This is only a preview of the June 2015 issue of Silicon Chip. You can view 33 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Bad Vibes Infrasound Snooper":
Items relevant to "Audio Signal Injector & Tracer":
Items relevant to "The Multi-Role Champion Preamplifier":
Articles in this series:
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Audio Signal
Injector & Tracer
. . . with optional tiny add-on RF probe
This Audio Signal Injector/Tracer is ideal for
troubleshooting AM radio and audio circuits.
It comprises a 1kHz oscillator (the Injector)
and an in-built preamp and amplifier with a
headphone jack (the Tracer) so you can trace
signals right through an amplifier or radio
circuit to locate faults.
By JOHN CLARKE
This photo shows the complete Audio Signal Injector/Tracer
together with its optional RF Demodulator Probe at right.
A
T SOME STAGE, everyone involved in electronics will need
to find a fault in an audio circuit. It
might be a circuit you have just built,
a repair job for a friend or a job to be
done in your workplace. And while
you can often check voltages if you
have a circuit diagram, sooner or later
Main Features
• Hand held & battery powered
• 1kHz injector output
• Adjustable injector level
• Tracer input attenuator
• Tracer volume control
• Audio output to headphones or
small speaker
• Low battery indication
• Optional RF probe for AM
modulation detection
60 Silicon Chip
you will probably need to trace the
progress of an actual signal though the
various stages.
For example, you might feed a signal
into the input and then find that it
disappears as it feeds through a capacitor. The obvious conclusion would
be that the capacitor is faulty (open)
or it has not been properly soldered
into circuit.
To do this sort of fault-finding, you
need a suitable signal (one you can
hear) and a small amplifier so you can
listen to the signal at various stages in
the amplifier or AM radio being tested.
So our Audio Signal Injector/Tracer
has a 1kHz oscillator as the Injector
and a small amplifier as the Tracer.
AM radio adds an extra complication because you need to listen to a
modulated radio signal as it goes from
stage to stage in the circuit. For that
you need an optional RF demodulator
probe and we show how to build one
in the article on page 68 of this issue
– it’s tiny!
Mind you, if you are repairing an
amplifier, you may not need the Injector’s audio signal, provided you have
a CD player or even a smart phone
which has music tracks.
One the other hand, a music signal is
not always ideal if you are using an oscilloscope and want to see if the signal
becomes distorted at a particular stage
in the circuit. In that case, you might
find the Injector more convenient as
you trace a signal of known shape
through the circuit.
As mentioned, our Injector is a
1kHz oscillator and you can see the
shape in the accompanying scope
grab designated Scope 1. It looks a
bit like a sinewave but is actually a
somewhat “rounded” square wave. It
has a maximum amplitude of about
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The Audio Signal Injector/Tracer is ideal for tracking
down faults in audio amplifiers and preamplifier stages.
And by adding the optional RF Demodulator Probe, it
can be used to trace signals through the RF stages of
AM radios as well. You can listen in to the traced signal
via either headphones or an external speaker but the
latter should be used if checking high-voltage circuits.
Specifications
2V RMS but it can be adjusted down
to just few millivolts.
This means that it will cover virtually all signal tracing situations, from
sensitive audio preamplifiers and the
audio sections of AM/FM radios, right
up to high-powered guitar and public
address amplifiers.
Then we come to the Signal Tracer.
It needs a small amplifier to listen to
small signals in sensitive circuits but it
also needs an input attenuator so that
it is not overloaded by the much larger
signals, perhaps 50V or more, that you
might find in a high-powered amplifier. You also need a volume control
so that your ears are not blasted as you
step through a circuit.
