Nautilus DMC IIb schematic

[Mario-j23]

Sorry to rehash an old thread, but this detector is bugging me. I went back to trying to start the detector and I can't adjust the BG. I don't have a picture and I don't know the values of the marked capacitors. They are KEMET products but only the date code is visible. I did a Ltspice simulation and it seems to me that these capacitors have a value of 10-100pf?. Maybe one of the esteemed colleagues has pictures of the Nautilus Dmc IIB or IIba board or knows the value of the marked components.

[steven]

also with schematic is difficult to build because use a triaxial coil

[Mario-j23]

Building the coil wasn't difficult. My DMC IIB clone works! It detects and discriminates metals flawlessly. I can set the loop balance using the R and C pots. I'm just having trouble removing the ferrite. I don't want to close the coil until I can figure that out. Maybe I have the wrong capacitor value on Rx? But first I need to rule out other errors.

[Altra]

Can you post the schematic you used, your google drive link in previous post is not working. I have a copy that Jerry Tyndall sent me. It's 11"x 17" paper so I can't post until I digitize. But I can correct any difference between the two schematics

[Mario-j23]

Yes. I used the diagram that Dbanner posted but I redrew it. There is a small mistake in my diagram (v-) is -6.5v in reality it is -8v! Thanks for your help.

[kt315]

Can you post the schematic you used, your google drive link in previous post is not working. I have a copy that Jerry Tyndall sent me. It's 11"x 17" paper so I can't post until I digitize. But I can correct any difference between the two schematics

i did so... attach the paper at autobus halt's wall near you. by scotch tape, for exampe. do the pics.

[Altra]

I took a photo of the original. Tyndall, I believe used white out on the schematic around the part for nulling the coil. From what I can tell it is accurate for the rest of the circuit. I also found my redraw from over 30 years ago done on autocad. People take for granted all the great software we have now.

Mario, I noticed some different capacitor values on your schematic around the GB pot. I think Dbanner took some part values from the Aquasound schematic which does not have GB.

Coil - Tx 2.7mH 10ohm, Rx 8.0mH 32ohm
Tx should run between 13.7 - 13.9 kHz

[Mario-j23]

Thank you Altra for the diagrams. Once I make the corrections I will let you know if I managed to fine tune the ground balance.

[Mario-j23]

I compared the diagrams and have some thoughts. It appears that the Nautilus IIB came in three versions.
The first one from 1993 without the "Transmit Power" potentiometer which had a Tx Hi/Low power switch. (Scheme posted by Altra)
The second version, around 1997, had smooth power regulation with the "Transmit Power" potentiometer and changes to some elements on the PCB (the diagram posted by Dbanner).
The last version was mounted on a DMC BIIa board, probably after 2000, without automatic "Autotiune" coil balancing. The double-sided board that I am trying to run has a capacitor added between the LM393 comparators input. I compared all available diagrams and the only thing missing is the value of these capacitors! What value did Jerry add here?

[Altra]

I think you are correct about the different versions. I don't know the dates, but Jerry retired and sold the company to a guy named Steve.The new owner added the auto null circuit which became the 2Ba and also the newer units used the green pcbs. The older units did not have the green solder mask. Jerry continued to make the coils at home even after retiring.

The LM393 capacitors are probably in the PF range to stop oscillations?

[Mario-j23]

But is it 1-100pf or 100-1000pF? and are both the same because the output signal is phase shifted depending on the capacitor used?

[Altra]

Apparently the detector worked without these capacitors for a number of years. If you are unable to ground balance you need to connect a oscilloscope to the pre-amp output and to the switching pins on the cd4066. Check to see if the GB pot moves the sample point.

[Mario-j23]

Without capacitors the detector did not work! The correct value is 62pF. To set the sampling point to the cd4066 in the middle I had to add an 82K resistor in front of the C24-470p. The diagram I posted above is probably correct and applies to the last green double-sided board.