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Old 28th Jan 2021, 1:51 pm   #1
Al (astral highway)
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Default 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillator

I made a quick lash-up of the RCA955-based test oscillator which was described as a 'Special Installation for Blenheim Aircraft)

The details are illustrated in the attached cct. diagram, marked 'SECRET' at the time (15 June 1940).


I got this from the archives at the Royal Air Force Museum in Hendon, a few years ago. I have an interest in RADAR at war, among other things.

Two feed-throughs are to the right of grey home-made inductor with red wire. The power supply is variable 25-90V.

The loop L2 is not installed yet.

The Acorn RCA 955 has always intrigued me and here it was installed in a bomber of some strategic importance.

I will have an interesting time finding out the upper frequency limits of this valve/tube - my 'scope is only 100mHz bandwidth!

Anyway, putting this here to invite any comments on the likely specific use of this little set at the time.
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Old 28th Jan 2021, 3:19 pm   #2
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

A basic FM portable on the bench will act as a test receiver covering 88-130MHz with almost no gap. Acorns were used in the R1147A Homing Receiver, around 1938, covering 180-220MHz
What was it for? - what radio and radar installations were on that aircraft?
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Old 28th Jan 2021, 3:47 pm   #3
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Do you know what mark of Blenheim this would have been installed in? In 1940 the Bleheim would have been used in both bomber and night fighter versions so I imagine it would have been the latter.

My father worked on the night fighter version around that time - hence my degree of interest.

Regards,

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Old 28th Jan 2021, 4:14 pm   #4
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Quote:
Originally Posted by Julesomega View Post
A basic FM portable on the bench will act as a test receiver covering 88-130MHz with almost no gap.

Of course it will. But that's not where things get interesting at the sharp end here, e.g:
Half of the 3 turn inductor has an inductance of 0.07uH. Half the tuning capacitor vanes have a capacitance of 10pF.

So Fres (low)=190MHz, call it 200MHz.
With the vanes all out, let's give it a capacitance of 0.5pF
So Fres (high) =850MHz.
Now it starts to explain why this specialist UHF tube was deployed. It's potentially good to 900MHz.

Quote:
Originally Posted by Julesomega View Post
What was it for? - what radio and radar installations were on that aircraft
Interesting Q! I hope someone can answer it.
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Old 28th Jan 2021, 4:16 pm   #5
M0FYA Andy
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

What is on the other sheet of paper in your third picture, does it give any clues what it was used for?

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Old 28th Jan 2021, 4:17 pm   #6
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

I also note it is Figure 16, so there must be more!
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Old 28th Jan 2021, 4:20 pm   #7
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Quote:
Originally Posted by RogerEvans View Post
Do you know what mark of Blenheim this would have been installed in? In 1940 the Bleheim would have been used in both bomber and night fighter versions so I imagine it would have been the latter.

Hi Roger, yes, this makes sense. That is the mark of Blenheim that matches the date of the draughtsman's signature of the drawing.
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Old 28th Jan 2021, 4:22 pm   #8
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

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Originally Posted by M0FYA Andy View Post
What is on the other sheet of paper in your third picture, does it give any clues what it was used for?
Hi Andy, I had that at one stage but I passed it on to another forum member with a specialist interest. I will try to remember who it was.

I believe it included the RF final power amplifier design, tuned anode tuned grid, with big old lecher lines.
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Old 28th Jan 2021, 4:43 pm   #9
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Hi Roger, according to Wikipedia and other websites a modified version of the RDF 1.5 was fitted to Blenheim Mk1 tail number L6622 operating out of Martlesham Heath for radio direction finding (RDF) trials over Bawdsey in November 1938. The fighter version of the Blenheim with Air Intercept (AI) Radar later became the Mk 1F. Jerry
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Old 28th Jan 2021, 4:59 pm   #10
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Hello Al,

I think that it would be very optimistic to get a 955 to oscillate at 900 MHz even less likely with a lumped tuned circuit. The minimum tuning capacitance may be a few pF and with circuit strays including the 955 input the highest frequency is likely to be below 300 MHz. With a 100 MHz bandwidth scope you should see some response at that frequency but don't make direct contact with the circuit.
As Andy has said more information would be very useful.
As a night fighter the Blenheim had metric radar operating around 200 MHz so one possibility is a test oscillator for manually injecting a signal to demonstrate that the radar was working. A number of books written by the operators highlight that confidence in the equipment was not great.
Certainly an interesting topic.