Finally, both the Injector and Signal
Tracer need to be protected from any
high voltages that may be present in
a solid-state or valve circuit. If you
feed the Injector into a circuit operating at 300V DC, for example, you
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Power: 9V at 2.3mA
Tracer input impedance: ~6.45MΩ to 10MΩ, depending on attenuator setting
Tracer signal gain: adjustable from 2x to 20x
Tracer attenuator: 1:1, 1:10, 1:100 & 1:1000
Tracer signal frequency response: 70Hz to 3kHz
Injector signal: 1kHz rounded square-wave
Injector signal level: adjustable from 0-2V RMS (5.6V peak-peak) with a 9V supply
Headphone output: 6.6V peak-peak maximum into 16Ω with a 9V supply
Test circuit DC voltage: ±300V DC maximum recommended
don’t want it to be blown to shreds
and by the same token, if you touch
the Tracer probe onto a similar highvoltage point, you don’t want it to be
“cooked”. Our circuit takes care of
those possibilities.
Our Injector/Tracer is housed in a
compact plastic case with an internal
battery compartment. It has a pair of
jack sockets for the output of the Injector and a BNC socket for the input
to the Tracer. Next to that socket is a
4-position slide switch for the Attenuator which has settings of 1:1, 1:10,
1:100 and 1:1000.
Input impedance
The input impedance of the Tracer
is rather high, varying between about
10MΩ and 6.45MΩ, depending on
the setting of the input attenuator.
This means that the impedance of the
June 2015 61
Scope 1: the 1kHz waveform generated by the oscillator
looks a bit like a sinewave but is actually a “rounded”
square wave. It has a maximum amplitude of about 2V
RMS but can be adjusted down to just a few millivolts.
Tracer will not load down or affect the
operation of the circuit being tested.
The high-impedance input also means
that the Tracer probe can be used to
directly test ceramic (crystal) phono
cartridges or piezoelectric pick-ups on
musical instruments such as a violins.
To connect signal to the Tracer you
can use a 1:1 oscilloscope probe or
any shielded cable with a BNC plug
at one end and a suitable connector
at the other, such as an RCA plug or a
pair of alligator clips. More about this
later in the article.
The on/off switch, a power LED and
the two knobs for the Injector level
Warning!
When using the Audio Signal
Injector/Tracer with high-voltage
circuitry (eg, in a valve radio), take
care not to touch any part of the
circuit with your hand. Always treat
the circuit as though it has mains
voltage present.
As stated in the article, use a
small extension speaker rather than
headphones when using the unit
with high-voltage circuitry. Small
non-powered extension speakers
are available for use with iPods and
similar MP3/MP4 players.
The use of a small speaker will
remove the possibility of deafening
clicks or even a high-voltage shock
should there be a fault within the
Audio Signal Injector/Tracer or if the
earth lead becomes disconnected.
62 Silicon Chip
Scope 2: this scope grab shows the Schmitt trigger operation
of IC1a. The yellow trace shows the charging and discharg
ing of the 6.8nF capacitor from 3V to 6V etc, while the green
trace shows the resultant square-wave output at pin 1.
and Tracer volume controls are at
one end of the case while the 3.5mm
headphone jack is on the side, adjacent
to the 4-position Attenuator switch.
Circuit description
Let’s now take a look at the circuit
of the Audio Signal Injector/Tracer –
see Fig.1.
As shown, it’s based on an LMC
6482AIN CMOS dual rail-to-rail op
amp and a handful of other components. One op amp is used for the
Signal Injector while the other is used
for the Tracer. The output frequency
of 1kHz is set by the 100kΩ resistor
and 6.8nF capacitor connected to pin
2, the non-inverting input.
The three resistors connected to the
pin 3 inverting input set the threshold
voltage (at pin 3) at 1/3Vcc or 2/3Vcc,
depending on whether the output of
IC1a is high or low. So with Vcc = 9V,
the input (threshold) voltage at pin 3
will be either +3V or +6V.
When power is applied to the circuit, the 6.8nF capacitor at pin 2 will
be discharged (ie, 0V), so pin 2 will be
lower than pin 3. Therefore the output
at pin 1 will be high (+9V) and this
charges the 6.8nF capacitor via the
100kΩ resistor between pins 1 & 2.
