Vallon VMH3CS Mine Detector

[Rocketdemon]

Hi all,

Just read through this thread as my VMH3S is arriving on Monday, was going to get a 2nd hand CS4PI as an intro to PI detecting on wet sand but the Vallon looks like a far better purchase!

All very interesting stuff, 1st question that comes to mind is what effect does potting/sealing have on capacitance/inductance of a DIY coil, sorry to be slightly off topic.

Regards

[Rocketdemon]

Great I got it today, all looks good. When put in mineral mode it indicates full scale, all LED's, then it gradually drops down to zero LED's. Is this Auto Ground Balance?

Regards

[Carolina]

That is not soil compensation. It is very involved. If you PM me Your email address I will send you a manual. Your coil question is quite involved also. You have many issues to solve before you worry about potting.

Carolina

[Rocketdemon]

My version of Eric's headphone mod using LT44 transformer, empty conditioner bottle, bits of bicycle inner tube and an old microphone rubber grommet, works a treat!

[Rocketdemon]

I lied! There was a bit of heat shrink tubing as well!

[Rocketdemon]

Today, I thought I would have a go at making a 11in coil for the VMH3CS. I used the same gauge wire and found that 50 turns gave a similar inductance to the standard truncated ellipse coil. I then overwound it with PTFE tape and then helical cable wrap to space off the shield. The shield is Scotch 24 tinned copper shielding and this went on quite smoothly when I anchored the end with tie wraps.
I had a spare bottom shaft and connected the coil to the internal cable. This is where I thought failure was going to spoil the day, as the detector signalled a fault. I put the scope across the coil wires and nothing was there. I then checked the continuity of the internal cable in the shaft. That was the problem. The shaft has a six pin connector and when I removed this, the cable was totally disconnected. Someone had tried to unscrew the connector and twisted the cable to the point where it completely broke. This took a while to fix, but when the coil was reconnected everything works fine. Great range, very stable - success. The next problem is purely mechanical: mounting the huge swivel assembly onto the coil shell. This has already been cut off a dud Vallon coil and it looks as a bit more work on the mill will be needed. The swivel is very good though as the cabling is internal and sealed. Just the job for shallow water hunting.
[ATTACH]36214[/ATTACH]

[ATTACH]36215[/ATTACH]

Regarding post number 94

I am going to make one of these but never wound a detector coil before, what diameter helical cable wrap should I get? 3mm, 6mm or 12mm, see here https://www.ebay.co.uk/itm/Coloured-...MAAOSwe7BWxHGv

What diameter would 50 turns of 23AWG plus the PTFE tape equate to?

Why did you wrap with PTFE tape? All help appreciated.

Pete

[Rocketdemon]

Looks more the part for beach and nugget hunting with the 11in search head.[ATTACH]36223[/ATTACH]
The coil winding has yet to be mounted are then there are two stages of potting.

The swivel is held in place by two M8 nylon bolts and one stainless steel bolt and nut (non-detectable) [ATTACH]36224[/ATTACH] There will also be adhesive sealant between the underside of the swivel base and the coil shell.

The cable in the shaft goes through the swivel with three O ring seals and a two turn coiled section to allow free adjustments to the coil angle. The coil wire ends will come up into the recess on the top of the swivel in which there is a little PC with two pins. This is where the cable and coil are joined. The circular cover with O ring then seals everything in. [ATTACH]36225[/ATTACH]

The VMH3CSEF should be ready to test on the beach by the weekend.

Where can I obtain the coil housing?

Pete

[waltr]

https://sites.google.com/site/dbcoilshells/home

[KingJL]

Vallon VMH3CS... what we know.

I have spent the last couple of months doing an in depth analysis of the VHM3CS electronics. Eric Foster graciously made available an electronics unit for study. The unit displayed the dreaded LED error sequence 2&3 5&7 10&11 13&14, which I have repaired. I have kept Eric apprised of discoveries as they have occurred. This effort was not to reverse engineer the VMH3CS, but rather to gain an understanding as to it’s performance.

WhenI started this, I had assumed a complicated and sophisticatedtransmitter and simple analog pre-amp with digital sampling andprocessing of the receive functions. I also assumed that the reasonfor demonstrated performance on small resistive (fast tau) targetswas due to very early ADC sampling of the received signal. I couldnot have been more wrong… on all points! Most every day ofdiscovery brought with it another surprise.


Ihave since come to appreciate the VHM3CS as an eloquentimplementation of hybrid analog and digital processing. It utilizesthe appropriate application of 3 technologies... analog, digitalFPGA, and DSP processor. The transmit function is much more simplethan I had presumed and the analog processing of the receiver muchmore sophisticated than I had presumed. All timing signals aregenerated by the FPGA and are therefore ultra stable and precise. The DSP processor makes all determinations of the timing parametersthat are supplied to the FPGA as well as DSP receiver post processingand filtering.

Twoareas that truly impressed me are the efficiency and effectiveness ofthe pulse transformer MOSFET driver circuit and the sensitivity andperformance of the integrator circuits. I have documented myfindings/determinations in the following document.

Theattached “what we know” paper documents what I have discovered. The attached TX-RX (top)test points.pdf depicts the various testpoint that were amassed during this study. I present these to theforum to further discussion and discovery. Again, Eric Foster cannotbe thanked enough... as none of this could have been accomplishedwithout his gracious contribution of an electronics unit for study.

Letthe discovery begin… You do not necessarily need an ultra fast coilto detect small resistive targets!!!

[6666]

Excellent work, very dedicated.

[Altra]

Thank you for posting this information.

[Ferric Toes]

Hats off for patience. Not something I would have tackled.

Eric.

[Mdtoday]

Great work KingJL thank you for sharing.

Mdtoday

[Altair]

Excellent work, thank you for sharing this info.

[kingswood]

Great reading thanks
I dont understand it all...but learning!!