Picture 1: MCR1
Picture 1: MCR1
INTRODUCTION
The M.C.R.1 radio (Midget Communications Receiver or the Miniature Communications Receiver), also known as the Type 36/I was developed for the Special Operations Executive and dropped to Resistance teams in occupied Europe for reception of broadcast services. It was also used by the Army and Special Forces. It was the receiver part of the Jedburgh Set. When the MCR1 was dropped into occupied Europe, it was concealed in a Huntley and Palmer biscuit tin, and thus was nicknamed the "biscuit tin" radio. The radio was accompanied by four coil boxes, a pair of 120 ohm headphones, a power supply and an aerial. The power was supplied by a Type 38/I power pack and/or a combined HT/LT dry battery (90 volts and 7.5 volts). The MCR1 was replaced by the Mk.301 in 1954.
Picture 2: Biscuit Tin (permission from the Crypto Museum)
The radio was designed by Captain J. Brown, designer of the Type 3 Mk.II (B2) set. It was manufactured by Philco GB in late 1943 and more than 30,000 were made. There was a Belgium copy made in the 1950s by MBLE (Manufacture Belge de Lampes et de Materiel Electronique). The MBLE model is painted brown wrinkle, has different knobs, and has different internal transformers.
The radio is a superheterodyne and uses a 1730 Khz IF (Intermediate Frequency).
It has four coil boxes, to enable it to cover four frequency ranges. They are:
CONTROLS
The radio has four controls, all on one side. The large knob is the TUNING knob,
which allows the operator to tune in radio stations. There is a small window,
which shows a moving scale, graduated from 0 to 180. This value is then used, with
the coil box scale, to determine which frequency is selected. The small knob near the
tuning knob is an AE TRIMMER (aerial trimmer), which allows you to peak the received signal.
At the other end on the box, are two knobs used to control the gain of the radio.
One is labeled SENSITIVITY and controls the RF (Radio Frequency) gain. The other is labeled
REACTION and controls the amount of regeneration in the detector, which increases the gain
and then provides oscillation so that morse signals can be heard. Turn the control fully
clockwise for CW (Continuous Wave) reception, or back it off for AM (Amplitude Modulation)
reception.
The radio has no ON/OFF switch. It must be unplugged from the battery, or the power
supply if used. One of the four coil boxes is plugged onto one end. The headphones, aerial,
earth, and power supply plug into the other end.
MECHANICAL DESIGN
The radio is in a long steel box, painted grey, and held together with self tapping screws.
One end of the box has pins protruding to engage with the coil box. The other end has four
sockets and the power cable. The sockets are labeled PH for the headphones, A for the aerial,
and E for the earth.
When the case is removed, a long steel chassis is revealed. This has the large components
on one side; the tuning capacitor, IF transformers, audio transformer, trimmer, potentiometers
and valves. On the other side is an insulated board, which carries most of the small components.
The resistors and capacitors are mounted neatly in two lines. The pig tails protrude through
the board, and are joined by wires on the underside. They are then connected to the valve pins
and other components with short wires. Access to the valve sockets pins and the underside
components is difficult. The valve sockets are ceramic.
The radio uses single core plastic coated wire. This breaks when flexed too often.
The wire insulation burns and melts when soldered for too long a time.
The aerial filter and last IF transformer, are contained in aluminium shields. The shield is a
normal punched type, but appears to be mounted upside down, as it is open at the top, and the
covered end is at the bottom. The first IF transformer appears to be two transformers. However,
one of them carries the primary winding, and the other carries the secondary winding. They are
unshielded and mounted close together for coupling. Each winding is inside a ferrite pot core,
and has a tuning slug.
The IF amplifier valve has a circular shield, riveted to the chassis, that comes halfway up
the valve. The potentiometers have the normal three tags attached, one for the wiper and two
for the total resistance. They are different, as the wiper tag is at one end, and not in the
middle of the three tags, as is the norm. Also, the wiper is connected to the metal potentiometer
cover.
The main tuning knob, has a 3:1 reduction ratio. This is adequate for the two middle ranges,
but a greater reduction ratio would benefit the high frequency range. This has a planetary drive,
and a dial with graduations from 0 to 180. These graduations refer to the dial scale on the coil
box that is plugged in, so that the desired frequency can be set. This is a fairly complicated
dial drive, for such a simple receiver. Of note is an earthing wire connected to the reduction
gear.
ELECTRICAL DESIGN
The aerial enters the radio through a banana socket, passes through a rejector
circuit (1730 Khz), and then to the coil box.