When the capacitor voltage rises just
above 6V, pin 2 becomes higher than
pin 3 and so the op amp’s pin 1 output
switches low, to 0V (remember, this is
a “rail-to-rail” op amp).
So now pin 3 is at 3V and the
capacitor discharges via its 100kΩ
resistor until pin 2 is just below pin
3, whereupon the pin 1 output goes
high again to recharge the capacitor.
This continuing cycle generates a 1kHz
square wave which is filtered using a
6.8kΩ resistor and 22nF capacitor to
give a “rounded” waveform, as shown
in Scope 1.
The Schmitt trigger operation of IC1a
is demonstrated in Scope 2, which
shows the charging and discharging of
the 6.8nF capacitor from 3V to 6V etc
in the yellow trace. The lower green
trace shows the resultant square-wave
output at pin 1. Note that the amplitude
of the square-wave is shown as 9.8V –
we used a fresh 9V battery.
Potentiometer VR1 connects across
the 22nF capacitor to provide the Injector level control. This is AC-coupled to
the output terminal via a 100nF 630V
capacitor. We specified a high voltage
rating for this capacitor so that the
Injector output can be connected to a
high voltage on the circuit under test
without damage.
For the same reason, diodes D2 &
D3 clamp any high voltage from an
external circuit (eg, a valve radio being tested) at the wiper of VR1 to 0.7V
above or below the 9V and 0V supply
rails. The 10MΩ resistor across the
100nF capacitor is there to discharge
the capacitor when it is disconnected
from the circuit under test. The 1kΩ
resistor in series with the Injector output limits peak current to the clamping
diodes.
Tracer circuit
The input signal from the BNC
siliconchip.com.au
POWER
D1 1N5819
+9V
A
100k
100k
K
IC1: LMC6482AIN
3
1
IC1a
2
100k
100k
D2
A
6.8k
INJECT
LEVEL
VR1
10k
LIN
22nF
100nF
K K
INJECT
OUT
1k
100nF
16V
A
2.2k
GND
10M
D3
BANANA
SKT
A
BC 327, BC33 7
+9V
B
K
BNC
TRACER
INPUT
D4
100k
9.1M
910k
91k
1:1
ATTENUATOR
S2
1nF
100k
A
10M
1:10
E
8
5
7
IC1b
6
4
E
1:100
1:1000
C
10M
2.7k
A
100k
10 µF
16V
100 µF 16V
CON1
Q1
BC327
3.5mm
JACK
SOCKET
100k
VR2
50k LOG
D1: 1N5819
1 µF
16V
A
VOLUME
LED1
2.7k
K
A
SC
C
220pF
D5
10k
20 1 5
Q2
BC337
620Ω
B
K
E
C
B
1kV
9V
BATTERY
ZD1
5.6V
100 µF
BANANA
SKT
630V
K
6.8nF
A
K
S1
λ LED1
AUDIO SIGNAL INJECTOR & TRACER
K
ZD1
A
K
D2–D5: 1N4004
A
K
Fig.1: the circuit is based on dual op amp IC1. IC1a operates as a Schmitt trigger oscillator and this generates the
injector signal, with VR1 setting the output level. The traced signal is fed in via a switched attenuator and then fed to
op amp IC1b. Its output signal is then buffered by Q1 & Q2 and fed to CON 1, while VR2 sets the op amp gain.
socket is fed to 4-way slider switch,
S2 and the attenuator resistors. The
resistors provide for division ratios of
1:1, 1:10, 1:100 and 1:1000.
Following S2, the signal is coupled
via a 1nF 1kV ceramic capacitor to
the pin 5 non-inverting input of IC1b.
This is tied via two series-connected
10MΩ resistors to a voltage divider
(two 100kΩ resistors) which provides a
reference at 4.5V ie, half the 9V supply.
Diodes D4 & D5 clamp any high
voltage input signals to 0.6V above or
below the 9V supply rails.
