The GD-48 was the first metal detector produced by Heathkit, first listed in the 1969 Summer catalog, originally sold for $59.95. 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 vintage NEDA #1602 9-Volt battery (Heath part #GDA-48-1).
The Heathkit GD-48 uses eight 2N3393 transistors and a single 1N4002 silicon diode.
Q1 is a VLF oscillator that uses L1 and C2 to produce a 100 KHz signal. Feedback is provided through an RC network that includes R2 and C1 which causes Q1 to stop and start oscillating at around 650 Hz, modulating the 100 KHz signal with an audio tone.
The pickup coil is placed in a way so that the coupling between it and the search coil to be minimal. The trimmer capacitor C3 can be adjusted to balance out any stray coupling.
The pickup coil is tuned by C5, and the signal from the coil is capacitively coupled to two amplification stages (Q2 and Q3).
The output of Q3 is fed up to the audio board in the control assembly. On this board the signal enters into a differential amplifier composed of Q4 and Q5. The signal is fed to the base of Q4, while a voltage level, adjusted by R16 (the "Sensitivity" pot) is fed to the base of Q5.
This voltage sets the current through the emitter resistor R14 and hence the bias point of Q4. The sensitivity pot is normally set to the point where, with no input signal, Q4 is just cut off. When a signal appears, Q4 starts to conduct and amplifies it. The output of Q5 is further amplified by Q6.
The audio output and meter driving stage is handled by Q7 and Q8. The diode and associated resistors scale and protect the meter from excessive overloads.
The GD-48 has two operating modes, normal and high sensitivity. In normal mode the sensitivity control must be adjusted (with the coil away from any metal) until a tone is heard and then must be reduced until the tone just goes away.
In high sensitivity mode, the sensitivity control should be advanced until the meter reads about 2 on the scale of 1 to 10. As a metal object is encountered, the indicator will increase. Its movement is much more noticeable than a change in the level of the tone.
The most sensitive part of the search coil is underneath the spot just in front of where the swivel joins the coil housing.
Note: Schematic and circuit description was adapted from the original Heathkit service manual & the February 2014 issue of RF, the newsletter of the Orange County Amateur Radio Club.