ual warns that the
smaller metal bat-
t e r i e s c a n a l s o
short out the cir-
cuitry. Figure 2
shows the Eveready
246 #1602 battery.
The GD-48 is ca-
pable of detecting
dime-size metal at
6”, quarter-size
metal at 7-1/2” and
a 5” square piece of
aluminum at 20”.
The search-coil
housing is not wa-
terproof so it cannot
be used in shallow
water.
Assembly:
Most of the wiring for the GD-48 is on two cir-
cuit boards. The search circuit board mounts in
the search-coil housing and the audio board
mounts up in the control panel housing. The
components are first mounted on these two
boards. The search and pickup coils come pre-
wound and attached to the coil housing base.
After the search circuit board is mounted to the
coil housing base, the five leads of the search
coil, and the three three terminals of the pickup
coil are soldered to the board; the latter by
short jumper wires.
Next the control panel is assembled. The
speaker and its grill, the controls, phone jack
and meter are attached. A short length of 8-
conductor wire is then connected to the audio
board, as is the negative lead of the battery
connector. The handle is assembled and at-
tached to the control panel.
A four wire spiral cable connects the two circuit
boards (one of the wires is not used). This cable
is now fed through the handle assembly and
into the control panel before it is soldered to
the still-loose circuit board. Finally the audio
board is bolted to the back of the speaker and
the eight-wire cable is dressed and wired to the
meter, speaker, phone jack, and
SENSITIVITY
potentiometer which contains the
OFF
-on
switch. The red battery lead is also connected
to the switch.
The shaft and swivel are then assembled and
the shaft is attached to the control panel and
handle after the spiral cable is snaked through
the shaft. Next, the coil housing top is con-
nected to the shaft; the spiral cable is then
sealed with a tube of supplied sealant where it
enters the coil housing cover.
After the sealant is given time to dry, the spiral
cable end is wired to the search circuit board
and the two halves of the coil housing are
joined by four non-magnetic screws. Finally the
battery and the control panel cover are in-
stalled and the well known Heathkit “blue and
white” label containing the model and serial
numbers is placed on the outside of the control
panel cover.
Alignment:
Alignment involves adjustment of two capaci-
tors
C3
and
C5
located on the search board.
First they are moved to a preset condition, then
they are adjusted in an iterative manner until
the proper null is established. A “coin test” is
then conducted to check for proper phasing
and sensitivity. This test can be repeated occa-
sionally to assure the metal locator alignment
has not degraded.
Final Assembly:
Once you’ve determined the GD-48 is working
properly, the coil cover is removed, sealant is
placed around part of the circuit board to pre-
vent vibration or jarring, and the coil housing
cover is sealed to the coil assembly using the
remaining sealant.
Circuit Description:
The Heathkit GD-48 uses eight transistors, all
type
2N3393
(silicon small signal NPN), and a
Heathkit of the Month #52 - GD-48 Metal Locator
HOM rev.new
Page 2 of 7
Copyright 2014, R. Eckweiler & OCARC, Inc.
Figure 2: NEDA 1602
