Heathkit GD-48 User manual

Heathkit of the Month #53:
by Bob Eckweiler, AF6C
GENERAL PRODUCTS
Heathkit GD-48
Metal Locator.
Introduction:
Buried Treasure? Aye Matey, I was saving this
series of Heathkits for September as that is the
month of “Talk Like a Pirate Day” which falls
on the 19th. Alas, so does the OCARC gatherin’.
I expect a fun time!
However, our rotating editor this month is Ken,
W6HHC, and since he passed along manuals for
the GD-48 and GD-348, I thought I’d honor him
with an article on them for his turn as editor.
I’m sure many a pirate of olde would have liked
to have a metal locator for finding those treas-
ures that they buried, only to later realize the
map they made was later used at the bottom of
the parrot’s cage.
Heathkit Metal Locators:
Heathkit used the nomenclature “metal ‘loca-
tor’” instead of “metal ‘detector’” for their
treasure seeking products. Heathkit produced
four models over the years that I’m aware of.
The first was the GD-48 which was listed as
new in the 1969 Summer catalog. The Deluxe
GD-348 followed in 1972 and sold concurrently
with the less expensive GD-48 for five years.
Around 1978 Heathkit introduced the GD-1190,
a third model designed specifically for hunting
small coins. In ads it was called the “Coin-
Track”. Around 1979 the last of the Metal Loca-
tors, the deluxe GD-1290 “GroundTrack” was
released.
Table 1 gives the production dates as best I can
determine with the catalogs in my files.
The Heathkit GD-48 Metal Locator:
The first Heath-
kit metal locator,
the GD-48 origi-
nally cost $59.95.
By 1976 the price
had increased to
$69.95; but in
the fall of that
year it was offered
again for $59.95
on a “save $10”
special. The last
catalog (1977) I
have showing it
listed had the
price back at the
original $59.95.
The GD-48 is
shown in Figure
1. It weighs 4
lbs. and has a
10.5” diameter coil housing. The arm length is
adjustable from 26” to 36”. It is powered by a
NEDA #1602 9-volt battery (Heath part #
GDA-48-1). This is a different battery than the
common 9V battery used in so many products
today. It is rated at 850 ma/hr and measures
approximately 1.3” x 1.4” x 2.7”. (The Eveready
part # is 246). These batteries now run about
$11.00 each and will power the GD-48 for
about 80 hours of operation. The cheaper,
more available, NEDA #1604 may be used but
will provide shorter operation time. The man-
HOM rev. new Heathkit of the Month #53 - GD-48 Metal Locator
Copyright 2014, R. Eckweiler & OCARC, Inc.!Page 1 of 7
Heathkit Metal ‘Locators’:
Model Name From To
GD-48 (none) 1969 1977
GD-348 (none) 1972 1981
GD-1190 “CoinTrack” 1978 1983
GD-1290 “GroundTrack” 1979 1985
Table 1
Fig. 1 Heathkit GD-48

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

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

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 2,000Ωearphones
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
SENSITIVITY
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

