The Murphy receiver B40 has been developed for the British Navy after 1946,
it's characteristic backlit dials gave the set it's nickname "lighthouse receiver"
or "spiral staircase radio". The set has been improved in the fifties, filters
and a fine tuning option to make it ready for FSK (radioteletype) reception.
The shape of the receiver with it's height is quite unique, the width of
the receiver is a bit smaller then 19 inch, two heavy duty screws keep the
front panel and chassis in place it the heavy metal cabinet.
The mains cable is connected to the receiver with a screwed two pin connector
typical for British naval communications sets.
The antenna socket, as well in a quite unique norm, a 600 ohms audio frequency connector,
the main speaker switch (must be set to face towards the front front panel, if no external
speaker is connected), an I.F. connector and a screwdriver operated dial illumination
dimmer can be found at the rear upper part of the receiver.
The frontpanel has three sections, the middle part looking like a column with the
illuminated "spiral staircase" dial and the main tuning knob, a right and a left panel
with the controls.
At the left, You find the internal monitor speaker behind five wholes
as protection cover, just below two switches for A.G.C and the noise limiter
and a control for noise limiter intensity. The next two big knobs are the bandwidth selector
(standard bandwidths are 1 kHz, 3 kHz and 8 kHz) and the modes switch. The modes are
CAL (a crystal calibrator, the calibration mechanism to adjust the dial is found behind the "door"
at the top of the frequency dial, the bit carrying the "B 40" label. Position R/T
is used for normal reception, TUNE will add a internal carrier to help You tuning
in a station (the carrier is correctly tuned when tuned to zero beat), and the positions
FSK NARROW / CW HIGH and LOW for CW reception with two different settings of the NFO beat note
and FSK-WIDE HIGH and LOW for FSK reception.
FSK is a radio teletype protocol based on signals transmitted on two adjacent frequencies
with a well determined distance, the so called shift distance. The two tone Baudot
code is used - a code system similar to ASCII code. The BAUDOT code only supports
capital letters and a special code is used to switch over to numbers instead of letters.
To decode the message, the output of the receiver is fed to a FSK converter (this
will accept signals of a certain frequency and shift) and will drive a mechanical
teletypewriter. The OSC TRIM only found on B40 D series is used to shift the
note of the radioteletype signal to the exact frequency used by the FSK converter.
The volume control is the small knob labelled A.F. gain between the two big ones. Further below,
there are the switches for the internal speaker and the mains switch as well as the
mains power cable connector.
In the right front panel section, You find a crystal compartment with an internal heater
for high frequency stability when receiving on a fixed frequency with a plug in crystal
(desired reception frequency - 500 kHz),
when fixed frequency mode is "ON", the compartment and the "X" is illuminated with
a red light. The knob "ANTI-CROSS MOD(ulation)" acts on the grid voltage of the
first R.F. amplifier, for normal use, this should be on the maximum position.
With the BAND SWITCH, You activate the massive band selector, the dial light
will move to the corresponding dial to indicate, which band is active. Just below
is the R.F. gain control and the headphones socket.
In the middle, You find the frequency dials behind a curved transparent protector,
the dial of the active frequency band is backlit, a small black triangle in the middle
acts as dial pointer and the complete dial is rotating when the set is tuned. Below,
just above the massive tuning knob, You find a 0 - 100 logging dial, this will give
You the possibility to find a station on a known frequency again, when You have
written the setting of the logging scale in the station's logbook. The lever
of the dial lock mechanism is very rugged (it has a feeling like a gear lever of
a sports car) and the dial lock lever can be blocked in the LOCK position
by means of a knurled screw.
The location of the controls spread on the front panel is a bit odd, but this
is not the only British set with controls spread over the frontpanel in a peculiar
way... Use the bis MAINS switch in the left lower corner of the front panel
to switch on the set, the speaker switch LS should be on ON, the modes switch on
R/T (for radiotelephony) and the BANDWIDTH switch on 8 kc/s. Turn the band
switch to band No. 3 and search for stations from the 49 m shortwave broadcast
band in the region of the 6 MHz mark. ANTI CROSS-MOD. and (R.F.) GAIN should
both be in the full clockwise position, use the tiny A.F. GAIN at the left
to control the volume (I first thought, this tiny knob might be the line level
gain control, not the main volume control).
This single conversion set with an intermediate frequency of 500 kHz
gives You fair - good results on the shortwave bands, the sensitivity is
sufficient to catch international shortwave broadcasters in good quality and also
some marine and amateur radio CW communication.
Because of the non-linear dial, searching for a signal of a station with a known
frequency is not easy, especially in the high frequency bands, where frequency
marks are quite coarse.
The receiver is constructed in a very solid way and should remain on a selected
frequency with the dial LOCK on very stably; even if the D version was meant
for FSK (radioteletype) reception, I have not tried operation with a FSK
converter. I qould rather rely on a stable PLL based receiver for this purpose.
Just in case, someone has a matching FSK converter for sale, let me know,
I might give it a try.
The Murphy B40 D is a very peculiar set, a real "boatanchor" rarely seen
"on the continent". It's a collectors set which still can be used as a
secondary receiver for shortwave DXing, but for everyday use, radioteletype
decoding with a matching converter and a teletypewriter it is obsolete...