Why these trimmer pots? Bias adjustment?

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Well actually it looks ok. The resistor's position I can't say. I've seen this arrangement before on the Bounty Hunter junior schematic.
I thought I had a good photo of the underside of the PCB, turns out I have two top side photos of two different pcb's of the same model detector. Oh well.
With the base and emitter shorted, the base collector junction is just a diode.

Time to move on. I'll not linger on this schematic any longer.

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I found the coil data, very detailed information about the coil and also some pics.

So it is feasible to build the circuit. I also have the TX and phase shift circuit working good in ltspice.

Maybe I'll continue on a bit with further simulation of the preamp and filters. Try to work out any bugs in the schematic.

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The manual says it's a 13.77kHz detector, and if I work out the resonance of the coil based on the information provided by the coil data specs shown above, it's bang on.

O.595mH with 220nF capacitor works out at 13.8kHz.

So that verifies that it's the correct coil data.

Here is a video of the compass liberty 50, towards the end you can see the recovery speed. Very fast. Nice and sharp response. It's the same detector as the circuit I posted above.
https://m.youtube.com/watch?v=3irKo7UH9oE

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It's an old design. I know. Perhaps there is room for some tweaks to the circuit.

Most obvious candidate for upgrade is the power supply.

Next is getting rid of some of those trimmers used for offsets.

Also eliminate the battery check with a redesign of the audio amp.

A DD search coil would finish it off.

[WM6]

Perhaps there is room for some tweaks to the circuit.

What about to remain in genuine state and proudly exhibited in your own MD museum?

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The coil photos look interesting, the ratio of the RX coil diameter to the tx coil diameter just shows that a RX coil could be of any diameter, provided that it is nulled out by sufficient turns for the feedback coil.

There is much to learn from these older circuits, such as analog ground resting above the battery negative and also the novel use of one of the quad gates as an oscillator to drive the chopper.

I'm sure there are other little things that can be lifted from this schematic, one being a strange lamp in the audio section of the circuit.. all these little things can be worthy of note. It gives lots of ideas for modding other circuits.

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The Compass xp350 pro is essentially the same circuit, so why not cut and paste the pwer supply and tx oscillator and phase shifter parts of the circuit unto the liberty 150 schematic.

Yes, that'll work out just fine. Now it becomes an XP350 LITE version. This will be a possible upcoming project.