Heathkit IT-5283 - Circuit Description

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CIRCUIT DESCRIPTION
Rofer
to
the
Schematic:
Disgram
(Illustration
Booklet,
Page
5)
while
you
read
this
circuit
descriptiun.
‘The Signal Tracer is basically a hi-gain amplifier. It
features a diode-equipped probe which acts as an RF
detector. A relaxation oscillator provides an audible
volt/ohmmeter function.
A high-impedance input stage is formed by transis-
tors Q1 and Q2 which are connected in a complimen-
tary amplifier configuration. The stage also provides
a voltage gain. The high impedance is a function of
the Darlington transistor, Q1. Diodes D2 and D3 pro-
vide input voltage protection. Diode D1, which is
located in the probe, can be switched in to provide
detector action in RF circuits. Capacitor C1, which is
in series with diode D1, blocks any DC presant on the
RF signal.
The audible volt/ahmmeter (logic tracer) circuit is
provided by unijunction transistor Q3, which is con:
nected as a relaxation oscillator. The circuit charges
capacitor C10 ta the “firing potential” of transistor
Q3, at which time the transistor conducts and dis-
charges C10. This discharge provides a pulse to the
audio amplifier through capacitor C4. Any voltage
abuve the firing voltage of transistor Q3 (normally 410
6 volts) will be sufficient to operate the circuit. Since
this circuit returns to ~9 volts, voltage levels down to
approximately ~3 volts can be determined. The audi-
ble ohmmeter functions when the probe tip is
touched to the test paint. The frequency of the tone
produced will vary when more or less resistance is
inserted in series with the probe.
Function switch SW1 solects oither the audio
preamplifier (signal tracer) or the relaxation oscillator
Qa (Audible V/A). SW1 connects the signal to the
audioamplifier through Level contro! R10 and coup!-
ing capacitor C5. Transistor Q¢ is an audio amplifier
which connects the amplified signal directly ta the
base of the audio driver, Q5. The amplified signal
from QS is connected directly to the complementary
output
stage.
which
consists
of
transistors
Q7
and
Q8,
and diodes Dé and 07.
‘Transistor Q6 acts as a current limiter to prevent cur-
rent drain on the batteries. Resistor R21 and
capacitors C3 and C9 provide power supply isolation
between the preamplifier and power amplifier
cuitry. The Signal Tracer may be powered by batteries
or the optional power supply, Model IPA-5280-1.