I will make a summary of the advice I gave publicly on the forum and privately.
My LRL essentially consists of an oscillator, around TR1 and a stage with high gain, TR2, TR3 and TR4 that amplifies a small part of the signal supplied by TR1. In the absence of external disturbances the signal at the output is a stable DC voltage. The "heart" is around TR2, the resonant circuit formed by L1 / C10 is connected through C9 to the base of TR2 and in appearance a signal captured by the antenna should be amplified by TR2, TR3 and TR4, and then only a signal with a frequency equal to the resonance frequency could be amplified. But if this were true, there should be a radio station that creates a uniform field in a large area and independent of the direction of the receiving antenna, and I think it's unlikely. If we consider the signal provided by the oscillator it is applied to the base of TR2 and a part of it is brought to ground through C9, C10 and L1. If the output signal increases, that is, we are in the presence of the "phenomenon", it means that lowers the signal that goes to ground, perhaps because it increases the impedance of the resonant circuit L1 / C10.
For the sensor stage it’s better to use a double side PCB with the lower part that is connected to ground and this serves to prevent auto-oscillations. When you have finished building the sensor stage you have to disconnect C2, C3 and C4 and to measure the voltage at the output that must be zero, if not mean that there is too much amplification and you must reduce C13 and/or C14, for example 470pF or 390pF.
Afterwards check that TR1 oscillates at a frequency of quartz, at the TR1 emitter must be about 1 or 2V peak to peak or more, is not change C1 but often you do not need just the parasitic capacitance of the transistor. Then connected in place of C2, C3 and C4 a single capacitor 1pF and measure the voltage at the output that must be in the range 1-5 V DC. If the voltage is more than 5V connect C2 and C3 in serie (we have 0.5pF). if the voltage is still too high connect also C4. If the voltage is zero with the 1pF capacitor necessary to increase the gain by increasing C13 and/or C14. As regards the display stage, with P1 adjusts the threshold and with P2 the gain of IC1A. P2 may be a trimmer to adjust only once so that with P1 a maximum there is not the effect compass.
As for the transistors are all of the type BC183C, this is because I have a lot of it, but I think any type equivalent is fine, the important thing is that they are of the type "C" that is, with great amplification (high beta).
The LRL is not a metal detector and it does not behave as such. The maximum sensitivity is walking from south to north, it is minimal walking from north to south and is average walking from west to east and vice versa. During the search, the antenna must be parallel to the ground but as it approaches the target you may have to tilt it down. Just above the target signal disappears to reappear walking past the target. I hope this will be useful for those who have already built my LRL and for those who plan to build. I ask you only to let me know of your findings.
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