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MODULATED
OSCILLATOR
C1070
A.W.A.
Modulated Oscillator
Type C1070
THE PURPOSE OF THE INSTRUMENT.
This instrument has been designed for use in servicing all kinds of radio receivers.
Modulated or unmodulated signals of any frequency between 100 Kilocycles and 20
Megacycles are available.
THE FOLLOWING TESTS MAY BE CARRIED OUT RAPIDLY AND WITH
AMPLE SERVICE ACCURACY:
1. Alignment of I.F. and R.F. circuits at any desired frequency.
2. Adjustment of receivers provided with wavelength or kilo-cycle scales to
correct dial calibration by setting trimmers and padding condenser.
3. Examination of ganged T.R.F. circuits for errors in tracking
4. Measurement of overall sensitivity of all types of receivers at any frequency,
and gain of I.F. amplifiers.
5. Estimation of noise level at higher sensitivities by comparison of audio
outputs between modulated and unmodulated carrier of equal strength.
6. Determination of stage gain in I.F. or R.F. amplifiers.
7. Testing of valves for performance under working conditions by insertion of
several in succession in a given socket in a receiver and noting the change in
stage gain or overall sensitivity.
8. Checking the performance of A.V.C. in receivers.
9. Measurement of selectivity of I.F. or R.F. amplifiers in terms of band width
in kilocycles, for input signals one hundred or one thousand times larger than
the signal on tune required to give some chosen value of audio output.
10. Determination of image ratio, or the ratio of the micro-volts input at the
image or second spot frequency at the micro-volts at the wanted signal
frequency, both giving equal audio output.
This compressive list of tests covers more than is usually required in receiver servicing.
None of these tests requires an instrument with capabilities of accuracy available in
a signal
8
1 CONTROLS
generator. Consequently the controls and methods of calibration can he modified in the direction
of speeding up and generally simplifying operation. This object has been the aim in the design of The oscillator has four controls (from left to right):
the Type C1070 modulated oscillator, and has been achieved with but small reduction in accuracy
below average signal generator performance. No. 1. Range Switch. This control selects the coil for use on any desired range.
No. 2. Battery Switch. The battery switch has three positions.
Design and Performance
(i) The first position, "OFF"; both the A Battery and B Battery circuits are open.
(ii) In the second position, "MOD. OFF," the A Battery is connected to both
FREQUENCY RANGE. valves but the B Battery is connected to the R.F. oscillator only.
(iii) In the third position, "MOD. ON," the B Battery is connected to the
100 K.C. to 20,C00 K.C. in six ranges, selected by a switch. Modulator valve also; thus modulated and on modulated carrier are
Range A, 96 K.C. to 250 K.C. available.
Range B, 240 K.C. to 600 K.C.
Range C, 560 K.C. to 1420 K.C. No. 3. Frequency Selector. By turning this knob any desired frequency throughout the
Range D, 90 metres to 220 metres (1,360 K.C. to 3,330K.C.). range corresponding to the setting of the range switch may
Range E, 37 metres to 96 metres (3,120 K.C. to 8,100 K.C.) he obtained. Use the scale corresponding to the setting of the Range Switch.
Range F, 15 metres to 38 metres (7,900 K.C. to 20,000 K.C.)
No. 4. Attenuator. By turning this knob the amount of the output signal may be adjusted
All ranges are directly calibrated on a rotovisor type dial: A, B and C are marked in to any desired level. The approximate value of this signal may be
kilocycles and D, E and F in metres. estimated from the data given on page 3.
Calibration accuracy is better than 1per cent on range C. and within 2 per cent on the other ranges. OPERATION.
1. Set the modulated oscillator about one foot to the left of the receiver under test.
OUTPUT.
The attenuator calibration indicates directly the approximate microvolts output on Ranges B 2 Connect the output cable to the terminals of the oscillator. Connect the braided lead with
and C. It is calibrated 1, 3, 10, 30, etc., up to 100 M (100,000) with two extra markings not coloured tracer in the braiding to the insulated terminal of the oscillator by means of the
carrying accompanying figures. The one adjacent to the 100,000 Mark is 300,000 and the Inst one spade terminal. Connect the plain lead to the earthed terminal.
is the extreme position for the pointer when turned clockwise. 3. Connect the other end of the cable to the receiver under test. The plain lead is to be
connected to the earth terminal and the other lead (tracer) to the aerial terminal or grid
The approximate output signal on the other ranges may be estimated from the following table cap of a valve as required. When connected to the grid cap it is necessary to connect a
Range Multiplying factor for Attenuator Calibration resistor between the grid cap and earth to complete the bins circuit (about 250,000 ohms
or larger is suitable).
Range Multiplying factor for 4 Connect an Output meter in the plate circuit of the last valve. See Fig. 2.
Attenuator Calibration 5 Turn the battery switch to the "MOD. ON" position and select the desired test frequency
A 1.4 by means of the range switch and the frequency control.
B 1 6. Tune the receiver to the frequency of the oscillator and adjust the attenuator to give a
C 1 convenient deflection on the meter. Any tests listed on page I may now be carried out.
