Marconi CR.100 Release Note

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RECEIVER
Type CR.100
TECHNICAL HANDBOOK
No. T 1868/1
NorE.—The photographs, drawings and letterpress used in this handbook are copyright
Maicom
MARCONI'S WIRELESS TELEGRAPH COMPANY LIMITED
Head Office: Marconi House, Chelmsford • Telephone: Chelmsford
3221 •
Telegraphic Address: Expanse, Chelmsford

RECEIVER TYPE CR. 100/2.
A.
Fine and Coarse Tuning Controls.
E.
H.F. Gain Control.
B.
Band-Change Switch.
F.
L.F. Gain Control.
C.
Aerial Trimmer.
G.
Beat Frequency Oscillator Control.
D. Pass-band Switch.
H.
Operational Switch.

DATA SUMMARY.
TYPE ..
FREQUENCY RANGE . .
SUPPLY REQUIREMENTS
Self-contained Communications Receiver of superheterodyne type
with A.V.C. for use on C\V or Phone reception.
60 kc/s to 420 kc/s
500 kc/s to 30 Mcls
• '  in 6 bands.
• •
(A) D.C. Supplies.
(1)
H.T. and L.T. Batteries.
H.T. 250 volts 100 mA. reduced to
160 Nolts 60 mA. if desired.
L.T. 6 volts 4 Amps.
or
(2)
Battery and Rotary Converter.
6 volts 8 Amps. total supply.
(B) A,C.
Supplies.
200/250 volts 50 c/s. 85 watts.
RECEIVER INPUT 
. .  .. 
100 ohms, balanced or unbalanced, or high impedance aerial.
SENSITIVITY . .

. . 
. . 
For 20 db signal-to-noise ratio on C.W.

(Average Receiver)

60 kc/s to 11 Mc/s. 1 to 2µV.
11 Mc/s to 30 Me/s. 1.5 to 4
µV.
VALVES 
.. 
. .  . . 
See page 9.
RECEIVER
OUTPUTS
Loudspeaker (3 ohms or 1,000 ohms) 2 watts maximum, approx.
Line (600 ohms) of the order of 2 mW.
Phones (high or low resistance) depending on impedance of Phones
and Receiver Edition.
MODIFIED EDITIONS

A number of Editions exist carrying modifications to suit the needs
of various Services, the principal models being CR. 100, CR. 100/2,
CR. 100/4, CR. 100/5, CR. 100/7, CR. 100;8 and CR. 100;8 Mod.
WEIGHT AND DIMENSIONS ..

Dimensions overall : Width 16 in ; Depth 161 in ; Height 12+ in ;
Weight 82 lb.

MARCON/
TYPE CR. 100 SERIES
OF RECEIVERS
SECT.ION 1.
G ENE't AL CHARACTERISTICS.
The receiver when operated from an A.C. supply is self-contained. It combines extreme
flexibility with high sensitivity and select-y:7.y, and is equally suitable for providing good quality speech
output at loudspeaker level as for the recep'..ion of C.W. signals under difficult conditions. it may be
operated from Batteries or Rotary Converter if desired.
SALIEi , ir FEATURES.
(1) Suoitieily and Image Protection.
By the use of two stages of high frequency amplification preceding the mixer, great sensitivity
and prctection against interference on image frequencies are obtained. image protection is of the
order of 30 db. (30 to Oat 23 Mc/s, and is greater than 60 db. (1,000 to 1) on frequencies below 11 MO.
(2)
Se!ectifi4 (Variable).
Protection against adjacent channel interference is made high by the use of a crystal gate and
three stages of Intermediate Frequency Amplification employing coupling circuits of high Q. A low
frequency filter preceding the output valve can be used to reduce further the pass-band width to 100
cycles if desired.
(3)
Automatic Volume Cont.:ol.
A.V.C. may be used on both phone and C.W. reception, and a suitable delay voltage and time
constants have been provided.
(4)
CLI:brF.tion and Log Bing Scales.
Easy tuning and accurate re-setting to any known frequency are ensured by the iliuminaLeci
scale co'dbiated directly in frequency, and the separate Logging Scale which has a high discrimination
and is driven from the same control spindle.
(5)
Side-Tone Facility.
Facilities are provided on certain models for muting the receiver during transmissions from
associated equipment.
( 1 )

