Conar 280 Manual

Operating and Maintenance
Instructions for the
CONAR
Signal Generator
Model 280
Price $1.00
QUALITY EQUIPMENT BUILT ON A HALF CENTURY OF SERVICE IN ELECTRONICS

CONAR
Model 280 Signal Generator
Operating and Maintenance Instructions
SPECIFICATIONS
OutputAmplitude:
BandsA, B, and C:
at least 1 volt peak to peak.
Bands D, E, and F:
at least 20,000 microvolts.
Audio:
at least 5 volts peak to peak.
RF and Modulated RF continuously
variable within six bands.
BandA170 kc to 550 kc
Band B 550 kc to 1600 kc
Band C 1.6 mc to 5.0 mc
Band D 4.5 mc to 15 mc
Band E 15 mc to 30 mc
Band F 30 mc to 60 mc
7-l/2" high x 9-7/8" wide x 6-1/2" deep
9 lbs (shipping weight)
115 volts, 60 cyclesAC, 10 watts
(1) 6BE6, (1) 12AU7
Power Transformer and Selenium rectifier.
RF; 170 KC to 60 MC,
RF Modulated; 170 KC to 60 MC by 400 cps,
Audio; 400 cps audio signal
Cabinet Size:
Weight:
Power Requirements:
Tube Complement:
Power Supply:
Output:
Accuracy: Better than 3% after calibration.
Attenuator: Course: High-Low switch.
Fine: Continuously variable potentiometer.
Output Cable: Permanently attached,
low loss coaxial, terminated in alligator clips.

Operating Instructions:
DESCRIPTION OF CONTROLS
Function Switch:
Band Switch:
Tuning Control:
Attenuator Control:
High-Low Switch:
Output Cable:
HOW TO READ THE SCALES:
The CONAR Model 280 Signal Generator is easy to
operate. The front panel labels for the controls and
scales enable you to set up the instrument for the
output signal you desire. The following discussion of
the controls, the scales, and the circuit is included to
help you become thoroughly familiar with the
instrument.
The operating controls of your CONAR Signal
Generator are conveniently grouped for easy
operation. Let's consider the function of each control
as it relates to the operation of the instrument.
This three-position switch, marked MOD, RF, and
AUD is used to select the type of output that you get
from the instrument. in the AUD position, the output
is a 400-cycle sine wave used for testing audio
equipment. In the RF position, the output is an
unmodulated RF signal at the frequency determined
by the band switch and tuning control, in the MOD
position, the output is an RF signal amplitude
modulated by the 400-cycle audio signal.
This six-position switch is used to select the band of
frequencies produced by the generator. Each position
is marked to correspond with one of the six scales on
the dial. The bands cover the following frequencies:
BandA170 kc to 550 kc
Band B 550 kc to 1600 kc
Band C 1.6 mc to 5.0 mc
Band D 4.5 mc to 15 mc
BandE15mcto30mc
Band F 30 mc to 60 mc
*1600 kc and 1.6 mc are the same frequency.
The tuning control moves the pointer across the dial
and adjusts the frequency of the generator within the
band of frequencies selected by the band switch, The
control has a vernier type drive so you can easily make
accurate settings of the output frequency.
This is a continuously variable potentiometer used
for adjusting the size of the output signal. An "ON-
OFF" switch is ganged to this control. In the extreme
counter-clockwise position, the switch is "OFF" and
the instrument is de-energized. As you rotate the
control clockwise from the OFF position, you will
hear a slight click. This turns the instrument ON. As
the control is turned further clockwise, the signal
generator output increases.
This is a coarse attenuator for varying the signal
generator output over a wide range. The HIGH
position is mainly used when testing a receiver or
when you want a very large signal to drive through a
defective or badly misaligned stage. The LOW
position is normally used in alignment work because
you need to keep the output signal at a LOW level.
The attenuator control is then used to further attenuate
the signal to the desired level.
Acoaxial cable comes through the center of the front
panel under the tuning control; The cable is
permanently attached to the attenuator circuit so that
the cable cannot be misplaced. The shielding of the
cable confines the output signal to the center
conductor of the cable. This enables you to inject the
signal where you want it in the receiver and prevents
unwanted signal radiation,
The tuning dial has six scales, labeled at each end
with letters corresponding to the positions of the band
switch. Each scale covers a given band of frequencies.
