R1294 UHF Receiver
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This receiver (primarily
used for identifying radar signals) is exceedingly rare. Looking
through various post-war magazine adverts it's not to be found
very often.
A search will reveal countless
requests for purchase during the 1950s. Could this have been
loosely associated with the arms trade because their primary
use was for supporting radar jamming or for scrap value? I did
discover a few of these for sale from dealers in the early 1950s,
then later, as they became redundant in the radio ham world,
a few were sold off by 70cm enthusiasts. In one article in a
1950 Short Wave Mag G2RD is mentioned as using an R1294 on 70cm.
I imagine that valuewise, and because of their limited appeal,
most surplus receivers would have been scrapped for their EF50
valves and non-ferous metal. Our local government surplus dealer
"Super Radio" or "Benson's Better Bargains"
yard had a veritable mountain of scrapped WW2 radio equipment
(the proprietor said "for the transformers" or "copper"
and, sure enough, when I tested the R1294 metalwork some 50%
of its weight was non-ferous). Interestingly of the two companies
advertising them, unlike their other goods, they were not priced,
suggesting that ringing up to buy one, any interested radio ham
would be subjected to a bit of sales pressure and price negotiation.
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Talking about scrap metal...When
I bought my first car, an Austin 16, the dealer said the price
is £10 but if you want the heater it's an extra 50 bob
cos its made of copper, so I paid him £12 and ten shillings.
He filled up the carburettor float chamber and pushed me out
onto the road... downhill to the nearest garage next door to
the Liverpool Adelphi Hotel. Petrol was three shillings and elevenpence
halfpenny a gallon which equates nowadays to less than 4.5p per
litre. I bought the car the same day as I passed my driving test.
I used the driving school car after my eleven lessons (plus use
of the car for the test) which had cost me seven pounds nineteen
and sixpence. Prior to driving the Austin I insured it 3rd party
fire and theft for a little under £11. I didn't need to
tax it because it still had 3 months remaining when I bought
it.
Hey ho....
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Unlike many "UHF"
receivers from WW2 this one really was designed for UHF. It tunes
to a basic range of 500MHz to 1000MHz with further settings employing
harmonics enabling reception up to 3000MHz, which bearing in
mind the specifications of vacuum tubes at the time, is pretty
miraculous.When this receiver was being dreampt up the common
VHF valves were typically the 955 triode and 956 pentode which
fizzled out at 300MHz if you were lucky. A few very specialised
types were being manufactured but in general none would really
be suitable as a tuneable UHF oscillator.
Physically this UHF oscillator
valve has roughly the same shaped glass top as a "modern"
B7G type such as an EF91 but it's bigger at 36mm in diameter
and 60mm tall. The electrode position and layout results in a
decent tuned circuit size for 500-1000MHz. I'll show more detail
of the E1231 or CV52 later.
That metal tubular component
at the side of the valve is the holder for a UHF mixer crystal
purported to be made of silicon with a tungsten cat's whisker.
The last crystal mixer I came
across was in the R216 receiver and this
proved to be duff. I fitted a tiny modern glass equivalent made
by HP which worked perfectly. Unfortunately, early microwave
mixer diodes are generally destroyed if checked with a standard
multimeter.
Goodness knows whether the E1231
or the mixer are still functional, but I do have a solid-state
design lined up if one or both are faulty.
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Below is a picture of the
oscillator tuning dial of the R1294.
It shows a logging scale 0 to
180 around its periphery with a cursor marked "500-1000MC/S"
under which you can read up to six sets of markings, of which
the outer reads from roughly "4.7" to "10"
(500 to 1000MHz). For each waveband (ie, 1GHz/2GHz/3GHz) there's
a duplicate set of markings with higher start frequencies (eg.
500MHz). I reckon that the set would need expert handling to
make sense of received signals.
As the dial is rotated it tunes
500 to 1000MHz, but as the mixer is tuned (500 to 3000MHz in
a single range), you need to read the inner harmonic calibrated
scales.
To explain the staggered duplicate
scales I initially thought that the UHF oscillator is either
being pulled by the mixer tuning (that adjacent knob) or most
likely associated with the front panel control "OSC ANODE
VOLTAGE" which is affecting the capacitances between the
CV52 electrodes. That being so it's unlikely a really accurate
frequency reading can be made without say an accompanying calibration
chart?
In fact this isn't the case.
The reason for the duplicated scales is twofold. Because the
IF is relatively low (=13.5MHz) it's not straightforward to determine
whether one is tuned to the correct signal or its image. In other
words a superhet will produce two strong IF signals, one at Fosc
plus IF and the second at Fosc minus the IF. I say "twofold"
because the oscillator and RF tuning controls are not ganged
as in most superhets. At least with a correctly tuned normal
superhet the true frequency will be a lot stronger than the image.
In the case of the R1294 receiver An 800MHz signal would respond
equally to an oscillator frequency of 813.5MHz or 786.5MHz. Either
response can be peaked to maximum by tuning the RF dial but only
one response would be correct in terms of the tuning dial reading.
A signal will be indicated by either of two dial settings, the
correct reading of 800MHz or its image of 827MHz.
In order for the operator to
figure out which response is correct two scales are engraved
on the dial. By taking readings from both it's possible to work
out the correct response. Paraphrased below is the method advised
in the operators manual.