IC1b is connected as a non-inverting
amplifier and its pin 7 output drives
a complementary emitter follower
stage using transistors Q1 & Q2. These
provide a buffered output to the headphone socket via a 100µF coupling
capacitor.
Note that the emitter follower output
stage is operated with no quiescent
siliconchip.com.au
current but is within the negative feedback loop of the op amp to minimise
crossover distortion.
The 50kΩ volume control (VR2) is
also in the op amp’s feedback loop,
connected in series with a 2.7kΩ
resistor. In conjunction with the 1µF
capacitor and series 2.7kΩ resistor
from pin 6 to 0V, this allows the AC
gain to be varied from between two and
20. The DC gain is unity, by virtue of
the 1µF capacitor.
Note that while the amplifier is
mainly intended to drive headphones,
it can also be used to drive a small
speaker and we recommend this if
you are doing signal tracing in a highvoltage circuit which might cause
deafening clicks when you touch the
probe on high voltage points.
ates from a 9V battery, fed in via toggle
switch S1. Diode D1 gives protection if
the battery is inadvertently connected
the wrong way around. A high-intensity red LED is used for power indication. It is bright when the supply is at
9V but drops to a dim glow when the
battery is flat, by virtue of ZD1, a 5.6V
zener diode in series with the LED.
When the battery is fresh, ie, putting out 9V or maybe as much as 10V,
we will have 1.8V across the red LED,
5.6V across ZD1 and 1.6V or more
across the 1kΩ resistor so that 1.6mA
or more flows through LED1. As the
voltage falls, the voltage across the
1kΩ resistor also falls. At a battery
voltage of 7.4V or less, there is very
little voltage across the 1kΩ resistor
and so LED1 will be dim.
Power supply
RF demodulator probe
As already noted, the circuit oper-
As previously noted, if you want
June 2015 63
VR1
100 µF Q2
+
This photo shows how switch S2 is
mounted. It’s soldered to a pin header
so that its top metal face is 12.5mm
above the PCB.
BC337
620Ω
2.7k
4004
4004
D5
1kV
220pF
2.7k
1 µF
D4
LMC6482
100k
6.8k
100k
2.2k
D3
22nF
Q1
PHONES
Inject
fitted. These are installed at the five
external wiring points, at TP GND
(near LED1) and at the bottom right of
S2. IC1 can then be soldered in place.
Do not use a socket for this IC, as this
would exacerbate noise pick-up.
CON1
100k
10M
10M
100k
1k
4004
S
100nF
630V
–
(-)
TO BATTERY CLIP
to troubleshoot an AM radio with
the Tracer, you need to have an additional demodulator probe for the
amplitude-modulated (AM) RF signals
that should be present in the circuit
being tested. As stated, a suitable RF
demodulator probe is described on
page 68 of this issue.
Construction
The Audio Signal Injector/Tracer
is built on a double-sided PCB coded
04106151 (85 x 63mm). This is housed
in a plastic remote control case measuring 135 x 70 x 24mm. A panel label
measuring 114 x 50mm is attached to
the front of the case.
To make the assembly easy, the PCB
100k
TO SHIELD PCB
10k
S2
91k
+
910k
9.1M
10M
+
GND
Tracer
GROUND
SOCKET
1nF
R
TRACER
INPUT
BNC SOCKET
10 µF
T
CUT LUGS
SHORT
(SEE TEXT)
AUDIO SIGNAL INJECTOR & TRACER D2
INJECTOR
OUTPUT
SOCKET
4004
6.8nF
+
100 µF
100k
100k
5 V6
ZD1
100k
TP GND
D1
50k LOG
10k LIN
100nF
BC327
LED1
S1
VR2
15160140
K
IC1
A
5819
Fig.2: follow this parts
layout diagram to build
the PCB assembly. Be sure
to install the 100nF 630V
and 1nF 1kV capacitors in
the positions indicated and
note that S2 is mounted on
a pin header (see photo).
