This is an old Soviet beat frequency oscillator metal detector, used in the Soviet army around 1950's.
The abbreviation UMIV-1 means "High-frequency Improved Metal detector". The frequency of operation is 20 kHz.
The power supply of the metal detector includes two 1.5 V cells (the Russian 373 battery is a "D" cell), they provide 3 volts for filaments, and an anode battery 70-AMCG-U-120, it provides 60...70 volts for anode voltages.
The circuit diagram of the metal detector UMIV-1 (see Fig. 1.) comprises two oscillator circuits - the search oscillator circuit based on the valve V1, and the reference oscillator circuit based on the valve V2. Both valves are 2J27 (this is a high frequency pentode, made in USSR). The inductance coil L1, capacitors C1, C2 and the primary winding of the transformer Tr are the resonant tank elements for the anode circuit of the search oscillator. The secondary winding of the transformer Tr is used to connect the resonant tank with the grid of the valve V1.
The tapped inductance coil L2, capacitors C5 and C6 are the resonant tank components of the second oscillator. Two bypass capacitors C3 and C7 are intended to prevent AC currents flowing between oscillators through power supply wires. The capacitor C3 is used to bypass the headphones from the point of view of the high frequency. The resistor R4 is used to decrease the filament voltage, this resistor is used when anode battery is fresh. After 5..6 hours of metal detector operation, the resistor should be shorted out with the switch S1.
In the reference oscillator circuit the headphones Tf and the upper part of the coil L3 are anode load of the tube V2. When the power supply is on, both oscillators produce oscillations of high frequency (about 20 kHz) that can't be heard in the headphones.
Fig. 1. Circuit diagram of the metal detector UMIV-1
V1, V2 - 2J27; R1 - 51 K; R2, R3 - 220 K; R4 - 6..120 Ω;
C1 - 6..25 pF; C2 - 10 nF; C3 - 25 nF; C4 - 430 pF; C5 - 200 pF; C6 - 47 pF; C7 - 0,01 uF;
L3 - 0.27 uH
The signal of the search oscillator mixes with the signal of the reference oscillator, and as a result there will be a beat frequency. If both frequencies of oscillators are slightly different, the beat frequency can be heard as a tone in the headphones.
The frequency of the reference oscillator can be adjusted within some range, this is the coarse tuning. It can be done by using the ferrite slug that changes the inductance of the coil L3, therefore the frequency of the resonant tank L3C5 will be changed.
The variable capacitor C1 of the searching oscillator is used for the fine tuning of the metal detector.
The larger a metal target, the closer the search coil to this target, the more frequency shift it will produce, therefore more noticeable frequency shift of the beat frequency will be heard in the headphones.
Fig. 2. Common view of the metal detector UMIV-1
To set up the metal detector, adjust the beating frequency to make it as low as possible, but steady. It will provide the maximum sensitivity of the device.
The operator can use the metal detector standing in upright position, by adding two additional sections to the rod. One of the rod has inside an additional inductance coil L2 that compensates the capacitance and inductance of the cable.