GD-48 Accessories:
Heathkit offered two accessories for the GD-48,
the GDA-48-1 NEDA 1602 battery ($1.30 in
1969) and the GDA-396 2,000
Ω
earphones
made by Superex ($3.50 in 1969). These head-
phones were also offered as accessories for
many of the shortwave radios sold at the time.
The Heathkit GD-348 Deluxe Metal Locator:
The GD-348 (Figure 3) is a refined version of
the GD-48. It was introduced in late 1972 at the
price of $89.95. It remained at this price in the
catalogs I have until 1980 when the price actu-
ally dropped to $79.95, which may have been a
clearance price as it didn’t appear in the 1981
catalog.
Overview:
The GD-348 weighs 3-1/2 lbs.; a half-pound
lighter than its predecessor. It uses the same
NEDA #1602 9-volt battery to provide 50
hours of operation. The sensitivity is given in
the specifications as detecting “dime-sized
metal at up to 6 inches in air”, on a par with the
non-deluxe model. The search coil housing is
specified as waterproof up to 2’ depth. Like the
GD-48 the shaft is adjustable from 28” to 36”.
The circuitry is significantly changed from the
GD-48. The GD-348 uses ten transistors and
four diodes. The relaxation oscillator itself uses
two transistors to provide more stable opera-
tion. It also operates at a frequency of 100 KHz,
but the modulation frequency has changed to
500 Hz. The amplifier that buffers the signal
from the pickup coil now incorporates an active
filter to reduce harmonics, and a diode detector
to remove the 100 KHz carrier before it leaves
the coil housing. Like its predecessor, the GD-
348 also has two circuit boards, one in the coil
housing that uses four transistors and handles
the relaxation oscillator and active filter-
amplifier, and one in the control handle that
uses six transistors and processes the audio.
The audio processing board contains a fixed high-
gain amplifier, followed by a temperature com-
pensated variable gain amplifier that is set by the
S E N S I T I V I T Y
control. This am-
plifier drives a
single transistor
that drives the
speaker and me-
t e r . T h e f i n a l
transistor is a
shunt across the
meter that con-
ducts on strong
signals and pro-
tects the meter.
Another feature of
the GD-348 is the
coil nulling circuit.
Instead of using
adjusting capaci-
tors in the head
housing that time-
to-time requires
removal of the la-
bel and readjust-
ment. The GD-348 uses a
BALANCE
potenti-
ometer located on the handle control box. This
pot is across two taps of the search coil and is
capacitively coupled to the pickup coil. The two
taps are located so as to be of equal voltage but
opposite polarity resulting in a small signal of
adjustable amplitude and phase that can be ad-
justed to cancel any residual coupling between
the search and pickup coil. The advantage of
this circuit is that the coil housing can be
sealed and made waterproof. The older GD-48
has two holes for access to the adjusting capaci-
tors which are sealed merely by a metallic label
and is not considered waterproof.
The GD-348 continued production until 1981.
GD-348 Accessories:
Heathkit offered three accessories for their de-
luxe metal locator. The GDA-48-1 battery, the
GDA-396 headphones, both described above,
and the GDA-348-1 carrying case. These acces-
sories cost $3.25, $5.95 and $11.95 respectively
in the Spring 1977 mail order catalog.
Heathkit of the Month #52 - GD-48 Metal Locator
HOM rev.new
Page 4 of 7
Copyright 2014, R. Eckweiler & OCARC, Inc.
Fig. 3: Heathkit GD-348