/1
/10
COM
Installing switch S2
/100
/1000
Switch S2 does not mount directly
onto the PCB but is instead raised off
the PCB using a 6-way DIL pin header.
Before installing this DIL header,
remove a pin from each side so that
there are three pins, then a gap, then
two pins (ie, on each side of the header
to correspond with the switch pins).
That done, position the header on
the PCB with the longer pins facing
upwards, then push each pin down so
that it extends only 5mm above the top
of the PCB. The pins on the underside
EARTH PC STAKE FOR
S2's METAL COVER
is designed to mount onto the integral
mounting bushes within the case. The
top of the PCB is also shaped to fit
around the case mounting pillars at
that end – see Fig.2 and photo.
Fig.2 shows the parts layout on the
PCB. Begin by the installing the resistors. Table 1 shows the resistor colour
codes but it’s also a good idea to check
each one with a digital multimeter
before soldering it to the PCB.
The diodes can go in next. Note that
there are two different types – D1 is a
1N5819, while D2-D5 are 1N4004s. Be
sure to mount them with the correct
polarity, then install zener diode ZD1,
again taking care with its polarity.
The seven PC stakes can now be
Table 2: Capacitor Codes
Value µF Value IEC Code EIA Code
100nF 0.1µF 100n 104
22nF 0.022µF 22n 223
6.8nF 0.0068µF 6n8 682
1nF 0.001µF 1n 102
220pF NA 220p 221
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
o
No.
3
1
1
8
1
1
1
2
1
1
1
64 Silicon Chip
Value
10MΩ
9.1MΩ
910kΩ
100kΩ
91kΩ
10kΩ
6.8kΩ
2.7kΩ
2.2kΩ
1kΩ
620Ω
4-Band Code (1%)
brown black blue brown
white brown green brown
white brown yellow brown
brown black yellow brown
white brown orange brown
brown black orange brown
blue grey red brown
red violet red brown
red red red brown
brown black red brown
blue red brown brown
5-Band Code (1%)
brown black black green brown
white brown black yellow brown
white brown black orange brown
brown black black orange brown
white brown black red brown
brown black black red brown
blue grey black brown brown
red violet black brown brown
red red black brown brown
brown black black brown brown
blue red black black brown
siliconchip.com.au
can then be soldered to their respective
pads, making sure that the header itself
is flush against the PCB.
Once it’s in position, switch S2
can be mounted by soldering its pins
to the top of the header pins, so that
its top metal face sits 12.5mm above
the PCB (see photo). The best way to
do this is to lightly tack-solder two
diagonally-opposite pins first, then
make any necessary adjustment before
soldering the remaining pins. Don’t
forget to resolder the first two pins, to
ensure reliable connections.
Once it’s in position, the adjacent
earth PC stake is soldered to the earth
tag on S2’s metal cover.
Completing the PCB
Now for the capacitors. Install the
100nF 630V polyester and 1nF 1kV
ceramic capacitors in the positions
shown, then install the remaining
MKT polyester types. The electrolytics
can then go in, taking care to fit each
one with the polarity as indicated on
Fig.2. Note that the tops of the electrolytics must be no more than 12.5mm
above the PCB, otherwise you will not
be able to fit the case lid later on.
Follow with potentiometers VR1
& VR2, toggle switch S1 and the
3.5mm socket. VR1 is a 10kΩ linear
potentiometer while VR2 is a 50kΩ
log potentiometer, so don’t get them
mixed up. LED1 can then be installed
– it mounts horizontally with its leads
bent down through 90° exactly 7mm
from its lens, so that they go through
the PCB pads. Push it down so that
its horizontal lead sections sit exactly
6mm above the PCB (use a 6mm-wide
cardboard spacer) and check that it is
correctly orientated before soldering
it to the PCB.
That completes the PCB assembly. It
can now be checked and placed to one
side while the case is drilled.
Preparing the case
Figs.3 & 4 show drilling templates
for the front panel and for the top of the
case. They can either be photocopied
from the magazine or downloaded as
PDF files from the SILICON CHIP website
and printed out.