For ranges 2 and 3, the coil box contains an RF transformer, resonated by
the variable tuning capacitor. The signal is then capacitor coupled to the mixer valve.
Range 4 is the same, except that the transformer secondary is not tuned.
Range 1 does not have an RF transformer. It contains an LPF (low pass filter).
This is not connected to the variable tuning capacitor. The signal is capacitor
coupled to the mixer valve.
Each coil box also contains another RF transformer, used by the local oscillator
and resonated by the other half of the tuning capacitor. Note that the oscillator
tuning range is a 1.8 ratio of upper to lower frequency. The aerial RF transformer
has to track the oscillator frequency with an IF offset (1730 Khz). This is normal
superheterodyne practice, and used for the coil boxes 2 and 3. The MCR1 does not attempt
this for coil boxes 1 and 4. Coil box 4 has no tuning. Coil box 1 has a frequency
tuning range ratio of 10, which would be difficult to achieve. Instead, the LPF is
used to ensure there is no high image received. This is the "single span" method
(ref Langford Smith and Sturley). The oscillator still has a 1.8 ratio (1880 Khz to 3.33 Mhz),
even though the RF has a 10 ratio (150 Khz to 1.6 Mhz).
The radio only has two types of valve used, a mixer being one type. All the other
functions are performed by a pentode. This would make spare parts supply relatively simple.
You would expect a battery radio with five valves, to have an RF amplifier, or a second IF
amplifier. This radio does not. The first valve V1A is a pentode (1T4 type), which is
strapped as a triode and used as the local oscillator. The mixer V2A (1R5 type) is not
self oscillating, but has the local oscillator injected into its first grid. The manual
notes this different mixer/oscillator arrangement but offers no comment. Langford Smith
talks about the 1R5 having a similar structure to the 6BE6, but the absence of a cathode
complicates a mixer circuit. He mentions that shortwave operation may require neutralization,
to reduce squegging. The external oscillator can be a method to reduce these problems,
reduce oscillator pulling, improve reliability, and allow for ageing components.
The IF signal is taken from the plate circuit, through the dual IF transformers,
to the IF amplifier V1B (1T4 type). The transformer is two individual coils using proximity
coupling. There is a voltage divider across the HT line, consisting of R3B plus VR1B.
This sets the fixed screen voltage of V2A and V1B. The potentiometer VR1B adjusts the
screen voltage of V1C to control oscillation of the detector (REACTION). The potentiometer
VR1A is across the filament supply, and adjusts the grid voltage of V1B to control the RF
gain (SENSITIVITY). This radio has no AGC (Automatic Gain Control).
The signal from V1B plate, is capacitor coupled by C2A to the grid of the detector
valve V1C (1T4 type). There is also coupling from V1B plate via the transformer L2A/L3A
to the screen of V1C which is used for reaction. The plate of V1C is capacitor coupled
to the grid of the audio output valve V1D (1T4 type). The plate of this valve is then
transformer coupled to the low impedance headphones.
The filaments of the five valves are connected in series, and run from 7.5 volts DC.
The positive filament battery connection, goes to V1B, then to V2A, then to V1A, then to
V1D, then to V1C and then to the negative filament and HT battery lines. The filaments are
arranged in this fashion to help with bias. There are resistors across the V1A filament,
the V1C filament, and the V1D filament. The filament and HT negative lines are not connected
to the chassis, but to an insulated rivet on the chassis. This can be used as a reference for
measuring voltages. The chassis/case/earth is wired to nothing, except the bottom of the
aerial winding in the coil box. There is a large capacitor C8B which electrically connects
the chassis to the HT negative line. This is not a problem when using the HT/LT battery.
When using the power pack, this radio becomes a "transformerless radio", so this is why
these precautions have been taken.
RESTORATION
I had two radios to restore, so comparing them made the task easier. One was complete,
but the other was in pieces. The complete example was dismantled and inspected. It appeared
to be un-molested. It was clean and all valves and components were present. The other had
evidence of several changes.
All the valves were removed and tested and all had good emission. The modified radio had
a 3V4 wired as the audio output amplifier. This was changed back to the original design,
using a 1T4. The filament wiring was checked on both radios, with a multi-meter. The modified
radio needed several filament wiring changes corrected. All the wiring was checked, and several
other changes were reversed to normal. It was discovered that both audio output transformers
were open circuit. Both radios were temporarily rewired to bring the plate circuit out to
the headphone terminals. This allowed testing using an external output transformer and speaker.
The resistors were checked, and some were a little high in value, but were left in place.