For example, when the band switch is set at positionA,
readings are made on scale A of the tuning dial. Note
that the scales A and B are marked in kilocycles
(thousands of cycles), while the other scales (C, D, E,
F) are marked in megacycles (millions of cycles). It is
easy to convert the frequency from megacycles to
kilocycles. All that needs to be done is to move the
decimal point three places to the right. For example,
1.6-mc is 1600-kc; 1.8-mc is 1800kc, etc.
As an aid in selecting the correct setting of the band
switch, the lowest and highest frequencies covered by
each band are indicated at the extreme ends of each
corresponding scale on the tuning dial. Example:
Beneath the left end of Scale A is printed 170-kc.
Beneath the right end of Scale A is printed 550-kc.
Therefore, if any frequency between 170-kc and 550-
kc is desired, the band switch is set to position A. The
tuning control is then turned so that the red "hairline"
on the plastic pointer is directly over the desired
frequency on scaleA.
Where a number representing the desired frequency
is directly marked on the dial, the transparent pointer

is moved over the dial until the hairline is directly over
the long marker line representing that frequency.
If the signal generator is to be set to a frequency not
directly labeled on a given scale, then the short lines
between the longer marker lines are used. For
intermediate readings, an estimation is made between
the short lines. The values represented by each
division (short line) between calibrated or marked
lines for the different bands are given below:
BandA, each division represents 10-kc.
Band B, between 550-kc and800kc, each division
represents 10-kc.
Between 800-kc and 1600-kc, each division
represents 20-kc.
Band C, each division represents .05-mc or 50-kc.
Band D, each division represents .1-mc or 100-kc.
Band E, each division represents .1-mc or 100-kc.
Band F, each division represents .2-mc or 200-kc.
Let's take an example of selecting a desired
frequency. Suppose you want the signal generator to
produce a frequency of 1430 kc, Referring to the main
dial scale, we see the minimum frequency covered by
band B is 550 kc and the maximum frequency is 1600
kc. Since the frequency we want falls between these
limits, the band switch is set at position B which
covers the standard AM broadcast band of
frequencies. Next we find the line marked 1400 kc on
the B band. The next higher marked line is 1600 kc.
Halfway between 1400 and 1600 we find a long line.
We know this line is 1500 kc because It is halfway
between 1400 and 1600. The short graduations
between 1400 and 1500 mark off five spaces. Each
space must represent 20 kc because the five spaces
cover the 100 kc from 1400 to 1500. Therefore, the
first graduation above 1400 kc is 1420 kc and the
second graduation is 1440 kc. Since we want the 1430
kc, we move the hairline of the pointer halfway
between the first and second graduation above 1400
kc. The signal generator is now set to produce 1430 kc.
When setting this plastic pointer to a given
frequency, be sure that you are looking directly
toward the pointer and dial scale. If your eyes are not
squarely in front of the pointer, an error in reading the
scale, (called parallax error) will result. Be especially
careful when placing the signal generator at one side
of the receiver under test. There is a tendency to read
the dial from an angle.
The CONAR Model 280 Signal Generator uses
simple, reliable circuitry to produce an RF, modulated
RF, or audio output. The RF signals are generated In
six bands by switching different coil and capacitor
combinations to the oscillator tube. Within each band,
the frequency is continuously variable by adjusting
the tuning capacitor. The dial pointer is attached to the
tuning capacitor to indicate the exact frequency
within the band. The RF signal from the oscillator is
coupled to a cathode follower output stage. The
output signal then passes through an adjustable
attenuator to the output cable.
The modulated RF output is produced by
modulating the RF signal In the oscillator stage. A
400-cps (approximately) audio signal is applied to the
mixer grid of the RF oscillator stage to amplitude
modulate the selected RF signal. The modulated RF
signal is coupled to the cathode follower in the same
way as the RF signal.
A fixed frequency (approximately 400 cps) audio
signal is generated by an audio oscillator stage. When
the audio output is selected, the signal is applied to the
cathode follower, passes through the attenuator
circuit, and appears at the output of the output cable.
As previously described, the audio signal is also used
internally to modulate the RF signal when the
modulated RF output is selected.