It’s just a matter of cutting the templates out, temporarily attaching them
to the case panels and then drilling
the various holes. The top of the case
requires holes for potentiometers VR1
& VR2, switch S1 and LED1, while the
front panel is drilled to accept the two
siliconchip.com.au
banana sockets, the BNC socket and
slide switch S2.
The rectangular cut-out for S2 is
best made by drilling a row of holes
inside the cut-out area, joining these
and then filing the job to shape. The
two banana socket holes can simply
be drilled and reamed to size but the
BNC socket hole needs to be shaped
as shown on the template. It can be
made by first drilling a small hole in
the centre, then finalising its shape
using small files, with the flat side
positioned as shown.
A hole must also be cut in one side
of the case to accept the 3.5mm jack
socket. To do this, temporarily position the PCB in the case, mark out
the socket position, then remove the
board and make a semi-circular notch
in the base using a small round file.
Once that’s been done, temporarily
assemble the case and complete the
hole by filing a matching semi-circular
cut-out in the lid.
Finally, you have to remove an
internal pillar inside the case lid so
that it doesn’t foul the nut for the
earth banana socket. This can be done
using side cutters. Note also that, as
provided, the banana socket terminals
are too long for the case and have to
be shortened by 5mm. A fine-tooth
hacksaw blade is the best tool for this
job – do not bend the terminals, as they
will break. File off any sharp edges
after cutting them to length.
Having drilled all the holes, the
front panel label can be attached. This
can be downloaded from the SILICON
CHIP website, printed out (preferably
onto photo paper) and affixed to the
lid using either glue or neutral-cure
silicone.
Alternatively, for a more rugged
label, print it out as a mirror image
onto clear overhead projector film (be
sure to use film that suits your printer),
so that the printed side will be on the
back of the film when the label is affixed. The film will have to be attached
using a light-coloured silicone applied
evenly over the surface, as the lid is
black.
Another option is to print the
panel onto either an A4-size “Dataflex”
sticky label (for ink-jet printers) or a
“Datapol” sticky label (for laser printers) and directly attach this to the case
lid. These labels are available from
http://www.blanklabels.com.au and
sample sheets are available on request
to test in your printer.
INJECTOR
OUT
+
TRACER
VOLUME
INJECT
LEVEL
POWER
WARNING!
Do not use
headphones
or earbuds
when testing
high voltage HEADPHONES
circuits.
Use extension
speaker instead.
1
10
100
1000
+
+
GROUND
TRACER
IN
TRACER
ATTENUATOR
SILICON CHIP
Audio
Signal Injector
& Tracer
Fig.3: this front-panel artwork can
be copied or downloaded from the
SILICON CHIP website and used as a
drilling template.
End Panel Drilling Guide
5mm 3mm
7mm
7mm
Fig.4: the end panel drilling template.
Drill pilot holes first to ensure they are
accurately positioned, then carefully
enlarge them to size.
Once the label is in position, cut out
the holes using a sharp hobby knife.
Making a shield PCB
Since the tracer has such a high
input impedance, it has the potential
to pick up hum from transformers but
it will also pick up the injector signal
as well, due to direct radiation of the
injector signal into the input attenuator and other components in the op
amp’s input circuitry.
We can reduce this by a significant
amount by installing a small shield
board, made from copper laminate,
underneath the PCB, with its copper
side earthed to the PCB’s GND stake.