A 220 and a 470 ohm resistor in the complete radio were replaced, with identical vintage looking
components, of the correct value. A 220, 330, and 470 ohm resistor in the other radio were also
replaced with vintage parts of the correct value. The capacitors were checked for leakage, but
none were changed.
A four pin socket was found and connected to a DC power supply. The correct valves were inserted
and 7.5 volts DC applied. The filament current was measured as normal (53 mA). The filament voltages
were checked and found to be correct on both radios. A speaker and output transformer was connected.
HT (High Tension) was connected and applied slowly, increasing the voltage from zero to 90 volts
DC.
The modified radio consumed normal HT current (6 mA). The oscillator was checked with an
oscilloscope and found to be working. There was some grid hum, but no received stations. A
broken wire was found on the screen of the AF amplifier. This had broken since checking with
the multi-meter.
The unmodified radio had a short circuit on the HT line. This was found in the plug and the
wires were reinsulated. It now consumed normal HT current. This oscillator was checked with an
oscilloscope and found to be working. Stations were heard, but low in volume and distorted.
The signal generator was set to the IF frequency and alignment attempted. On one receiver,
all three tuned circuits aligned initially. When checked a second time, two of the adjusting
slugs suddenly had no effect. The transformers were removed and the ferrite slug was found to
have broken off from the brass adjusting screw. The ferrite slug was re-glued to the adjuster
with epoxy.
The IF transformers were realigned. There was some doubt as to the correct peak, so a sweep
generator was connected. It showed a double peak at the wrong frequency.
After some experimentation, it was discovered that the SESNSITIVITY control needed to be low,
less than 50%, so that noise and the mixer did not affect readings. It was also discovered that the
REACTION control needed to be at about 30%, so that it did not break into oscillation during the
sweep. The changing signal level during the sweep caused it to oscillate at different points.
The Tuning also affected the sweep and provided false peaks. Once the controls were set,
a normal sweep could be performed and a nicely shaped peak could be established.
Both the audio transformers were removed and the laminations taken out. The secondary was
unwound and saved. On one transformer, the primary winding was broken where it was attached to
the pigtail. It was re-soldered to the pigtail, re-insulated, and the secondary was rewound. The
laminations were inserted, the transformer installed, and tested. It performed properly. Primary
turns were removed from the other transformer, until the break was found. It was only about 5%
into the winding, so this wire was discarded. The transformer was re-assembled and worked
satisfactorily.
A signal generator was connected, and the receiver sensitivity was checked, for a 10 dB
signal to noise ratio. The sensitivity was:
Audio on one receiver, was lower, even through the sensitivity was good. It was tracked down
to a broken wire (another one!) that went to the audio output grid. Even though the wire was
broken and short by a millimeter or so, the audio was still getting through, and the valve was
still amplifying, just with lower output.
POWER SUPPLY
There was a mains power supply available with the radio. It supplied 90 VDC and 7.5 VDC.
It could be set for AC or DC operation, and allowed taps to be set for the appropriate mains
voltage. The taps were protected by a cover.
There is a plug panel on one end to allow the selection of the mains voltage. The top row
was for selecting 107, 127, 205, and 235 volts DC. The bottom row was for selecting 107, 127,
205, and 235 volts AC. A small RED plug is screwed into the appropriate hole. There was no
method to determine if the voltage was correct. The operator went to the electricity meter box,
and read the markings, to establish the mains voltage. This information is used to set the tapping.
For AC operation, a plug selects a tap on an autotransformer. From here, is produced a
fixed voltage, connected to a single selenium rectifier. The rectified AC is then filtered
with a two capacitor and choke filter, to produce the DC high tension of 90 volts DC. When
set for DC operation, the plug selects dropping resistors for the mains voltage. The voltage
passes through the selenium rectifier, and the choke filter. The mains connections must be
reversed if the DC mains is the incorrect polarity. The selenium rectifier protects the filter
capacitors, if the DC mains polarity is wrongly connected. The filament supply is merely a
dropping resistor from the HT supply, and a filter capacitor.
The filament smoothing capacitor is not connected, until the radio is plugged in. The filament
load is required. If the power pack was energized and the radio was not plugged in, the filament
filter capacitor would be subject to the full voltage and would fail. To avoid this possibility,
the capacitor is wired through the plug, so that it can not be connected until the radio is
plugged in.
The instructions for setting the mains voltage tapping is problematical. There is some
discussion to verify if it is AC or DC, but nothing to prevent setting the incorrect voltage.
This may result in over voltage on the filaments, or the filter capacitors in the power pack.