The Heathkit GD-1190 “CoinTrack”:
In 1978 Heathkit introduced a new metal loca-
tor. It is believed to originally have sold for
$119.95. This device was designed specifically
for finding small coins. The GD-1190 weighs
3.4 lbs. The over all length is adjustable from
22-1/2” to 45-1/2”. The small coil housing
measures 6” in diameter. The meter may be as-
sembled for left or right-hand operation. Power
is supplied by 6 AA batteries. An optional nicad
battery pack and charger are available. The cable
from the coil housing to the control unit is external
to the adjustable shaft and wraps around it. The
cable can be unplugged at the control unit.
Unlike the coil system of the previous metal lo-
cators, this locator uses a search coil but no
pickup coil. Instead of relying on the metal be-
ing located creating its own magnetic field that
is then sensed by a pickup coil, the GD-1190
relies on the metal detuning the resonance of the
coil and associated capacitance. The coil is ex-
cited by a stable 3.59 crystal oscillator divided
down to 56 KHz. Detection is done by a dis-
criminator circuit.
This is the first Heathkit metal locator to use
integrated circuits. The five ICs are an LF211
FET comparator, LM2902 quad op-amp,
CA3130 op-amp, CD4011 CMOS quad NAND
gate and a CD4024 CMOS binary counter. One
JFET and five bipolar transistors (one a Dar-
lington) are also used, along with 4 diodes (one
general purpose, one varactor and two zener
diodes).
External controls include VOLUME, DIS-
CRIMINATE, TUNING, a 4-position MODE
switch and auto TUNE pushbutton. A meter,
speaker and PHONES jack mount on the con-
trol cabinet which also supports a handle. The
“CoinTracker” custom balances for less fatigue
during use.
GD-1190 Accessories:
Three accessories were available for the GD-
1190. The GDA-1190-1 nicad battery pack, the
new GDA-1190-2 Superex headphones and the
GDA-110-3 carrying case. In the Fall 1980
catalog these sold for $19.95, $11.95 and $11.95
respectively.
The accessory nicad battery pack can be
charged either from a car cigarette lighter
socket or from a standard 120V AC outlet.
The Heathkit GD-1290 “GroundTrack”:
In 1979, shortly after the GD-1190 was intro-
duced, Heathkit introduced another metal loca-
tor, the GD-1290 “GroundTrack”. In the Fall
1980 catalog it sold for $189.95. (Later it sold
for $219.95. The ad called this locator “Our fin-
est Locator”; it was also the last locator that I
am aware they built. Unlike the “CoinTrack”
this unit uses search and pickup coils (induc-
tion balance) like the older units, though its
circuitry is significantly advanced. Physically
this locator is similar to the “CoinTrack” in
size, weight and appearance, with some minor
variations in controls.
GD-1290 Accessories:
The GD-1290 uses the same three accessories
(the nicad battery pack, the Superex head-
phones, and the carrying case) that were avail-
able for the GD-1190.
Comments:
While I was is possession of manuals for the GD-
48 and GD-348. Finding information, or even a
clear schematic, for the other two units proved
inadequate to give a more thorough description.
GD-48 User Interview:
I got a chance to interview Ken - W6HHC on his
thoughts about the GD-48 Metal Locator.
Mostly his comments were positive. The kit was
easy to build, calibrated easily and operated as it
was supposed to do. He mentioned he liked to
take it along on the family outings to the beach;
his kids enjoyed searching in the sand too.
Ken did comment on two faults he had with the
GD-48. The first was that it needed to be ad-
justed whenever he took it out. However he
said that once adjusted it stayed in adjustment
HOM rev. new Heathkit of the Month #53 - GD-48 Metal Locator
Copyright 2014, R. Eckweiler & OCARC, Inc.!Page 5 of 7

until it was put away for another day.
Ken’s second complaint is one that
probably most metal locator owners,
who go looking for coins or treasure,
have. The GD-48 has a poor ROI.
For those of you who don’t know the
phrase ROI, it is a business term that
stands for Return on Investment! Ken’s
ROI was nominal. Maybe if aluminum
recycling was in affect in the seventies,
he might have been able to return all
the can tabs he found. But alas, Ken
never mentioned gold coins nor pieces-
of-eight he located under the search
head during his explorations.
I personally know of one dime he did
find. I hid it under the carpet when we
were checking his GD-48 out one after-
noon.
When I asked Ken what happened to
his metal locator, he said the swivel
broke between the sense-head and the
arm and he finally abandoned it. How-
ever he said it gave him good service for
many years.
Acknowledgements:
I’d like to thank Ken - W6HHC for sav-
ing and passing along his GD-48 man-
ual after his unit broke beyond repair.
He also provided the GD-348 manual
that he acquired from a friend.
73, from AF6C
Heathkit of the Month #52 - GD-48 Metal Locator!HOM rev.new
Page 6 of 7!Copyright 2014, R. Eckweiler & OCARC, Inc.
Figure 4: GD-1290 “GroundTrack”. The GD-1190 “CoinTrack” is
similar in size and appearance. (From 1984 Catalog)
Remember, if you are getting rid of
any old Heathkit Manuals or Catalogs,
please pass them along to me for my
research.
Thanks - AF6C
This article originally appeared in the
February 2014 issue of RF, the news-
letter of the Orange County Amateur
Radio Club - W6ZE.

HOM rev. new Heathkit of the Month #53 - GD-48 Metal Locator
Copyright 2014, R. Eckweiler & OCARC, Inc.!Page 7 of 7