BC348-O
Communications Receiver
|
|
This is a receiver I've always
wanted since seeing them advertised as government surplus back
in the 1950s. Prices were never low as there always seemed to
be a strong export demand with as many people advertising to
buy rather than sell. Originally designed by RCA in the mid-30s
this marque was to be built by various manufacturers and supplied
mainly for airborne use, such as in the B17, and some mobile
use in vehicles such as Jeeps. Earlier examples carried the number
BC224.
You can tell who made a particular
receiver by the letter which follows the number.
Stromberg-Carlson: E, M &
P: Belmont Radio Corp: H, K, L, R & S: RCA: B, C & O:
Wells Gardner: J, N & Q
It is said that versions "A,
D, F, G & I" do not exist. |
 |
 |
The manufacturer's label
tells me it's a Version "O"
All was not so clear cut because
many receivers were recalled or subsequently rebuilt after first
manufacture to add an extra MF waveband. These usually carry
internal labels showing the progression from one type to another. |
 |
On the inside of the outer case
there's this label so maybe the chassis and case were once swapped
around or, more likely the set was modified more than once.
Possibly this example was originally
a Version "C" made by RCA, converted to a Version "S"
by Belmont Radio, then converted to Version "O", again
by RCA?the inside |
|
|
The valves used depended on
who made the set and as the original line-up was invariably military
coded, both these and their commercial equivalents, are quoted.
The "O" type uses:
VT86 (6K7), 1st RF Amplifier;
VT86 (6K7), 2nd RF Amplifier; VT91 (6J7), Frequency Changer;
VT65 (6C5), Oscillator;
VT86 (6K7), 1st IF Amplifier;
VT70 (6F7), 2nd IF Amplifier & CW Oscillator; VT93 (6B8),
3rd IF Amplifier, detector & AVC;
VT48 (41), AF Amplifier; RCA991,
Voltage Stabilizer
The sets six wavebands are 200-500
KHz, 1.5-3.5 MHz, 3.5-6.0 MHz, 6.0-9.5 MHz, 9.5-13.5 MHz, 13.5-18.0
MHz |
 |
|
The BC348 has a few interesting
features. The volume control is actually a pair of potentiometers
in tandem. One operates as an RF gain control and the other an
AF gain control. When the AVC switch is set to manual or "MVC",
the RF potentiometer is in circuit. When AVC is selected the
AF pot is operational. In an ordinary RF circuit, internal noise
increases as the frequency is raised so in the BC348, in order
to keep the audio output flat, there is a compensating circuit
governed by a potentiometer driven from the end of the tuning
condenser. Pretty unusual?
Because the set was originally
designed for use in a 14 or 28 volt environment in an aircraft
the set used a built-in dynamotor (rotary transformer) to develop
the HT line. In my example this has been removed and a small
mains transformer fitted for this purpose (top left). To simplify
the heater supply requirement a standard 6.3 volt heater transformer
has also been fitted and the valve heaters rewired for parallel
instead of series/parallel operation.
In operation the tuning control
is extremely smooth with no backlash and so resolution of SSB
is very good. Getting from one end of a band to the other is
quite a chore involving some 100 revolutions of the knob, but
tuning resolution is superb, to the extent that even though Droitwich
is only 2KHz lower than the lowest dial reading, it is too far
for the receiver to tune it properly. Stability is excellent
as the chassis and general construction are very solid being
based on welded aluminium castings. A crystal filter may be switched
into circuit to improve selectivity and the BFO is adjustable.
The Intermediate Frequency is unusual being 915KHz and fault-finding
is possible via test points accessible under a detachable plate
which can be seen on the right of the front panel. |
 |
Early sets didn't have the lower
band and had slightly different tuning ranges to the later models.
To help with dial reading, only the band in use is visible through
a window coupled to the wavechange switch (the spoked knob lower
centre). This was presumably a useful feature in a darkened aircraft
when the only illumination was in the tuning window of the receiver. |
|
|
Under the chassis you can
see the 4-gang tuning condenser and aluminium screening cans
for the coil sets. |
 |
|
Below: The tuning condenser
anti-backlash gears. |
 |
 |
|
Above: The wavechange
gearing. |
|
Click
to see a repair of a Model BC342 for a customer |
