Ex Govt RF_IF_Unit 10U/16654

[SteveCG]

A request for information on this Unit please.

I've attached two photos of the Unit. It is missing some of its Valves (B7G bases) and a Crystal (Holder on the bottom LHS in the photo). The id NL/EX/1307 has been written on its side.

It seems to have one RF stage, a LO/Mixer followed by 4 IF stages and then other valve processing steps. The original input/output sockets have been replaced with Belling & Lee type sockets.

[M0FYA Andy]

I've checked the issues of AP1086 Section 10U I have, but unfortunately 10U/16654 isn't present.

Andy

[davidw]

Its an IF amp in an R7351 receiver. (200-300Mhz am) Its mentioned in this (very long) document https://www.blunham.com/Radar/Signal...PDFs/R7351.pdf scroll down to pdf pages 12,13
There is probably more but I'll leave you to search the rest!

[SteveCG]

You got it in one davidw ! Many thanks.

See Chapter 4 - Construction, Fig 11, for the confirming photo (On page 43 of the .pdf scan)

The circuit diagram is on page 29 of the .pdf scan.

[SteveCG]

Just to wind up this thread:

I eventually got this unit working. The crystal controlled LO required a slight adjustment of the LC section of it for it to burst into life. Also a couple of 0.02uF decoupler capacitors failed short circuit after a couple of mins. I've no doubt more will fail; but I tried dodging this by reducing the HT voltage down to 200 Volts, as my purpose was to see if there was life at all in the unit, not to run it for any length of time.

[Radio Wrangler]

Originally Posted by davidw

Its an IF amp in an R7351 receiver. (200-300Mhz am) Its mentioned in this (very long) document https://www.blunham.com/Radar/Signal...PDFs/R7351.pdf scroll down to pdf pages 12,13
There is probably more but I'll leave you to search the rest!

So, it's the IF section from a crystal-controlled single-channel ground station receiver. For aircraft comms use in the military comms band. Channel spacing 100kHz, receiver bandwidth 60kHz to accommodate frequency errors in early equipment. (Civilian stuff later exploited the over-wide receivers to allow offset transmitters to share a channel to extend coverage without pilots having to retune)

The document gives the IFs at 24 and 1.975MHz for a double superhet. And that 60kHz bandwidth has a skirt spec that gives it a shape factor no more than 2.333:1 so it will be LC filtering at the second IF.

These sort of things weren't just at aerodromes, they sometimes went in out stations with GPO lines to control centres. Easier to move audio around than 300MHz.

This is also still done with civilian comms ground stations. E.G. The receivers for Edinburgh airport are on a hill a few miles away. It gives better coverage for low flying aircraft, and it gives the receivers some protection from the intensity of signals that would be found on the airport itself.

Aviation has stuck with AM precisely because it does NOT have the FM capture effect. If two aircraft transmit at once, the audio and carrier beat from the quieter one can still be heard in the background, so that a pilot or controller can hear that someone else is trying to speak and can be invited to try again after the loud one has ended. Having remote receivers reduces the probability of simple RF blocking due to proximity robbing them of this characteristic. There are back-up receivers in towers.

David