(in
O
O
SECTION 2.
OPERATION.
Assuming that the receiver has been correctly installed the following instructions give all the
information essential for its correct use. Most of this information is also to be found in concise form
on a bound card supplied with later receivers.
Fig. 1. The numbered controls correspond to the numbers in brackets below.
PRELIMNARY ADJUSTMENTS FOR OPERATING THE RECEIVER.
(1)
MAINS SWITCH to ON. Dial lamps should light up. Allow a few minutes for warming up,
and longer if possible. This switch is not in circuit if operating the receiver from batteries.
(2)
OPERATIONAL SWITCH to " C.W.—A.V.C."
(3) PASS-BAND SWITCH to " 3,000 cis."
(4) H.F. GAIN at or near maximum clockwise.
Fig. I. Panel Controls.
L.F. GAIN adjusted to give a comfortable level of noise in phones or loudspeaker.
BAND-CHANGE SWITCH to the frequency band required.
TUNING. Adjust the pointer on the calibration scale to the desired frequency by the larger tuning
knob, and rock the smaller knob
slowly,
about one revolution on either z,ide, until the carrier of
the wanted station is heard. If RiT (telephone) is to be received change the Operational Switch (2)
to " MOD—A.V.C." and retune slightly if necessary.
B.F.O. Do not use the B.F.O. for fine tuning. When the receiver has warmed up (15 minutes
or longer if convenient) this should be adjusted and thereafter not varied, except for occasional
checking. See General Notes below.
AERIAL TRIMMER. If signals are weak this should be adjusted for best results. The optimum
setting for any frequency in use is found by reducing the H.F. Gain to a level at which strong
incoming signals do not sump the amplifiers or work the A.V.C., and then adjust for maximum
signal strength or maximum 1st circuit noise. The setting is more eritieed and important at the
high frequency end of the band in use.
( 2 )
(5)
(6)
(7)
(8)
(9)

GENERAL NOTES ON OPERATING THE RECEIVER.
Adjustment of Beat Frequency Oscillator.
To enable full use to be made of the selectivity of the receiver the B.F.O. must be accutately set
to give the optimum beat note of about 1 kcfs in the phones when the receiver is accurately tuned to the
signal. A convenient method of finding this setting is to make use of the characteristics of the L.F.
Filter, which is adjusted to have maximum response at this frequency. Proceed as follows : After
the preliminary warming up period put the Pass-band switch to 100 cis, the Operational switch to
C.W.--MAN and both gain controls at or near maximum so as to have a high level of receiver noise.
Adjust to a frequency where no signals are received, or disconnect the aerial. For maximum receiver
noise choose the high frequency end of Band 4. Rotate the B.F.O. Control knob to one of the points
at which the maximum rinning noise is heard, and leave it in this position. Checks should rarely be
necessary if the receiver is left switched on for long periods.
Use of Pass-Band Switch.
Once the wanted signal has been found the operator should endeavour to use a narrower pass-
band so as to get greater protection from interference. A very slight and careful readjustment of tuning
may be necessary when switching to the narrower pass-bands.
6,000 c/s Pass-band. Gives best intelligibility of speech, and makes tuning broader, but can only
be used when signals are strong and there is little interferenee. Used for C.W. signals only
in exceptional citcumstances.
3,000 c/s Pass-band. Better selectivity and less background noise. Recommended for use
on speech and when searching.
1,200 c/s Pass-band. High selectivity. Not used on speech. Useful chiefly on Bands 4, 5
and 6.
300 c/s Pass-band Higher selectivity. Used on C.W. only, chiefly
on
17.!:vids 1, 2, 3 and 4.
Can be used with care on Band 5, but not recommended for use on Band 6.
100 cis Pass-band. Hi Lest possible selectivity. For use on C.W. only, chiefly on Bands 1
and 2. If used on Bands 3 and 4 greater care in tuning is demanded as I the correct setting
of B.F.O. becomes increasingly important. Not recommended on Bands 5 and 6.
The narrowest pass-bands can only be employed where the frequency of the transmitted signal
is reasonably constant, and the speed of sin-nallin.
,
relatively low.
Vnen receiving C.W. on the broad pass-bar;ds and tuning through zero bmt it will be found that
the signal is equally strong on both sides, but en narrower pass-bawds one side will give a stronger
note than the other. The weaker signal may even be inaudible. Always tune to the stronger of the
two.
Should a signal be tuned-in on a wide pass-band and the wrong side of zero beat be selected,
the signal will probably be lost if the operator decides to change to a narrow paso I:and. Because of
this and the difficulty of distinguishing the stroneer signal it is recommended that the 6 kc/s pass-band
should not normally be used when tuning-in or searching for a C.W. signal.
Use of A.V.C.
The use of A.V.C. will be determined chiefly by the conditions prevailing and the shill of the
operator. It is given as a general rule,however, that A.V.C. should always be 
except whore the
wanted signal is weak and in danger of being lost because the gain has been reamed by the at tion of a
strong interference on the A.V.C. diode. Examples of this are peaky types of static, and pulse trans-
missions.
The II.F. Gain must be controlled ?nanually when transmitting on the same frevency, wiless the
receiver is being muted by the Side Tone Facility (as in the Type CR. 100/2
receiver) or other effective
device.
On C.W. the A.V.C. is given an increased recovery tine-constant. This will tend to broaden
the apparent selectivity when searching tnrousgh strong signals, and thus make tuning slower. To
switch off A.V.C. put the Operational Switch to MAN
( 3
)

