Heathkit GD-48 - User Manual

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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

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

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Summary

Heathkit Metal Locators: An Overview

Introduction and Model Listing

Article introduction, mention of "Talk Like a Pirate Day", and listing of four Heathkit metal locator models.

Heathkit GD-48 Metal Locator Details

Introduction to the GD-48, its price history, weight, coil size, and battery requirements.

GD-48 Assembly and Circuitry

Details on how the GD-48 is assembled, including circuit boards, coils, and control panel components.

GD-48 Operation, Alignment, and Final Assembly

Explanation of GD-48 operating modes, sensitivity adjustments, alignment procedures, and final assembly steps.

Heathkit GD-348 Deluxe Metal Locator

GD-348 Overview and Specifications

Details on the GD-348, its weight, battery life, coil housing, and adjustability compared to GD-48.

GD-348 Circuitry and Features

Explanation of the GD-348's advanced circuitry, including transistors, diodes, and the coil nulling circuit.

GD-348 Accessories

Listing of accessories available for the GD-348, including battery, headphones, and carrying case.

Heathkit GD-1190 CoinTrack Metal Locator

GD-1190 Overview and Technology

Introduction to the GD-1190, its design for coins, weight, dimensions, and unique coil system.

GD-1190 Integrated Circuits and Controls

Details on the GD-1190's use of ICs, transistors, diodes, and external controls like volume and mode.

GD-1190 Accessories

Accessories offered for the GD-1190, such as a nicad battery pack, Superex headphones, and a carrying case.

Heathkit GD-1290 GroundTrack Metal Locator

GD-1290 Overview and Features

Introduction to the GD-1290, its price, advanced circuitry, induction balance system, and comparison to GD-1190.

GD-1290 Accessories

Accessories for the GD-1290, which are the same as those for the GD-1190.

GD-48 User Interview and Feedback

An interview detailing user experiences with the GD-48, including ease of build, calibration, and operational issues.

GD-1290 GroundTrack Marketing and Specifications

GD-1290 GroundTrack Features

Description of the GD-1290's capabilities in finding valuables, its professional design, and waterproof search head.

GD-1290 Specifications and Pricing

Technical specifications for the GD-1290, including sensitivity, detection method, power requirements, and kit price.

Summary

Heathkit Metal Locators: An Overview

Introduction and Model Listing

Article introduction, mention of "Talk Like a Pirate Day", and listing of four Heathkit metal locator models.

Heathkit GD-48 Metal Locator Details

Introduction to the GD-48, its price history, weight, coil size, and battery requirements.

GD-48 Assembly and Circuitry

Details on how the GD-48 is assembled, including circuit boards, coils, and control panel components.

GD-48 Operation, Alignment, and Final Assembly

Explanation of GD-48 operating modes, sensitivity adjustments, alignment procedures, and final assembly steps.

Heathkit GD-348 Deluxe Metal Locator

GD-348 Overview and Specifications

Details on the GD-348, its weight, battery life, coil housing, and adjustability compared to GD-48.

GD-348 Circuitry and Features

Explanation of the GD-348's advanced circuitry, including transistors, diodes, and the coil nulling circuit.

GD-348 Accessories

Listing of accessories available for the GD-348, including battery, headphones, and carrying case.

Heathkit GD-1190 CoinTrack Metal Locator

GD-1190 Overview and Technology

Introduction to the GD-1190, its design for coins, weight, dimensions, and unique coil system.

GD-1190 Integrated Circuits and Controls

Details on the GD-1190's use of ICs, transistors, diodes, and external controls like volume and mode.

GD-1190 Accessories

Accessories offered for the GD-1190, such as a nicad battery pack, Superex headphones, and a carrying case.

Heathkit GD-1290 GroundTrack Metal Locator

GD-1290 Overview and Features

Introduction to the GD-1290, its price, advanced circuitry, induction balance system, and comparison to GD-1190.

GD-1290 Accessories

Accessories for the GD-1290, which are the same as those for the GD-1190.