The circuit of the Model 280 is shown in Fig. 1. The
Hartley type oscillator circuit was selected for its
stability. Six separate coils and capacitors form the
oscillator tank circuits. A different tank circuit is
switched to the oscillator tube for each of the six
bands of frequencies. The band switch, S1, shorts out
all the coils except the coil for the selected band. A
ganged variable tuning capacitor varies the frequency
over the selected band. The tuning capacitor, C7, has
two sections, one section for the four low bands and
one section for the two highest bands. This
arrangement assures a favorable L-C ratio for the
oscillator circuits over all the frequency bands.
The RF oscillator tube, V1, is a 6BE6 pentagrid
converter tube. This tube functions as an electron
coupled oscillator. The cathode, the first control grid
(pin 1), and the screen grid (pin 6) act as a triode to
operate the RF oscillator, The electrons that pass
through the grids and reach the plate are the electron
coupled output from the oscillator, in this way the
load placed on the output of the oscillator does not
Avoid Parallax Error:
CIRCUIT DESCRIPTION

affect the frequency of oscillation. Grid 3 (pin 7) is
used to amplitude modulate the oscillator frequency.
A 400-cycle signal is applied to this grid when the
function switch is set to the modulated RF position.
The oscillator output from the plate of V1 is coupled
through C9 to the grid of the cathode follower. The 1-
2-3 section of the 12AU7, V2, forms the cathode
follower output of the signal generator. The use of a
cathode follower provides a low impedance output
from the signal generator and acts as an additional
guard to prevent the output load from affecting the
frequency of the oscillator.
The 6-7-8 section of V2 and the associated
components form a Colpitts type audio oscillator. L7,
C12, and C13 are the frequency determining elements
of the circuit. The oscillator produces a sine wave at a
frequency of approximately 400 cps. In the
modulated RF position, this signal is fed through R6,
C14, and S3 to grid 3 (pin 7) of the oscillator tube. In
the audio position, the 400-cycle signal is fed to the
grid of the cathode follower. Notice that In the audio
position, B+ is removed from the plate and screen of
V1 so there is no RF output.
The output from the cathode follower is fed to the
output cable through an attenuator network. The 3K
potentiometer, R11, provides a continuous variation
of the output, High-Low switch, S4, provides a large
step-change In the output signal, Both the High and
the Low output are continuously variable, The output
from the attenuator network is coupled through C17
to the output cable. This capacitor isolates the signal
generator circuitry from the receiver you are working
on. Thus it is safe to inject a signal to the plate of a tube
in a receiver even though the plate has B+ voltage on it.
The output cable is a low capacitance coaxial cable.
The grounded coaxial shield prevents signal radiation
from points other than the output clip on the end of the
cable.
The power transformer provides isolation of the
instrument from the power line. The voltage from the
high-voltage secondary on the transformer is rectified
and filtered to provide B+ for the instrument. The
low-voltage secondary provides filament voltage for
the tubes.
Your CONAR Signal Generator can be used for
testing and aligning AM and FM radio receivers. The
400 cycle output is a convenient test signal for use on
all types of audio equipment.
If you have never done any alignment work you will
want to try out the instrument before you use it to
repair or align a defective receiver. The section on
alignment in this manual is to be used only as a
general guide. When aligning a specific receiver we
recommend that you follow the manufacturer's
instructions,
Skill in aligning all types of receivers takes practice
and experience. One of the best ways to start getting
this experience is to practice alignment on receivers
that are already properly aligned. Set up the
equipment and follow the alignment steps without
making any adjustment to the receiver. This will give
you practice injecting signals and observing
indications. Next, you can move adjustments In the
receiver and observe the effect. This will give you a
feel of how the adjustments should respond. Mark the
position of the adjustment before you move It so you
can put it back in case you are on the wrong
adjustment. Most inexpensive AC-DC radios are
slightly out of alignment even when new. Realigning
several of these sets can give you valuable experience
in using your instrument.
Alignment is the process of adjusting tuned circuits
to respond to a desired frequency or band of
frequencies. In a superheterodyne radio receiver, the
tuned circuits consist of the RF stages, the IF stages,
and the local oscillator circuit. The RF stages are
aligned to produce maximum response to the band of
frequencies covered by the particular receiver. The
local oscillator must be adjusted so it will "track".