Tune in one signal response and note the dial
reading for each of the six scales.
Tune in the second response and again note the
readings for the six scales.
In the six readings one will be common to both
responses and this is the true frequency.
If the BFO is in use. Set this to 13.5Mc/s and
zero beat. This is the correct frequency having ascertained the
correct response.
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Lowest setting near "180"
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Set to about 610MHz.
Max frequency of 3GHz is "0"
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Left, the rather confusing tuning
dial for the mixer. You'll note that this dial covers the complete
range from 500 to 3000MHz.
Further down the page I've shown
the method used for tuning the receiver.
Below is the meter for setting
oscillator grid current. Will the reading determine the tuning
scale to be used?
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Here's a view inside the case. Along
the rear are six VR91 valves (V2/V3/V4/V5/V7/V8 = EF50) and having
those codes would likely make this receiver an RAF example. I
understand the Army used a few of these intercept receivers but
would these use Army codes ARP35 rather than VR91? Third from
the left is a can concealing a VR119 (V6 = DDL4 an odd choice)
And for that matter using EF50s (V7/V8) for audio amplication
seems a wasteful thing to do except that I understand they're
employed not for audio per se but for video fed to an oscoilloscope.
On the right you can see the E1231 (= CV52). Inside that square
topped box on the right is the BFO which uses a VR65 (the ubiquitous
SP61). Being a superhet receiver... what's the IF? This isn't
straightforward as the designers were looking for more bandwidth
than normal. The IF has is 13.5MHz plus/minus 1.75MHz making
its response 11.75MHz to 15.25MHz. The BFO has a commensurate
range of 11MHz to 16MHz. Looking at the circuit diagram below
you'll see there are four flatly tuned stages of IF amplification
to produce sufficient gain over the wide bandwidth.
Below, click to see full size.
Component values are listed below the underside view (later).
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Now some views of the tuning
arrangements. Separate RF (mixer) and oscillator with no anti-backlash
gearing.
The upper tube tunes 500 to
3000MHz and the lower 500 to 1000MHz.
The two pictures show the receiver
tuned to about 600MHz.
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Here you can see mechanical
details of the CV52/E1231 and the way it connects to its tuned
circuit.
The cathode and grid connect
to the two thinner metal tubes with that for the cathode the
middle of the three.
The electrical chracteristics
are given on the right. The heater supply is relatively high
at 750mA and its heater isn't isolated being connected to the
cathode.
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Next, a peek underneath...
click to see the minutest detail. |
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Underneath it's a bit
surprising because it's untouched and original (is it 1944?).
If the receiver had been in the hands of a serious microwave
enthusiast I'd have expected the IF strip to have been doctored
to reduce its bandwidth.
Before powering it up it's going
to need a lot of effort. I'll tackle it by using my favourite
method of using 500 volt chip capacitors in place of those waxed
paper ones. There are several larger types as well which will
need attention.
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The bulging block on the
right (picture above)is C46-49 and the pair of large cylindrical
caps are C38/43 on the left with C50/51 on the right.
I've not shown these on the
circuit diagram, neither the pair of chokes, treating these as
part of the power supply circuitry. See below.
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Now I'll cover the power
supply arrangements in more detail. The PSU paired with the R1294
carried an octal socket with a cable terminated in a 4-pin "Plessey-style"
connector. I don't imagine many of these PSUs would have been
around and its loose cable would have been even scarcer so any
amateur user after WW2 would have made his own as appears to
be the case in this example.
The low voltage for the valve
heaters is 6.3 volts AC with the original design having a separate
heater supply for the CV52 probably because it has a connection
between its cathode and heater. The HT is 150 volts for the oscillator
and 340 volts for the remainder of the circuitry. This arrangement
would make the receiver more reliable in terms of frequency readings
and avoids the use of a voltage stabiliser although relying on
mains voltage regulation.
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The above picture explains
how the RF tuning control is used to adjust the aerial "tapping".
A return spring is fitted to the tuning shaft so either of two
different functions can be selected. |
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Below... cleaning the
engraved dials. I discovered the grub screws in both tuning knobs
are completely broken posing a problem to the (later) removal
and restoration of the front panel. The labels are engraved so
can be refilled but are riveted to the panel so will need masking.
I might have to make new parts carrying the cursors if the plastic
is permanently clouded. This also applies to that for the BFO
which is completely opaque. |
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Out of the blue I was
asked to repair a lift board. I retired from this business about
a year ago, but always up for a (paid) challenge I agreed and
was presented with this circuit board. Now; "What's the
connection with this R1294 UHF receiver?", you might ask. |
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Well, in the bottom left
corner of the circuit board is a tiny daughter board carrying
a couple of chips, an 8L05A (=5 volt regulator) and a 28-pin
TDA5210 |
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That TDA5210 is a "UHF
Superheterodyne Receiver", measuring only 9.7mm x 4.4mm.
It's designed for frequencies in the 400MHz, 800MHz and 915MHz
ranges
Complete with the required crystals,
filters etc plus its 5-volt power supply, that daughter board
is only 34mm x 18mm and weighs in at less than one thousandth
that of the R1294.
Performance-wise that black
wire is its aerial so can't be bad (better than -107dBm).
Who knows how the R1294 will
compare..
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The repair was accomplished
by fitting a new 12-volt relay.
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In progress |
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