The dimensions of this shield board
are shown in Fig.5. It fits between the
June 2015 65
Parts List
1 remote control case, 135 x 70 x
24mm (Jaycar HB-5610)
1 double-sided PCB, code
04106151, 85 x 63mm
1 single-sided shield PCB, code
04106153, 62 x 63mm
1 panel label, 114 x 50mm
1 9mm square PCB-mount 10kΩ
linear potentiometer (VR1)
1 9mm square PCB-mount 50kΩ
log potentiometer (VR2)
1 SPDT PCB-mount toggle switch
(Altronics S1421) (S1)
1 DP4T PCB-mount slider switch
(TE Connectivity STS2400PC04)
(element14 Cat. 1291137) (S2)
1 PCB-mount 3.5mm stereo jack
socket (CON1)
2 knobs to suit VR1 & VR2
1 panel-mount BNC socket
1 blue insulated banana socket
(Jaycar PS-0423)
1 green insulated banana socket
(Jaycar PS-0422)
1 9V alkaline battery
1 9V battery snap connector
4 No.4 x 6mm self-tapping screws
7 PC stakes
1 DIL 6-way pin header
7
7
7
7
ALL DIMENSIONS IN MM
62
BLANK PCB
COPPER ON UNDERSIDE
20
Semiconductors
1 LMC6482AIN dual CMOS op
amp (IC1)
1 3mm high-intensity red LED
(LED1)
1 BC327 PNP transistor (Q1)
1 BC337 NPN transistor (Q2)
1 5.6V 1W zener diode (ZD1)
1 1N5819 Schottky diode (D1)
4 1N4004 diodes (D2-D5)
Capacitors
2 100µF 16V PC electrolytic
1 10µF 16V PC electrolytic
1 1µF 16V PC electrolytic
1 100nF 630V polyester
1 100nF 63V or 100V MKT polyester
1 22nF 63V or 100V MKT polyester
1 6.8nF 63V or 100V MKT polyester
1 1nF 1kV ceramic
1 220pF disc ceramic
Resistors (0.25W, 1%)
3 10MΩ
1 6.8kΩ
this, solder a short piece of wire to
the copper side and then connect its
other end to the earth pin (GND) for
the BNC connection, on the PCB. The
shield PCB is then secured inside the
case using silicone adhesive.
Final assembly
63
28
7
4
Fig.5: this diagram shows the dimen
sions of the blank shield PCB.
four integral pillars used to mount the
PCB and it has a cut-out to clear the
back of the Injector jack sockets.
Alternatively, if you don’t wish make
your own shield board, you can buy
a ready-made board from the SILICON
CHIP Online Shop (code 04106153).
The shield board is installed in
the case with its copper side facing
downwards, away from the underside
of the PCB (otherwise it would short
the component pigtails!). Before doing
66 Silicon Chip
1 150mm length of hookup wire
1 50mm length of single core
shielded wire
Now for the final assembly. First,
attach the sockets to the front panel,
then solder short lengths of hook-up
wire to the Inject and GND terminals
on the underside of the PCB. That
done, pass these leads up through their
respective holes in the PCB, ready to
solder to the banana socket terminals.
Next, attached a short shielded
cable (for the BNC socket) to the GND
and Tracer PC stakes on the top of the
PCB. The 9V battery snap can then be
fitted. Its leads are fed through from
the battery compartment before being
looped through stress relieving holes
in the PCB and soldered to the “+” and
“–” terminals.
The next step is to fit the end panel
to the potentiometers, switch and LED
and install this into the base of the
case. The PCB is then secured using
four No.4 x 6mm self-tapping screws
that go into integral mounting pillars.
1 9.1MΩ
1 910kΩ
8 100kΩ
1 91kΩ
1 10kΩ
2 2.7kΩ
1 2.2kΩ
1 1kΩ
1 620Ω
Test Leads
Tracer In
Option 1: 1 x 1:1 oscilloscope probe
Option 2: 1 x BNC plug-to-RCA plug
lead fitted with a PC stake and 5mm
& 10mm heatshrink tubing (see text)
Option 3: 1 x BNC line plug, 1 x
RCA line plug, 1 x 500mm-length
of single core shielded audio cable,
1 x M4 nut, 1 x PC stake and 2mm,
5mm & 10mm heatshrink tubing (see
text)
Injector Out
Option 1: 1 x multimeter lead set
with accessory alligator clips
Option 2: 1 x red banana plug, 1
x black banana plug, 1 x red alligator clip, 1 x black alligator clip, 1 x
500mm length of red medium-duty
hookup wire, 1 x 500mm length of
black medium-duty hookup wire
(made into two banana plug to alligator clip leads).
