R1392 Crystals

[Metropolitan]

Can anyone explain in laymans terms my query?
I bought the Air Ministry receiver R1392 because it covers the Airband frequencies, 118 to 137 Mc/s. But it needs 6 crystals according to the relevant Air Publication 2555F. There are plenty of crystals on ebay, but none of the required frequencies. Does each crystal just receive one broadcast or would it cover a range of frequencies? There is no wavechange/range switch on the receiver, so each time you want to hear a different airfield, do you have to change crystals? What would happen if a crystal of a different frequency to those specified in the AP were to be plugged in? As you will have gathered, I've never had anything to do with crystals before. Any advice geatfully received.
Thanks, Metropolitan.

[G8HQP Dave]

I think this is a single channel receiver, so you have to swap the crystal and retune for a different frequency. Under normal use it would have stayed on the frequency allocated for that airfield. If you plug in the 'wrong' crystal it won't do any harm but you won't hear anything.

According to this site the crystal you need is (f-4.86MHz)/18, where 'f' is the frequency you want to receive.

[HMV 1120]

Does it have a seperate tunable scale as well as a crystal holder on the front panel? I believe you can switch between "VFO"/"manual" (i.e. manual tuning) and "crystal" or "Xtal" - if this is the case then no crystals are needed for you to operate this set, as long as it is set to VFO.

In ground station use each squadron would have had a limited number of frequencies assigned. The crystal enabled the set to be locked to this channel with a high degree of frequency stability. In use the set would normally have been left monitoring this assigned frequency, with the (less stable) VFO allowing an operator to manually tune another frequency and then switch between the two as required.

The frequency marked on a crystal is usually its fundamental (F). This does not, in VHF receivers, correspond to its frequency of operation in your receiver. The formula for determining frequency will be found in your technical publications and will be along the lines of:

F = (Desired frequency-I.F.)/multiplication factor. Typical values of I.F.* and multiplication factor are 10.7 and 9 respectively (although they may be different in your set). For example, supposing I want to listen on 122mhz:

122-10.7 = 111.3
111.3/9 = 12.366 for the fundamental.

Hope this helps somewhat, I'm sure others can correct me where I've gone wrong.

Robs

*I.F.: Intermediate frequency, seehere.

Edit: I see Dave beat me to it! That'll teach me to be long winded.

[G4XWDJim]

It's a long time since I had one of these sets but I remember there was a mod for the VFO which made it very usable on airband frequencies and the set performed well. Much better than the R1132A.

No doubt others will remember better than I.

Jim

[Metropolitan]

Hello everyone,
Many thanks for replies and helpfull info. According to the link provided by G8HQP, the R1392 is asingle conversion superhet single channel xtal locked receiver with front panel tuning to obviate quick channel changes, Frequently, these receivers were convertaed to VFO operation.
So i assume my R1392 does not have VFO and will need a different xtal for each airfield frequency that I visit. (On WW2 re-enactment w/e's and Classic Wheels and Wings shows).
The formula for calculating the xtal frequency is.... fcrystal = fsignal -4.86 divided by 18
I've since been told that the R1132 receiver would have been a better set as it has VFO control, but at it's highest frequency of 125 Mc/s. it doesn't extend high enough to cover some of the airfields I visit. eg Sandtoft 130.425 and Sturgate 130.300 Mc/s.
I'll keep hunting for the xtals. Newark Air Museum Aero-jumble tomorrow Saturday 3rd March.
Cheers,
Harry Webster.

[Sean Williams]

Harry,

With a little work you could easily hide a small solid state DDS VFO inside the set - they are available ready made and tested - wouldnt take a lot of work to get this going - trying to find obscure crystals that work in the 1392 will become a serious bore!

[HMV 1120]

On the above information crystals between 6.285 and 7.285 should put you somewhere in the airband. It should be possible to use amateur crystals designed for the 7 Mhz band if you can fit them inside the apropriate holder - most WW2 era crystals can be dismantled and a modern HC-49 size fitted inside.

