Heathkit Groundtrack GR-1290 VLF metal detector

Schematic diagram of the Heathkit GR-1290 VLF metal detector
Fig. 1: Heathkit GR-1290 VLF metal detector schematic diagram

Parts list


  • U1: CA3130S (BiMOS Operational Amplifier)
  • U2, U3: LM308 (Precision operational amplifiers)
  • U4: LM2901 (Quad Single Supply Comparator IC)


  • Q1, Q4, Q9, Q10, Q11, Q12: MPSA20 (NPN 40V 100mA)
  • Q2, Q7, Q8, Q13: MPSA55 (PNP 60V 500mA)
  • Q3, Q5: 2N4117 (N-channel JFET transistors)
  • Q6: MPSA13 (NPN Darlington transistor)


  • D1, D3, D4: 1N4149 or 1N4148 (switching diodes)
  • D2: 1N5230B (4.7V, 0.5W Zener Diode)
Heathkit Groundtrack GR-1290 VLF search coil diagram


All resistors are 1/4 watt, 5% tolerance and all capacitors are in microfarads (uF) unless marked otherwise.

Voltages were taken with a 9 Volt supply and a setting with no sound coming from the speaker. The controls were set as follows:

  • Volume -- fully counterclockwise
  • Pushbutton tune -- center of rotation
  • Discriminate -- center of rotation
  • Audio Tune -- center of rotation

The original coil that came with the Heathkit project was preassembled at the factory. It had a diameter of around 15cm (6 inch). A larger (30cm, 12inch) coil can be made using the following data:

L1 = 300mm diameter, 90 turns, .4mm wire
L2 = 140mm diameter, 90 turns, .4mm wire
L3 = 130mm diameter, 550 turns, .1mm wire

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