Chris
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Old 28th Jan 2021, 7:23 pm   #11
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Quote:
Originally Posted by geeoboeh2s View Post

With a 100 MHz bandwidth scope you should see some response at that frequency but don't make direct contact with the circuit.

Hi Chris, thank you for clarifying. I didn't mean so much that this circuit as 900MHz capable, just reading around the max design frequency of the tube itself, under ideal conditions - including the use of Lecher lines, for sure.

Now that I've looked at the 'scope trace with an RF probe right on the anode, there is a strange train of oscillations - see pic - which arises and decays. So the cct, simple as it is, isn't yet behaving predictably.

( I don't want to take this OT into a troubleshooting direction - I can resolve this easily, I imagine - I'm more interested in the context and the use of the design in the B. Blenheim)

Thanks for your input including on the likely use as a RADAR function checker and also on the documented unreliability of the kit used on the aircraft. I hope others will be able to add to this.


Cheers!
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Old 28th Jan 2021, 7:42 pm   #12
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Is there by any chance a document number at the top of the page?

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Old 28th Jan 2021, 7:46 pm   #13
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

The circuit initially reminded me of a super-regenerative receiver...

Perhaps your 'strange train of oscillations' is intended? Super-regens are _meant_ to squeg so hard the oscillation ceases when the grid builds up so much negative-bias that the valve cuts-off, then after a period resumes.... essentially generating a train of pulses - which could be just what you need in a piecr of RADAR test-gear.
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Old 28th Jan 2021, 7:47 pm   #14
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Quote:
Originally Posted by M0FYA Andy View Post
Is there by any chance a document number at the top of the page?

Hi Andy, there may have been but the A3 photocopy may have cropped it.


It would be interesting to go back to the archive when that is permitted...
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Old 28th Jan 2021, 7:49 pm   #15
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Quote:
Originally Posted by G6Tanuki View Post

Perhaps your 'strange train of oscillations' is intended? Super-regens are _meant_ to squeg so hard the oscillation ceases when the grid builds up so much negative-bias that the valve cuts-off, then after a period resumes.... essentially generating a train of pulses - which could be just what you need in a piecr of RADAR test-gear.
That's interesting, G6Tanuki - a good theory there.

It's certainly biased at an interesting point compared to the reference data.
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Old 28th Jan 2021, 8:17 pm   #16
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Two 955s were used in the APN-1 altimeter, around 450MHz fwiw
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Old 28th Jan 2021, 8:27 pm   #17
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

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Two 955s were used in the APN-1 altimeter, around 450MHz

Hi Jules, interesting. Can you say more? was that an altimeter with telemetry? I'm just wondering how the circuit might have worked ?


Cheers
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Old 28th Jan 2021, 8:30 pm   #18
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

There's a very good write-up at vk2bv.org/archive/museum/apn1. I had one in my teens but it was no use to me. It used the Doppler principle to get a range without vertical movement, by applying FM to the tx with a loudspeaker coil which moved a capacitor plate
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Old 28th Jan 2021, 8:34 pm   #19
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

Looks like a Colpitts oscillator.

Lawrence.
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Old 28th Jan 2021, 9:37 pm   #20
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Default Re: 'Special installation for (Bristol) Blenheim aircraft' - lash-up of test oscillat

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Looks like a Colpitts oscillator.

Hartley? The inductor in the tank circuit is tapped halfway.

But then again, the stators of the tuning capacitor are also a voltage divider.

So what does that make it? A bit of Hartley and a bit of Colpitts?
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