The instructions do emphasise that the tapping screw must be screwed all he way in, to operate
the switch. This is to short out a resistor when operating from AC. This resistor acts as a
bleeder for DC operation, but is not required for AC operation.
One of the power packs was in pieces when acquired. There was a filament wiring change
that was corrected. This was probably for the 3V4 that was fitted to the modified radio.
The modern mains cord was replaced with figure 8 cord. The original cord had two banana plugs
for insertion into the mains socket. The new cord had a normal mains plug fitted to it.
The power pack was slowly powered up on AC on a variac, and checked for heating, over several
hours. The voltages appeared to be correct. A 150 ohm resistor was placed across the filament
supply, to draw 50 mA, and the voltage measured at 8 volts DC. A 15K ohm resistor was placed
across the HT supply, to draw 6 mA, and the voltage measured near 90 volts DC. After an hour,
the voltages were checked and found to be correct and the power supply was only warm.
MANUAL
The manual is interesting. It is a reprint offered by the CLYDESDALE SUPPLY CO. LTD, The Radio
Man's Shop, 2 Bridge St, Glasgow, C5, Scotland. Branches in Scotland, North of England, and
Northern Ireland.
There is a fault finding section. It also has practical tips, on how to wear your MCR1 under
a raincoat, and how to operate it in a car. There is a set of tables that relate the dial
graduations to the frequency, for each coil box. There is a list of UK stations and their dial
settings for range 1.
There is also a circuit for an audio amplifier to drive a speaker. The kit of parts was
available, which includes a box, a speaker, a 1A5 valve, all for 5/19/6. It mentions that
the dry batteries, although available then, may not be available in the future. The original
power pack was available for 4/15/0.
The manual has a few errors. The IF frequency is mentioned in error in one place as 1700 Kcs.
The last diagram shows the coil box windings, but the numbering is incorrect.
CONCLUSION
I found the radio not very nice to use. The tuning was good for bands 2 and 3, but finiky
for band 4, as there was backlash evident. Using the scale on the coil box, and the graduations
on the dial, made accurate setting of the frequency difficult. The REACTION control required
getting used to, as it had to backed off to about 75% for AM reception. Operation on CW was
reasonable. Operation on SSB was difficult. The RF gain control affected the local oscillator,
and it could be used to fine tune the radio for resolving SSB and CW. The radio was sensitive,
but there was broadcast band breakthrough on the short wave bands. The reception was broad due
to the IF frequency chosen. There was microphonics when the radio was bumped. There were random
clicks and pops, and the headphones made them very loud.
The design is unusual. It has an external oscillator for the mixer. It uses a 1T4 as the
audio power output valve, instead of a 3V4. It has no AGC or BFO (Beat Frequency Oscillator),
but uses reaction, instead of a 1S5 detector and BFO. It uses "single span" tuning for one
frequency range. The IF transformers are pot cores with single windings, and the shields are
upside down.
Battery operation should be safe, but mains operation may result in over voltage on the radio,
or tingling with no earth connection, or a mains wire reversal. A coil box should always be
attached, to protect the pins on the end of the radio from damage. The practice of plugging
and unplugging the power, to serve as an ON/OFF switch will eventually cause wear and damage
to the cable and connectors.
REFERENCES
WIRELESS FOR THE WARRIOR - Volume 4, Clandestein Radio. Louis Meulstee
Copyright
Range 1
Range 2
Range 3
Range 4
Picture 3: Sockets
Picture 4: Top
Picture 5: Side
Picture 6: Bottom
Picture 7: Other side
Picture 8: Dial
Picture 9: Coil Boxes (1,2,3,4)
Figure 1: Circuit (permission from Louis Meulstee)
Picture 10: LHS - IF Transformer L1A (in pieces) RHS - IF Transformer L2A and L3A (dismantled)
Picture 11: LHS Sweep with two peaks (reaction problem), RHS Sweep with correct peak
Picture 12: Audio Transformer (dismantled)
Range 1
Range 2
Range 3
Range 4
Picture 13: Power Pack
Picture 14: Top
Picture 15: Side
Picture 16: Under
Picture 17: End
Figure 2: Power Supply Circuit (permission from Louis Meulstee)
Picture 18: Switch (operated by screwing the tapping plug)
MIDGET COMMUNICATION RECEIVER M.C.R.1 manual
RADIOTRON DESIGNER'S HANDBOOK, Fourth Edition, F. Langford Smith, page 1001, page1020.
RADIO RECEIVER DESIGN, part 1, 1943, K.R. STURLEY, page 183
Ray Robinson