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 - Page 7