Signal Tracer and Injector

    This audio signal tracer/injector will undoubtedly prove to be very useful
for many routine servicing operations. The unit consists of an audible signal
monitor for "listening" to the signals present in an electronic device (such
as an audio system, receiver, amplifier, or tape deck) at circuit points
inside these devices. It also includes an RF detector probe for use with HF
modulated signals, such as those found on an antenna, RF amplifier, or IF
section of a receiver. In much service work, the task of finding the problem is
greatly simplified by putting into the item being serviced some form of input
signal and checking to see what happens to the signal as it goes in and out of
various stages or circuits. If something is amiss, eventually the signal will
disappear or suffer a high loss, such as in an audio stage with a defective
IC, transistor, or other component. At this point, another signal from an ex-
ternal source can be "patched in" and followed through around the "roadblock"
to check to see if subsequent stages or circuits have any other problems. If
not, it is safe to assume that you have located the problem. At this point,
testing of individual components, voltages, or currents will reveal the defect-
ive component that is causing the problem at hand. 

The signal tracer is simply a high gain, low noise audio amplifier, with a
small speaker for audible monitoring. A diode detector probe is furnished for
checking RF and IF @tgnals. In addition, a signal injector dtrcuit, which is
a simple audio oscillator, is provided to act as a signal source if no input
signal is available or practical to obtain. This feature makes for convenience
in field service work because it eliminates the need for an extra audio osc-
illator. Both the audio amplifier and oscillator operate from either an AC
source or a +9 to +12V battery pack if isolation from the AC lines is desired.
A PC layout can be used or the circuit can be constructed on a piece of G-10
material breadboard style, or if preferred, vectorboard. Adequate grounding and
shielding should be provided because this is a high gain circuit. It is best to
enclose the finished instrument in a metal enclosure, or in an aluminum foil
lined plastic case. Refer to Fig 1 (schematic) The signal tracer portion oper-
ates as follows: Input signal at Jl couples through Cl, a DC blocking capaci-
tor, to R2, a current limiting resistor to the gate of Ql. C2 and R4 provide
source bias and set the DC operating point for Ql. An amplified signal appears
at the drain of Ql across R5. R5 provides a DC return to ground for the gate
of Ql. C4 provides an AC path to ground and bypassing for any power supply
stray signals appearing at the bottom of R5. S2 and Rl provide a means of in-
jecting dc bias to Jl for the purpose of noise testing components connected to
Jl (input) or as a means of providing DC bias to the external RF probe (see
Fig 3) to increase its low level RF detection sensitivity. R9 provides supply
decoupling to C4.

    The audio signal across R5 is coupled to R6, the amplifier gain control,
through C3. The setting of R6 determines the overall amplifier gain. Signals as
low as 10 microvolts at Jl can be plainly heard at full gain setting. Audio
signal from the wiper of R6 is connected to the input of ICI, a small audio
power amplifier with about 250 mW audio capability, sufficient to drive LSl,
a 16 to 32 ohm speaker (transistor radio type), which is not critical as to
impedance. A 3" (76 mm) size is adequate. R7 and C5 suppress a tendency for
IC1 to oscillate at high frequencies. C6 is a DC blocking capacitor for coup-
ling audio from ICI to LSl. C7 provides DC supply bypassing for ICI. R8 and
LED 1 serve as a power on indicator. Sl is a DC on/off switch. If convenient,
it can be part of R6, the gain control, although a separate mounted switch
for Sl is probably preferable to save wear and tear on R6. Dl is a diode to
protect ICI against accidental reversal of power supply. It also serves as a
rectifier if an AC power source is used to power the unit. In this case, C7
should be changed to at least a 1000 uF unit to reduce 50 or 60 Hz AC hum,
and C4 should be increased to 470 uF to further reduce AC hum.

    The detector probe can be constructed inside a plastic tube, medicine bot-
tle, or a piece of half inch (13mm) PVC plastic pipe obtained at a plumbing or
hardware store (see figure). A ground wire (6" to 8" long) fitted with an
alligator clip is suitable, but make sure flexible stranded wire is used to
reduce breakage. A 5 ft (1.6 m) length of RG-58/U coaxial cable can be used
as a test lead to the signal tracer. Either a BNC or phone plug (RCA style)
can be used at Jl and Pl. We prefer a BNC, but this is up to the discretion of
the builder. If only audio work will be done, just the probe assembly shown in
Fig.3 is needed. However, having both probes is desirable in any case.

