single 1N4002 silicon diode (Heath part# 57-
65). Figure 5 shows the block diagram and Fig-
ure 6 shows the schematic diagram.
The heart of the GD-48 metal locator is the
search and pickup coils, L1 and L2 respectively.
Q1 is a VLF oscillator that uses L1 and C2 to
produce a 100 KHz signal. The transistor’s bias
is provided through R1. This oscillator has an
additional feature; feedback is provided through
an RC network that includes R2 and C1 which
causes Q1 to stop and start oscillating at an
audio rate of about 650 Hz, effectively modulat-
ing the 100 KHz signal with an audio tone. This
circuit is called a relaxation oscillator.
The pickup coil is situated so that the coupling
between it and the search coil is minimal.
Trimmer capacitor C3 and fixed capacitor C10
each provide some coupling between the two
coils; however, they are out of phase with each
other so C3 can be adjusted to balance out any
stray coupling. The pickup coil is tuned by C5,
and any signal from the coil is capacitively cou-
pled to two stages of direct coupled amplifica-
tion provided by Q2 and Q3. All this circuitry is
on the search board located in the coil housing.
The output of Q3 is fed up the three conductor
spiral coil (which carries battery voltage,
ground and the amplified pickup coil signal) to
the audio board in the control assembly. On
this board the signal is fed to a differential am-
plifier composed of Q4 and Q5. The transistors
are coupled through a common emitter resis-
tor. The signal is fed to the base of Q4 while a
voltage level, adjusted by R16 the SENSITIV-
ITY pot is fed to base of Q5. This level varies
from approximately 2.2V fully CCW to 0.9V
fully CW (maximum sensitivity). This voltage
sets the current through the emitter resistor
R14 and hence the bias point of Q4 with re-
spect to the 0.37 volts on the base of Q4 (estab-
lished by R11 and R12). The sensitivity pot is
normally set to the point where, with no input
signal Q4 is just cut off. When a signal becomes
present Q4 conducts and and amplifies the
changes in the input signal. Transistors Q4 and
Q5 share the same metal heatsink; this is not
for heat dissipation, but to keep the two tran-
sistors at the same temperature and reduce
drift in the differential amplifier. The output of
Q5 is further amplified by Q6, which is a basic
common emitter amplifier.
The audio output and meter driving stage is
handled by Q7 and Q8 which are wired as a
single Darlington transistor running class B. A
class B amplifier is biased so the transistor is
just cutoff with no signal. You might be more
familiar with class B push-pull amplifiers that
are used in hi-fidelity audio. However, the GD-
48 uses just half of the push-pull design since
audio quality of the tone is not important in
this case. Class B also means that there is little
current used until metal is detected improving
battery life. The meter effectively measures the
collector current of Q7 and Q8. The diode and
associated resistors scale and protect the meter
from excessive overloads. The collector current
also passes through the speaker. When ear-
phones are plugged into the PHONES jack the
speaker is disconnected and the current passes
through the earphones.
Operation:
The GD-48 has two operating modes, normal
and high sensitivity. In normal mode the sensi-
tivity control is adjusted, with the coil away
from metal, until a tone is heard and then re-
duced until the tone just goes away.
In high sensitivity mode, the sensitivity control
is advanced until the meter reads about 2 on
the scale of 1 to 10. As metal is encountered the
meter will increase. Its movement is much
more noticeable than a change in the level of
the tone.
Heathkit recommends that you practice with
known pieces of metal to become familiar with
the operation and sensitivity of the instrument.
The most sensitive part of the search coil is un-
derneath the spot just in front of where the
swivel joins the coil housing.
HOM rev. new Heathkit of the Month #53 - GD-48 Metal Locator
Copyright 2014, R. Eckweiler & OCARC, Inc.! Page 3 of 7
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