D 0.5
E 0.2 CAUTION.-Be sure that the Battery Switch is turned "OFF" when the oscillator is
F 0.08 not in use.
7
2
the average maximum R.F. output on each hand is therefore:
A ............................. 400 Millivolts
B ............................. 300 "
C ............................. 300 "
P ............................. 150 "
E ............................. 60 "
F ............................. 25 "
The attenuator reacts noticeably upon the carrier only at the higher outputs. The frequency
change caused through advancing the attenuator control from low outputs up to the
position marked l00M is approximately as follows:
High frequency end of hand 0.3 per cent.
Middle of band 0.1 per cent.
Low frequency end of band 0.05 per cent.
On position 300 M the shift of carrier frequency will be three times greater, as the
change of frequency is directly proportional to the indication of the attenuator control.
It is important that the special shielded cable supplied should be used with the
oscillator always, otherwise the above figures for R.F. output on each hand will not hold.
This cable has the correct characteristics to suit the attenuator, and to behave as a dummy
aerial on the broadcast ranges (for alignment purposes only).
Leakage signal through the attenuator cannot be detected except at about 20 M.C.
where it is of the order of less than one microvolt. The relative ratios of attenuation as
marked on the attenuator scale are almost unaffected by the value of the operating radio
frequency.
MODULATION.
The modulator oscillator is adjusted to approximately 400 C.P.S. by adjusting the air
gap of the TAl060 Transformer. The modulation depth is set at about 30 per cent. on the low
frequency bands by means of a resistance in parallel with the "tank" circuit of the audio
oscillator. The only serious departure frown the figure of 30 per cent. for the modulation
depth occurs on band F where it rises to about 45 per cent. The battery switch has 3
positions, "OFF," "MOD. OFF," and "MOD. ON," allowing the radio frequency output to be
obtained free from modulation in the middle position.
CONSTRUCTION.
The coils and wave change switch together with the wiring and the oscillator valve socket
are individually screened, and form together a complete assembly. This assembly, the
variable condenser and modulator oscillator system are mounted to a rear panel. In this
way radio
frequency currents in this panel cannot give rise to external stray fields due to the shielding
6 effect of the front panel and outside metal case.
4
3
The variable condenser is operated by a smooth eight to one reduction drive situated
between the two panels. The piston of variable capacitance attenuator also lies between the two
panels and is of special design. It contains no fragile electrical parts, as in the resistance type
attenuators, that may require replacement.
At one end of the brass tube is placed an insulated metal disc that is connected to the
tank circuit of the R.F. Oscillator. A second and similar disc is carried on the face of an insulated
piston, and is connected to the output terminal on the front panel.
The batteries are strapped in the rear of the case, and are wired to a terminal panel at the side
where clearance holes (insulated) are provided for insertion of the prods of a voltmeter to check
voltages without opening the case.
EQUIPMENT.
Valves. Two type 30 Radiotrons are used - one as R.F. oscillator and one as 400 cycle Audio
oscillator.
Batteries. Four 4.5 Volt Bias Batteries (drain 60 M.A.) and one 60Volt Light Duty Battery (drain
3 to 5 M.A.) are required, The Bias Batteries are connected in parallel to operate the filaments in
series.
The condition of the batteries may be checked with a voltmeter through the holes in the side of the
case. UTPUT METER.
To make satisfactory use of the modulated oscillator it is necessary to connect an output
Satisfactory Voltage Limits are as follows: meter to the receiver under test.
"A" flattery 3.5 to 4,5 Volts. A standard rectifier type of output meter is most convenient, or a conventional design of
"B" Battery 50 to 60 Volts. valve voltmeter may be used.
It is not desirable to continue to use batteries of voltages below these limits. An economical substitute is suggested as follows:
To instal or change batteries or valves proceed as follows: A standard D.C. voltmeter of 1000 ohms per volt may be used in conjunction with a valve
1. Turn the tuning control (the knob second frown the right) so that pointer indicates operating as a diode rectifier. The valve may have a filament type or indirectly heated type.
100 K.C. on range A, otherwise the tuning condenser may be damaged. The plate and all grids are connected together to form a diode plate.
2. Lay the oscillator on the bench face up, and take out the screws round An output meter may be constructed according to fig. 2. Using a D.C. voltmeter of ranges 0-
the edge of the front panel. 3. 0-30 and 0-300 volts and resistance 1000 ohms per volt the following A.C. voltage ranges
3 Lift the oscillator out of the case by taking hold of the edge of the front were obtained using a type 76 and a type 30 valve:
panel.
4. See that a good 30 valve is held firmly in each socket. D.C. Range. R.M.S. A.C. Ranges
5. Assemble the batteries in the back of the case. See that they are held firmly
with the straps provided. See diagram Fig. I. 30 76
6. See that the battery switch is turned to "OFF" 0-300 0-250 0-290V.
7. Connect the batteries according to the diagram Fig. I 0-30 0-33 0-35V.
8. Reassemble the oscillator in the ease. 0-3 0-6.5 0-9.0V
5
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