IFF Receiver, W4790B

Why did I acquire this rather odd receiver?

Well, I bought something very similar not far short of 70 years ago and just used it for parts as was the custom back in the 1950s. The actual name on the label I remember as "RDF1". However, a review of information on the Web results in more questions than answers. For example, I'm sure my RDF1 had a hinged lid like the one below but didn't have a built-in power supply. Maybe a search of my numerous junk boxes will turn up the odd part to clear things up? Anyway the example below was very cheap and its free postage made it an attractive proposition, not for parts this time, but for powering up to see how it performs.

The receiver, whose design was initially carried out for the RAF and called an "R.D.F. No.1" ("Radio Direction Finding" obscuring the secret IFF term), was named in both the RAF and the RN as a "Responsor". The general description is a MkIII IFF System and in the RN was used for identification of both aircraft and ships and had the title "242 Interrogator Equipment". It was used in the range 165MHz to 185MHz and back in the mid 1950s appeared on the surplus market as being suitable for use in a homebrew TV and slightly later for reception of ITV sound in Band III.

I realised that the 1950s seller of my RDF1 might have had an advert so picked up a 1958 Wireless World and sure enough I found not only the "Responder" but also a BC1066 VHF Receiver whose purchase had also stretched my pocket money to a second 15 shillings. The responsor was actually described as "ZC8931" which from its code might be a British Army version, but the one I bought definitely had "RDF1" on the front probably indicating RAF. Looking at a picture of the RDF1 on the website of the "Kurrajong" radio museum in Australia I must admit to not recognising their example so maybe there were several versions around at the time I acquired mine? Maybe the "ZC8931" opposite was another version of the RDF1?

As I read the advert opposite I wondered if any articles on its modification had been published and was surprised to find, not only an exhaustive article written in 1954, but other code numbers allocated to the receiver. So, W4790=APR4790=ZC8931 (plus ZC13132) and a close match to the R3118 which appears to include an extra RF amplifier plus internal mains power supply. In fact there's evidence one was sold on Ebay described as ..

R 3118 Receiver, ground station, IFF Mk-III receiver. (10DB/545 weighs 40lb). 13+ valves

Below is the 1954 article (with a rough sketch of an RDF1?) followed by that for the R3118

I've reproduced these pages for nostalgic reasons as well as the fact that they include the redrawn full circuit (in two parts) of the IFF receiver Type W4790

The article actually shows modifications needed to receive signal in Band I rather than Band III (which is mentioned as an adjunct).

An earlier article from 1954 dealing with a similar IFF receiver (the R3118) is also reproduced below.

Below is a drawing showing component numbering probably taken from an official manual.

A picture and circuit of the RDF1 at the Kallajong Radio Museum in Sydney Australia http://www.kurrajongradiomuseum.com.au/index.htm

The circuit diagram (left) is a bit blurry but the RF amplifier appears to be a "twin-horn" triode, the CV63, which I recall is the WS19 VHF receiver RF valve. If I indeed had an RDF1 in the 50s it must have been more closely aligned to the W4790 rather than the example shown above. The EB34 rings a bell but I do not believe it used the CV63. Maybe, by 1944 or so the designs of the different receivers were rationalised?

The museum is too far for me to visit alas although I must have a lot of distant relatives in Sydney. My great great grand uncle emigrated to Sydney in 1842. Yes, willingly emigrated not transported!

Maybe there are some radio-responsive DNA particles in living relatives?

Below pictures of a ZC13312/RDF1 and a ZC8931 (subject of the article following) Note .. no relay present behind the EC52.

You can see in these articles that the receivers covered are basically similar. The R3118 has the additional EF54 RF amplifier and a mains transformer occupies most of the space taken by the RF chassis in the other models. A magic eye is provided in the R3118.

Many years ago I recall our local surplus shop sold an RF amplifier fitted with a pair of B9G valves which they said was suitable for long range reception of Band III TV transmissions. I did buy one and tried it out but at that time we hadn't invested in a Band III aerial so didn't gain its true benefits using a short length of wire. The valves were not standard EF54s and might have been EF55s. I wonder if this amplifier had been developed for use with the MkIII IFF receivers?

W4790B

A few valves are missing viz. the EF54 RF amplifier and EC52 oscillator, plus three VR65 (=SP61) IF valves.

The lower metal cover hides three VR65s.

I noticed that the metal plate covering the VHF front end in the centre of the chassis is missing.

There are also a few white unterminated wires sticking out of the circuitry. Apart from this, at first sight, the equipment looks untouched suggesting these mods were made scores of years ago.

Lots of waxed paper condensers are visible so no doubt the nominal HT current of 100mA will be a trifle higher until these are replaced.

If you look closely at the birds eye view of the top of the chassis you may spot the relay screwed to the rear of the RF box. Its purpose was to switch in a small preset capacitor at the oscillator anode thus lowering the oscillator frequency. No mention of this is made in the article on converting the APR4790 for TV use so I can't rely on a view of the purpose of this from the 1950s enthusiasts.

My theory is the relay was used to overcome oscillator pulling from strong signals. AVC isn't used in the receiver because the type of valves employed are "straight" types with a sharp cutoff rather than variable-mu types. It's quite easy to integrate AVC into an IF amplifier or RF amplifier if variable mu valves are used, but tricky if not. The absence of AVC means that a strong signal could feed into the oscillator grid from the EF54 and lower the internal parasitic capacity of the EC52 (increasing its oscillation frequency).

My guess is that the signal from an incoming target getting stronger and stronger would drift across the IF band until it disappeared. To make things simpler for the operator a relay was added which responded to increasing HT current and once engaged brought back the signal into the IF passband. Once the target flew further away the relay would drop out, governed by its hysteresis, restoring the oscillator frequency to that originally set by the operator. I understand that in fact IFF transmitters would customarily be tuned to one of two standard frequencies so the operation of the relay would make operation much simpler as tuning of the receiver would rarely need attention. That also means that, without a compensating relay, if an operator had changed the setting for a really strong signal and then left alone it he'd miss signals from weaker targets.

I intend to power up the receiver and see if the hypothesis is valid.