GD-48 User Interview and Feedback

An interview detailing user experiences with the GD-48, including ease of build, calibration, and operational issues.

GD-1290 GroundTrack Marketing and Specifications

GD-1290 GroundTrack Features

Description of the GD-1290's capabilities in finding valuables, its professional design, and waterproof search head.

GD-1290 Specifications and Pricing

Technical specifications for the GD-1290, including sensitivity, detection method, power requirements, and kit price.

The Heathkit GD-48 Metal Locator is a treasure-seeking device designed to detect metal objects buried in the ground. It was introduced in 1969 and remained in production until 1977.

Function Description:

The GD-48 operates on the principle of Very Low Frequency (VLF) induction balance. It uses a search coil (L1) and a pickup coil (L2). A VLF oscillator (Q1) generates a 100 KHz signal, which is modulated with an audio tone of about 650 Hz by a relaxation oscillator circuit. This signal is transmitted by the search coil. When a metal object is present in the search field, it creates its own magnetic field, which is then sensed by the pickup coil. The signal from the pickup coil is amplified through several stages, including a differential amplifier (Q4 and Q5) and a common emitter amplifier (Q6). The amplified signal then drives an audio output stage (Q7 and Q8), which produces an audible tone and deflects a meter, indicating the presence and relative strength of the detected metal. The device is designed to detect dime-sized metal at 6 inches, quarter-sized metal at 7-1/2 inches, and a 5-inch square piece of aluminum at 20 inches.

Important Technical Specifications:

Weight: 4 lbs.
Coil Housing Diameter: 10.5 inches.
Adjustable Arm Length: 26 to 36 inches.
Power Source: NEDA #1602 9-volt battery (Heath part # GDA-48-1, Eveready part # 246).
Battery Life: Approximately 80 hours with a NEDA #1602 battery.
Oscillator Frequency: 100 KHz.
Modulation Frequency: Approximately 650 Hz.
Transistors: Eight 2N3393 (silicon small signal NPN) transistors.
Diode: One 1N4002 silicon diode.
Waterproofing: The search-coil housing is not waterproof and cannot be used in shallow water.
Original Cost: $59.95 (1969), increased to $69.95 by 1976, then offered at $59.95 on special, and back to $59.95 in 1977.

Usage Features:

Operating Modes: The GD-48 has two operating modes: normal and high sensitivity.
Normal Mode: In this mode, the sensitivity control is adjusted until a tone is heard, then reduced until the tone just disappears.
High Sensitivity Mode: In this mode, the sensitivity control is advanced until the meter reads about 2 on a scale of 1 to 10. When metal is encountered, the meter reading increases, providing a more noticeable indication than just a change in tone level.
Controls: Includes a SENSITIVITY potentiometer with an OFF-on switch, a speaker, and a phone jack.
Coil Sensitivity: The most sensitive part of the search coil is directly underneath the spot where the swivel joins the coil housing.
Accessories: Optional accessories included the GDA-48-1 NEDA 1602 battery and GDA-396 2,000Ω earphones.

Maintenance Features:

Alignment: The device requires alignment, which involves adjusting two capacitors (C3 and C5) on the search board. This is done iteratively to establish a proper null. A "coin test" is then performed to check phasing and sensitivity, and this test can be repeated periodically to ensure alignment has not degraded.
Battery: Uses a NEDA #1602 9-volt battery. While a common 9V NEDA #1604 battery can be used, it will provide shorter operation time. The manual warns that smaller metal batteries can short out the circuitry.
Vibration Prevention: Sealant is applied around part of the circuit board within the coil housing to prevent vibration or jarring.
Adjustments: The device may require re-adjustment each time it is used, though it generally holds its adjustment well once set.
Repair: The swivel connecting the sense-head to the arm is a potential point of failure, as noted by a user who eventually abandoned his unit due to this part breaking.

Heathkit GD-48 Specifications

General

Brand	Heathkit
Model	GD-48
Category	Metal Detector
Language	English