sheet I found didn't have the circuit in it!

That shows the BBC followed BS530 (the predecessor of BS3939 & IEC617!) to the letter even as far as m.e. indicators go, as these indicator symbols had the most variations on a theme even amongst UK makers!

Therefore the symbol I use for my valve m.e.'s, which is the same as the RC1/1 circuit posted, is the official one from the BSI "horses mouth" of the period, and therefore the correct one for this Forum!

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Old 23rd Apr 2019, 1:50 pm   #5
Chris55000
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Default Re: Use of Magic Eye Indicators by the BBC?

PS!

Forgot to mention that the DM70 symbol shown in the Ficord 1A circuit is again, drawn the exact way BS530 recommended and is the symbol I use for these sub–minature "exclamation–mark" battery indicators!

In conclusion tho', I think the Practical Wireless/Newnes/Radio Constructor/Wireless World method of using a black spot where a valve electrode meets the envelope, which traditionally is the way of depicting a valve–pin, was deprecated by the BSI of the time, and therefore the BBC, Electronic Engineering magazine & Mullard always omitted them in this position!

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Old 23rd Apr 2019, 2:24 pm   #6
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Default Re: Use of Magic Eye Indicators by the BBC?

Magic eyes aren't really suitable for use in professional equipment because they can't be meaningfully calibrated, though Mullard claimed that their phosphor on glass valves like the EM84 were sufficiently consistent to be used with a scale.
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Old 11th May 2019, 2:42 am   #7
Synchrodyne
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Default Re: Use of Magic Eye Indicators by the BBC?

The NBS null indicator was an example of the use of a domestic receiving magic eye in a professional instrument application. It was described in Radio News 1948 May, excerpt attached.

NBS suggested that this could also be used as an FM receiver tuning indicator. And in fact Leak used this type of tuning indicator circuit in its original Troughline, with the added facility of showing (qualitative) signal strength as well. The NBS-type circuit seems not to have been carried over to the Troughline II, 3 and Stereo variants, but Leak did return to the notion of a single indicator (in that case a meter) to show both tuning null and signal strength in its solid-state Stereofetic.


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