That is, the local oscillator must produce the correct
frequency at all settings of the radio tuning dial. The
correct frequency is the frequency that when mixed
with the incoming signal from the desired radio
station produces a fixed intermediate frequency. The
IF stages are adjusted to give maximum response to
the intermediate frequency (usually 455 kc).
To align tuned circuits, It is necessary to have a
signal source and a response indicator. Your signal
generator provides the signal source at the frequency
that the tuned circuit is to be aligned. You can use any
one of several methods to indicate the response of the
tuned circuit to the applied signal. The simplest
method is to use the speaker of the receiver. The signal
How to Align Radio Receivers
with the Signal Generator
RECEIVER ALIGNMENT
--GENERAL

from the signal generator must be modulated to
produce an audible tone from the speaker. Then as the
circuits are aligned, you judge the response by the
loudness of the tone from the speaker, Since it is
difficult for the ear to detect small changes In
loudness, other methods are considered more
accurate. However, using the speaker as a response
indicator is entirely satisfactory for aligning radio
receivers.
Another method of indicating the response of the
tuned circuits in a receiver is to connect a scope or AC
voltmeter across the voice coil of the receiver speaker.
This gives you a visual indication of the response as
you adjust the tuned circuits.
Another popular method of indicating the response
is to connect a DC voltmeter to the output of the
second detector. The meter is very sensitive to small
changes in circuit response. With this method you can
use an unmodulated signal and you do not have to
listen to the tone from the loudspeaker. One
precaution: If a circuit you are aligning breaks into
oscillation, your indicating meter will show a high
false reading. It will be necessary to find and correct
the cause of oscillation (usually feedback) before you
can peak the tuned circuit. Otherwise you will have to
align the circuit slightly off resonance where
oscillation does not occur. When an oscillating
condition is suspected, use an unmodulated signal and
listen to the output. This enables you to detect an
oscillating circuit condition because the receiver will
produce squeals or motor boating noises. You can still
use your DC meter as the response indicator,
It is necessary to inject the proper amount of signal
into the tuned circuit without detuning the circuit to be
aligned. This is usually accomplished by injecting the
signal into a stage ahead of the stage being aligned.
For example, to align IF stages, the signal is applied to
the control grid of the mixer stage. The mixer tube
isolates the tuned IF stages from the signal generator
so there is no loading effect. The amplitude of the
signal is then adjusted by attenuating the signal
generator output.
Minimum coupling should be used to Inject a signal
into the RF stages or the pre-selector section of a
receiver. Placing the output clips of the signal
generator cable near the loop antenna of a receiver
will usually radiate enough signal Into the receiver. If
the receiver is badly misaligned you can start
alignment by connecting the signal generator output
directly to the antenna terminals. This will inject
enough signal to drive through even a badly
misaligned stage. When you get the set approximately
aligned, remove the clips from the antenna terminals
and radiate the signal into the set by placing the signal
generator clips close to the antenna. This prevents any
loading effect of the signal generator during final
alignment.
Accurate receiver alignment requires the use of a
minimum signal. You want the receiver to produce Its
maximum sensitivity when receiving weak radio
stations, Therefore, final alignment adjustments
should be made using a signal strength that is just
large enough to give a good indication on your
response Indicator. In a receiver using AVC, the signal
strength should be small enough so it does not
produce an effective AVC voltage. An alternate
method is to disable the AVC voltage by shorting the
AVC line to B-.
The oscillator of your CONAR Signal Generator is a
stable circuit that will show no appreciable frequency
drift during warm-up. However, it is good practice to
allow the generator to warm up for ten or fifteen
minutes before using It. The circuits in some receivers
will drift noticeably during warm-up, Therefore, turn
on the set and allow it to warm up for about fifteen
minutes before starting the alignment procedure,
When aligning the IF section of a receiver, it is
desirable to disable the local oscillator. This prevents
the local oscillator signal from beating with the signal
generator signal and producing confusing signals,
The local oscillator can be disabled by shorting the
oscillator section of the tuning capacitor.
To obtain maximum sensitivity from a radio receiver
you can use an alignment procedure known as
"rocking". This procedure may produce inaccuracies
in the dial readings of the radio, however, the dial
markings on most broadcast radio receivers are only a
rough indication of the frequency that the radio is
tuned to. Therefore, the disadvantage of small
inaccuracies in the dial settings is more than offset by
the improved receiver sensitivity.