    The signal injector schematic is shown in Fig 2. Ql is a 2N3565 low noise,
high gain NPN transistor. R14 provides dc bias for Q2 through switch 53, the
oscillator on/off switch. Rll, R12, and R13 provide base bias for Q2. C10 pre-
vents loss of ac gain via feedback through R12 and R13. Cl provides DC block-
ing and AC coupling of oscillator signals from the collector of Ql. C12, R18,
C13, R19, C14, and the input impedance of Q2 (around 10 k) form a phaseshift
network. At the frequency of oscillation, there is a net 180 of phase shift
and loss of 29x in the network. Therefore, a minimum amplifier gain of greater
than 29 is necessary for oscillation to occur. The theoretical frequency of
oscillation is given by:

                  F= 1/(15.4RC) approximately

This formula predicts that C12, C13, and C14 should be 0.0043 microfarads if
a frequency of 1000 Hz is needed, but also assumes that the input impedance of
Q2 is infinite and that Rll in parallel with R12 is also equal to R13 and R14.
However, this is not so, but is only approximately true. Actually, 1,000 Hz is
not mandatory, but is customarily used for tests. You can use 0.002 to 0.01 uF
for C12, C13, and C14 to get frequencies between about 400 to 2,000 Hz, if you
prefer, but 1,000 or 400 Hz are customarily used for most audio testing. R17
is a level control to vary the oscillator output. R15 and R15 provide a means
of obtaining low level output signals below 30 mV or so without crowding at
the low end of adjustment pot R18. J2 has a full 0 to 2 V (approx.) output,
while J3 has low (0 to 20 mv) level output. Either RCA phono or BNC connectors
can be used. The instrument should be constructed to minimize unwanted coup-
ling of audio from Q2 into amplifier Ql-IC1. A PC layout can be used or you
may use vectorboard or "ugly bug" construction. If desired, either the tracer
or injector circuit can be built separately. R8 and LED 1 is a pilot light and
it can be omitted if desired. Circuit checkout is simple. First, test for B+
shorts. The B+ line should read 300 ohms or more to ground. If not, reverse
the ohmmeter probes. If it is still low, check for a short or a defective ICI.
Next, apply power and check the voltage at the drain of Ql. It should be +4V
to +8V (to 10V is OK if a 12V supply is used). Also, pin 5 of ICI should read
half the supply voltage (+4.5V to +6V or so). ICI should not get hot. With R6
set halfway open, a hum or click should come from LSl as the center conductor
of Jl is touched with a screwdriver blade. Next, connect the RF probe assembly.
Set S2 to ON. Touch the probe to a 10 ft (3 meter) length of wire or to an out-
side antenna. A loud buzz should result, and possibly you might even hear radio
stations. Next, turn on S3. About +3 to +8 V should be present at the collect-
or of Q2, and +0.6 to 0.7 V should be present on the base of Q2. Connect a wire
from the center conductor of J3 to the center conductor of Jl (tracer input).
With R6 set halfway, a tone should be heard at all or most settings of R17
(except at the extreme low end, possibly). If not, check all components C12
to C14, R18 ,R19, and Q2. This completes the checkout.

       Complete details of suitable PC board layouts and construction details
are given in our book "Incredible Audio and Video Projects You Can Build" 

Parts list for the signal tracer

(All 5 % 1/4 watt unless specified)

R1                1 Meg
R2              100 k
R3              2.2 Meg
R4              3.3 k
R5, R10,
 R18, R19        15 k
R6          pot, 50 k 
R7               10 ohm
R8, R9            1 k
R11              47 k
R12, R13        150 k
R14             4.7 k
R15             220 k
R16             2.2 k
R17         pot, 25 k


C1              0.01 disc 50 V ceramic
C2, C10, C11    10 uF 16 V electrolytic           
C3, C5          0.1 uF mylar
C4              47 uF 16 V electrolytic
C6              220 uF 16 V electrolytic
C8              100 pF ceramic 500 V
C9              0.001 uF ceramic
C12,C13,C14    .002 to .01 uF mylar
                (see text)


Ql              MPF102 FET
Q2              2N3565
Dl              1N4002
D2              1N914B or 1N4148
ICI             LM386N
LED1            Red LED (any)
Miscellaneous items

LS1             Any small speaker 16 to 32 ohms
J1,J2,J3        RCA phono jack or BNC
P1              RCA phono jack or BNC
S2,S3,S4        SPST switch, any type

Additional items:

Alligator clips, tipjacks, as required
Chassis, PC boards, hardware to suit
l/2" plastic pipe fittings (for probes)
PO Box 200, Hartford NY 12838-0200
Tel 518-854-9280