A quick search for the company "QuartSlab" will give you a UK supplier able to cut crystals to order if you need particular frequencys.

[Sparky67]

Some of us on here will have a box of odd xtals liberated from various places over the years. If you would like to post the frequencies you are interested in, something might possibly turn up


And as an alternative to individual airfield freqs some of the en-route radar services can be heard over quite a wide area - and may be have more traffic than some of the smaller airfields, particularly on windy/wet days...

Martin
G4NCE

[geeoboeh2s]

Take care when selecting the crystal frequency.
The IF for the R1392A is 4.86 MHz as you have established but later models, R1392 D &E are 9.72 MHz.
British sets did not use 10.7 MHz at that time.

Chris

[Metropolitan]

Hello Chris,
According to the relevant Air Publication, both versions of the R1392 (A and D),used the same IF of 4.86Mhz. It was the Navy version, 62H, that used the higher frequency. (Dont know why the Navy set was also described in an AP) The versions B and E were merely tropicalised versions of the A and D.
The frequencies I was looking for were 6405, 6533, 6549, 6579, 6969, 6979, and 6942 khz.
The crystal frequencies listed in the AP for testing & aligning dont work out exactly to the formula they give. I mentioned this to a stallholder at Newark Aerojumble and he said some tolerance was acceptable. So perhaps the dont need to be spot-on? As I said in my first post, I dont know much about Xtals.
PS, Sean mentioned installing a DDS VFO. What is it
Thanks, Harry.

[Radio Wrangler]

Hi Chris

DDS = Direct Digital Synthesiser

An ordinary oscillator, VFO or crystal usually makes a sine wave voltage in pretty much the same way as a pendulum moves sinusiodally.

A DDS does things the hard way. It *computes* what the voltage of the wanted frequency would be at regular time intervals, and it instructs a digital to analogue converter to make that voltage. The stepped voltage levels from the converter need filtering to construct the real sine wave.

It sounds difficult, but the whole thing can now be fitted onto a single chip in a package less than 10mm square.

The frequency out is programmable in ludicrously small increments. The DDS still needs a crystal oscillator to give it a timing/frequency reference.

The hard part is that DDSs normally do binary arithmetic and you need to convert your nice frequency number into base-2, and then squirt it serially onto the DDS chip.

However, there are loads of kits where you get a small board with a keypad for typing in numbers, or a rotary knob and a little LCD display. The board has a small microprocessor to juggle the digits, and it also has the DDS chip and a crystal reference oscillator.

You get crystal stability AND VFO versatility.

One problem is that in an air band receiver with such a low IF, the front-end will be tuned to get a few adjacent channels and to reject things elsewhere in the band. You still won't get all 760 channels (now being increased by 3 as 8.3333kHz channel spacing is being introduced to replace 25kHz spacing)

The R1392 is intended to not only have a crystal fitted for the intended channel, but also to have its RF stages peaked for that channel. It then sits at an airfield monitoring that channel only. If they want another channel, they have another receiver. Only if frequency assignments were changed would a receiver need re-channeling by fitting a new crystal and realigning the receiver for that frequency.

Having a DDS or a bank of crystals doesn't get you around the realignment issue, I'm afraid. You'll be limited to a group of channels.

My current day job is designing aircraft radios. I've got to cover the full band and get full certified performance on every channel, and minimise current consumption for people running from batteries in gliders or balloons.

Sorry,
David

[Sparky67]

Finally found my box of spare xtals Harry... I have:

6583, 6890, 6906, 6890, 6960, 6970, 6990 in various can shapes and sizes. The 6970 is only 1kHz away from your wanted 6969 so might trim closer to your wanted final freq and still be within the bandwidth of the Rx. That is in HC6/U and I believe it is either 20, or more likely 30pF parallel capacity.

Also got a few odds at 13MHz, so if the first multipleir is x2 will check those as well...

Any of these any use ?

Cheers

Martin
G4NCE