Once it’s in place, complete the wiring to the banana sockets and the BNC
socket, then secure the lid to the base
using the supplied screws. You will
need to make sure that the wires do
not interfere with the banana sockets
– if they are sandwiched beneath the
banana sockets, they will prevent the
lid from fully closing.
Similarly, any wires running over
the battery compartment or over the
slider switch will prevent the case
from closing. If necessary, move the
wires out of the way using a small
screwdriver as the case is being closed.
Finally, fit the battery and the assembly is complete.
Test leads
As mentioned earlier, a 1:1 oscilloscope probe makes a suitable test lead
for the Audio Signal Injector/Tracer’s
BNC input. Alternatively, a cheaper
test probe can be made using a BNCto-RCA lead. This can be a commercial
lead but these tend to be made from
stiff large-diameter cable.
A do-it-yourself cable using a line
RCA plug, a line BNC plug and standard shielded audio cable will be much
more flexible. The connections to
siliconchip.com.au
The shield board is installed in the
case with its copper side facing down
and is secured in place using silicone
adhesive. Its copper side is connected
to the GND stake on the main PCB.
the BNC plug can be made using the
method described in the article on the
RF Demodulator Probe.
The tip of the RCA plug can be used
as the probe but note that the outer
metal earth shell must be insulated
using 10mm-diameter heatshrink tubing to prevent it making contact with
the circuit under test. In addition, a
PC stake can be soldered to the centre
pin of the RCA plug to extend it. That’s
done by first drilling a 1mm hole in the
end of the plug’s tip, then inserting the
PC stake and soldering it.
It’s a good idea to cover the RCA
plug’s centre terminal with 5mm diameter heatshrink tubing, leaving only
the PC stake “probe” exposed. This
will help prevent inadvertent shorts
when probing closely-packed circuits.
The injector signal can be fed out using a multimeter probe. Alternatively,
you can use a lead fitted with a banana
plug at one end and an alligator clip
lead at the other. A banana plug-toalligator clip lead can also be used for
the ground lead.
Testing
To check that the unit is working
correctly, connect the “Injector Out”
signal to the “Tracer In” (BNC) socket,
then plug in headphones (or earphones) and listen for the 1kHz signal.
Assuming that it’s present, check that
siliconchip.com.au
This is the view inside the completed unit. Make sure that the wiring leads to
the banana sockets aren’t squashed under them as the lid is closed (push the
leads towards the outer edge of each hole using a small screwdriver).
the level varies when the “Inject Level”
potentiometer, the “Tracer Volume”
potentiometer and the “Tracer Attenuator” switch are adjusted.
As noted above, if the tracer input is
disconnected from a circuit, the unit
will pick up hum and the 1kHz injector signal due to the tracer circuit’s
high input impedance (ie, the 1kHz
signal will be heard even when there
is no connection). The pick-up level
will depend on the capacitance of the
input cable, the attenuator setting (S2),
the injector level setting (VR1) and the
gain (Volume) setting (VR2).
Obviously, it will be at a maximum
when the attenuator is set to 1:1 and
VR1 & VR2 are at maximum but this
combination of settings would not be
used in practice. Basically, it’s just a
matter of choosing settings to suit the
job at hand and to minimise extraneous noise pick-up.
Under normal use and when connected to a circuit for testing, the
crosstalk from the injector will be
minimal and will be swamped by the
signal from the circuit under test.
Ground connections
Finally, note that when using the Audio Signal Injector/Tracer, the Ground
banana socket must be connected to the
ground of the circuit under test. This
can be done using a lead fitted with an
alligator clip as described above or by
using the earth lead on the 1:1 oscilloscope probe.
Now turn to page 68 for the optional
SC
RF Demodulator Probe.
June 2015 67
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