The "rocking" alignment procedure can be used on
any receiver having a padder capacitor or a tuning
slug in the local oscillator coil. In most cases, the
Injecting the Signal:
Use Minimum Signal:
Warm-up Time:
Disabling the Local Oscillator:
Obtaining Maximum Sensitivity:

receiver will be a transistor set because most modern
tube broadcast receivers do not have a padder. In the
following discussion, the word "padder" refers to the
padder capacitor or the tuning slug in the oscillator
coil.
First, adjust the oscillator and RF trimmers for
maximum output at 1500 kc with the receiver dial set
at 1500 kc. The RF section of the receiver is now
properly aligned and should not be adjusted again.
Next, set the signal generator tuning control to 600
kc. Tune the receiver to produce maximum output
Indication regardless of dial setting. Write down the
exact dial reading.
Change the setting of the padder slightly. Retune the
receiver for maximum output, and make a note of the
output. If the output has increased, you are adjusting
the padder in the correct direction; if the output has
decreased, you are adjusting the padder in the wrong
direction.
If you are adjusting the padder in the correct
direction, continue adjusting the padder In that
direction, retuning the receiver dials until maximum
output Indication is obtained. Of course, you must
tune the padder slightly beyond the correct point
(where the output begins to decrease) and then come
back to make certain you have the maximum.
If the original padder adjustment was in the wrong
direction, turn the padder In the other direction, retune
the receiver and note the output. Continue this
procedure until maximum output is obtained, again,
you will have to tune slightly beyond the correct point
to make certain you have the maximum output.
This "rocking" adjustment increases the receiver
sensitivity at the expense of exact dial calibration by
effectively adjusting the oscillator and the pre-
selector simultaneously. The combination of setting
the padder and the receiver dial adjusts the oscillator;
setting the receiver dial adjusts the mixer and the pre-
selector.
After performing the "rocking" adjustment at the
low end of the receiver dial, you must adjust the
oscillator trimmer capacitor at the high end of the dial.
Repeat these adjustments until the dial calibration is
correct at the high end of the dial, and the padder is
adjusted for maximum response at the low end of the
dial, when you have done this, the receiver will track
reasonably well and maximum sensitivity will be
obtained over the entire tuning range.
The following alignment procedure is presented as
an example that can be used for aligning receivers
when instructions are not available, if the
manufacturers alignment data is available, it should
be used. Fig. 2 shows a typical AC-DC broadcast
receiver; Use this schematic to follow the alignment
steps.
1. Plug in and turn on both the Signal Generator and
the receiver to be aligned, Allow a few minutes for
them to warm up.
2. Connect a DC voltmeter across the volume control.
You could, of course, use one of the other response
indicators discussed previously. The volume control
in Fig. 2 is the 500K potentiometer, R5. Connect the
voltmeter to terminals 20 and 18. The high end of the
volume control will have a negative potential so set
the meter to read DC on the 3-volt scale.
3. Short the oscillator section of the tuning capacitor,
C2. This can be done by connecting a jumper from
terminal 17 to the chassis. This disables the receiver's
local oscillator and prevents spurious signals during
IF alignment.
4. Disable the AVC voltage by connecting a jumper
between terminals 2 and 3, this shorts out capacitor
C7 and grounds theAVC voltage.
5. Connect the signal generator output to the terminals
of loop L1 at the antenna. Notice that L1 is a single
turn of wire near the built-in loop antenna, L2, The
output of the signal generator will radiate a signal
from L1 into the loop antenna, L2.
6. Set the Signal Generator to produce the receiver's
intermediate frequency, In this case, 455 kc, Set the
Band switch to A, Tune the Signal Generator so the
red hairline on the plastic pointer is over 455 on scale
A. Set the function switch to RF. Adjust the
attenuator (HIGH-LOW switch and ATTENUATOR
control) to produce about 1 volt reading on the
response indicator.
7. Adjust the AC-DC trimmers for maximum reading
on the response indicator. The IF trimmers are C5, C6,
STEP BY STEP ALIGNMENT OF
A TYPICAL VACUUM TUBE
RADIO RECEIVER