Use of Gain Controls.
The relative positions of H.F. and L.F. gain controls will depend largely on whether the operator
is controlling the gain manually to prevent overloading of the H.F. and I.F. amplifiers or relying on the
A.V.C. to do it. The rule laid down for general guidance is
On C.W.--A.V.C..kH.F. Gain Control at maximum, except for very strong signals.
or MOD.—A.V.C. f L.F. Gain Control as desired.
On C.W.—MAN. H.F. Gain Control as desired.
or MOD.--MAN.
f L.F. Gain Control at approximately mid position.
" Off " Position of Operational Switch.
Frequency drift of the oscillator due to temperature fluctuation is reduced to a minimum if the
receiver is left switched on for long periods.
During short stand-by periods the H.T. to certain stages may be cut off by putting the Operational
switch to the OFF position.
Use of Logging Scale.
Fig. 2. Logging Scale.
This scale is not for reading frequency directly but for noting how to reset the tuning control
to a station which has once been found. Some slight allowance for initial drift should be made if the
receiver has not been running for 2 hours or more.
The main divisions 0 to 25 are read on scale " A " and sub-divisions on scale " B." 1 he example
shows a reading of 12-61.
(4)

SECTION 3.
TECHNICAL DESCRIPTION.
(A) ELECTRICAL.
CIRCUIT ARRANGEMENT. (See Drg. WZ.1943, page 47.)
Two signal frequency stages of amplification employing variable-mu valves arc used before
the mixer valve, which is of the triode hexode type. The triode section is not used, and the frequency
change oscillator is a separate valve. The mixer output voltage at the intermediate frequency of
465 kc/s ± 2 kc/s is applied to the I.F. amplifier embodying three stages, with suitable couplings and
a crystal gate for varying the band width. The third T.F. amplifier operates a double-diode-triode
incorporating the signal detector, the automatic volume control rectiner and the 1st audio frequency
amplifier. An independent beat frequency oscillator is provided for continuous wave reception and is
coupled to the signal detector.
An output stage and the mains rectifier valve and associated circuits complete the circuit plan.
Aerial Input, Signal Frequency Circuits and Mixer.
The overall gain of the signal frequency stages does not vary greatly between bands, and is
sufficient to make shot noise from the first valve greater than that geeerated by the mixer valve. Up
to a frequency of about 11 Mc/s the dynamic impedance of the first tuned circuit is high enough to
ensure that thermal agitation noise from this circuit exceeds valve noise, but on Band 6 the circuit
noise has fallen below valve noise, and the sensitivity is then limitcd by the first valve noise alone.
The feeder or dipole connections are taken via double-pole contacts on the aerial section of
the Band-change switch to the low impedance winding of the aerial coupling coil. In the case of the
Types CR. 100, CR. 100/4 and CR. 100/7 receivers the input is suitable for a balanced input if desired,
there being two terminals marked " D.' For the Types CR. 100/2, CR. 100/5 and CR. 100/8 receivers,
one end of the coupling coil is earthed and the other connected to a coaxial socket marked " D."
The coupling is designed to present an impedance of approximately 100 ohms on all bands. The
secondary winding with the rear section of the main tuning condenser and the aerial trimmer condenser
in parallel form the grid circuit of the first amplifier.
The aerial socket or terminal " A " is connected to the top of this circuit through a capacity
of 10 pf, and a static leak of 2 megohms to earth is provided.
The grid connections of signal frequency and 1st Oscillator valves contain stabilizing resistances,
the value being 10 ohms for V1,
t
'V2 and V3, and 50 ohms for V4.
The rear plate of the rotor of the grid circuit Band-change switch short-circuits the idle
inductances while the front plate selects the active one.
The bottom of the grid circuit of the two Signal Frequency Amplifiers and the first two inter-
mediate Frequency Amplifiers is joined to the A.V.C. line through a .30,000 ohms resistance, the top
end of which is decoupled by a condenser of 0.1 ill
, " to earth. The &coupling condensers, both for
grid and anode circuits of Signal Frequency Stages are situated in the P..
..F. Coil Astembly,
The cathodes of all signal frequency and intermediate frequency valves have series resistances
of 400 ohms or the preferred value of 390 ohms ± 20 per cent., and are decoupled by 0.1 tiF
condensers to chassis.
The screen grid voltage of signal frequency amplifiers and other Salves is taken from a line fed
from a voltage divider and is of the order of 80 volts. Each valve Las its own screen grid decoupling
resistance and condenser.
The coupling from the first to second amplifier is by the conventional untuned anode—tuned
grid transformer, the anode being decoupled from the H.T. supply by a resistance of 2,0
1
)0 ohms and
0.1 aF condenser.
Each grid circuit inductance is fitted with a trimmer conde aser. All inductances in the coil
pack have adjustable iron dust cores. The second signal frequency str se is similar to the first and drives
the 1st grid of the mixer. The mixer cathode resistance is connected to earth and the hottom of grid
circuit is also earthed ; there is therefore no manual or automatic control of bias on this valve. The
triode anode of the X66 is earthed.
(5)

