Why these trimmer pots? Bias adjustment?

[dbanner]

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.

[dbanner]

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.

[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?

[dbanner]

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.

[dbanner]

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

[dbanner]

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.

[dbanner]

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.

[waltr]

Ok, good news.

Guess there is an error in the schematic you posted in post #1.

If so you may want to recheck the transistor from pin 1 to 2 of opamp U5.
This may be miss drawn and should be acting as a Log Amp.

[dbanner]

I got the oscillator running, turns out it was missing a resistor on the schematic. This was confirmed by photos of the PCB.

What's more, I have phase shifted sine waves going to the cntrl pins of the CMOS gates.

Quite a difference compared to the use of comparator chips in other circuits, which outputs a more rectangular phase shifted pulse.

I don't know how this is supposed to work, but I guess it does.

The PCB is a two sides board, and I'm not in the mood for tracing the schematic for errors.

[Carl-NC]

Since Spice solves for the operating point prior to the start of a time domain simulation, you can also try ramping the power supply. This will sometimes give the oscillator the necessary kick. Another trick is to set an initial condition on one of the capacitors. This is particularly useful if the oscillator is in a balanced state when the simulation starts.

True & true. Another trick that works with either the current pulse or the I.C. on the main tank cap is that you can usually determine what is needed to kick the thing into a nearly-settled running condition. That way, you don't have to wait for it settle out.

[Qiaozhi]

Oscillators often need a "kick" in Spice. Put a pulse current source in parallel with the coil; pulse, say, 1mA of current for a few microseconds, then off for the rest of the sim.

Since Spice solves for the operating point prior to the start of a time domain simulation, you can also try ramping the power supply. This will sometimes give the oscillator the necessary kick. Another trick is to set an initial condition on one of the capacitors. This is particularly useful if the oscillator is in a balanced state when the simulation starts.

[dbanner]

I added a 68k resistor from the base of NPN to the 6volt supply rail after the 10ohm resistor. The circuit now works. But the shifter outputs to the gates look wrong.
I will also try what you suggested.

[dbanner]

My laptop has crashed, so I'm using my desktop PC, as soon as I can figure out how to get it online, I'll post the spice file and the results.

I'm using my phone to connect to the forum. Lord have mercy!

[dbanner]

When I added a 68k resistor from the base of NPN transistor to the supply rail, the thing runs beautifully.

But now the two phase shifted outputs look bad.

[Carl-NC]

Oscillators often need a "kick" in Spice. Put a pulse current source in parallel with the coil; pulse, say, 1mA of current for a few microseconds, then off for the rest of the sim.