Oscillate)? aad Into rnet"ate Frequency Circuits.
A KTW.62 valve coanecred as a triode and having a tuned grid circuit mutually coupled to an
untum4 anode coil serves as the frequency change oscillator. The oscillator operates at a frequency
higher than the sirs al fr-ttaanicy on all bands. Series tracker condensers are contained in the coil
pack in addition to the meal decoupling condensers. On the three lowest frequency bands variable
trimmer condensers ar. uttad to the oerifiator inductanc,s, but on Bands 4, 5 and 6, where very snveil
values of capacity are required, fixed capacitors are connected instead, the required value being
determined on test.
The tuned anocla circuit of the mixer is decoupled from the H.T. supply and forms the first
side circuit of the cry.-.t4 resonator. The second side circuit is also tuned and is coupled to the 1st
interrayliate fraquency
vane.
The crystal whose frequency may be between 463 kcis and 467 kcis,
is neutralised, a fraction of the voltage present in the 2nd side circuit being fed back into the 1st side
circuit in the correct phase. The neutralising condenser is situated in I.F.1 assembly and the crystal
is moanted in I.F.2 assembly, The crystal is not in circuit when using the 3,000 cis and 6,000 cis
positions of the Pass-band switch.
The normal band-width with the crystal in circuit is 1,200 cis. By switching the condenser C.43
(value 7 pF) from the first to the second side-circuit and altering the phase and impedance the band-
width is reduced to 300 cis.
The couplings between the three intermediate frequency amplifiers consist of loosely coupled
tuned transformer of high Q, both primary and secondary coils having variable permeability tuning
and fired value capacitors. A si all, tightly couplet: aueitiary coil is introduced in series with the
secondary of the I.F. t: ansformers, 1.F.3 and I.F.4, when the Pass-band switch is placed to 6,000 c/s.
The rain of the 
amplifiers is of the order of 28 db per stage (25 to 1). The conversion gain
of the mixer is about 16 db (6 to 1).
I.F. Sensitivity ft sues will be found in Section 5, page 30.
The selectivity switches S.10, S.11, S.12 and S.15 in Diagram WZ.1943, page 47, are all operated
by a common spindle.
All switches are shown on the diagram in the counter-clockwise pcsition.
Signal ratect.or
 A.V.C. Rem tiler.
The signal detector is driven from the secondary of the last I.F. circuit, and works into a series
connected load of 0.3 M fI with suitable low-pass filtering components, a portion of the rectified
voltage being applied to the L.F. gain control potentiometer via a capacity of 0.1 aF. The slider of
the potentierneter is connected to the grid of the triode section of the DH.63 valve and the bottcra is
tapped on to a suitable biasing point of the cathode resistance, giving a voltage of 1.8 veils negative
with respect to the cathode.
The auto-gain voltage is derived from the second diode of the DH.63 driven from the anode of
the 3rd I.F. valve via a capacitor of 100 pF. The A.V.C. diode load of 0.5 M 0 is in shunt with the
valve. In order to have the necessary delay the cathode is biased positive by about 16.8 volts relative
to the anode. Suitable decoupling is provided, and an additional capacity of I al 7
for increasing the
recovery time constant is introduced when the Operational switch is placed to C.W.
The time constant is 0.1 sec. on MOD, and 1.0 sec. on C.W.
Beat Frequency Oscillator.
The B.F.O. is of the electron-coupled Colpitts type, and is coupled to the signal detector through
a capacity of 30 pF. Tie core cf the inductance is adjustable as in the I.F. circuits, and a variation of
several kilocycles above and be'ow the intermediate frequency is obtained by the variable condenser
which is under the control of the operator. The correct adjustment of the B.F.O. condenser is a
matter of importance when using narrow pass-bands and is dealt with in Section 2, page 3.
( 6 )