Vallon VMH3CS Mine Detector

[Asgard]

Thank you for your reply Carl, good progress on your projects do not hesitate to come to me, if you have needs, I would be happy to help you.

[Ferric Toes]

More features on the VMH3CS

This detector has more features than I thought. When you switch on there is first a self check, after which it defaults to a standard high sensitivity mode with no compensation. By pressing the + button and holding down the sensitivity level is indicated by the number of LEDs illuminated with the default at 12. If you keep holding the button down it goes up to 13 and finally 14. Sensitivity is reduced in the same way by pressing the - button and can be brought down to level 1. Any changes that are made when the detector is switched on, are cancelled when switched off and the default setting of 12 returns.

The operation of the pinpoint mode is different to what I previously thought. When the button with the crosswires is held down the detector changes to a non-motion mode. This means that the signal does not disappear when the coil is stationary over the target and the maximum signal is more easily heard to give best pinpointing.

Various combination of the buttons can bring up different features. There is a surf mode for using the detector in sea water. This, I guess, retards the sampling a bit to reduce the effect of sea water conductivity. I will check this out when I go to our local beach shortly. Apparently this setting is stored when unit turned off, until reset, and may account for the lower range on a nickel that Carl was getting on his unit. The pinpoint button and - have to be pressed together for surf operation. Pinpoint button and + for land. For compensation in water, the C button is pressed, then released, with the head in water and prior to searching.

There is what is called 'channel adjustment' this is to eliminate outside interference and crosstalk with other detectors. This is done with a combination of buttons and the unit switched to speaker mode. Channel settings are stored until changed.

There are 6 filter settings with different bandwidths to decrease the effects of electromagnetic interference. Default is 6Hz and minimum bandwith 0.5Hz. This has yet to be investigated, and how this affects performance. Filter settings are not stored at power down and return to default of 6Hz.

More later on the ground compensation facility.

[greylourie]

Maybe the picture in Janes of the VMH3 is of an earlier version. Because a look at images in the respective pdf brochures on their website for both models, shows them sharing identical buttons and knobs. Vallon also has evaluation software available for download, so with the right cable it should be possible to log the output of the detector to a computer/laptop. (http://www.vallon.us/download/download.html)

[Ferric Toes]

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.

[daverave]

very nice workshop

[green]

Did a simulation in spice. Don't know if it means anything since I couldn't enter coil diameter. Compared to coils, 300uH(1MHz resonance) and 1.5mH(170kHz resonance). Target TC 3.3usec which could be close to a .3gm nugget. Could see target decay on both simulations. Tested a 6x6mm piece cut from the bottom of an aluminum coke can(TC=3.1usec) and a piece 6x6mm cut from the side(TC=1.3usec) awhile back. Couldn't see the target decay on the 1.5mH coil simulation with a target TC of 1.3usec.

[Monolith]

very nice workshop

I would trade all my secrets for a workshop like that.

[Davor]

When I grow up, I'll have such a workshop of my own.

[WM6]

When I grow up, I'll have such a workshop of my own.

Probably fertilization is poor to grow up faster.

When asked in quiz, I would say that it is workshop of our valued member Mechanic.

[Ferric Toes]

very nice workshop

That is just one corner, the rest was too untidy to show. I always get in a mess when working on something interesting. The big tidy up comes tomorrow.

[Chet]

I checked a US nickel on my VHM3CS and get an air range of 15 inches for a definite response and 16 inches for an iffy response that you would likely miss. I think that is good for a coil which is about equivalent to an 8 inch diameter. That is why I want to make a 12in and see what I get then. What foil are you using, Carl? Is it baking foil?
I hope to get another VHM3CS shortly, which I will be happy to open up and look at the works.

Hi Eric, Carl
I just tried a US Nickel with My White's TDI-SL with the 12" Dual Field Coil. It gets it solid sound at 9.5" and an iffy sound at 10.5". The VHM3CS has been in production for many years. Do your units have manufacturing dates on them? The date may be on an inspection seal. The one that Carl has may be an earlier model or an earlier software version.
Have a good day,
Chet

[bbsailor]

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.
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Eric,

Your use of Scotch 24 is very good to see as I discovered this stuff many years ago when doing independent coil research. Lead shields seems to have fallen into safety and health issues these days. I see that you chose to spiral wrap the double layer Scotch 24 around the coil. Another method is to cut it in a single layer a bit narrower than the coil cross section bundle circumference and put it on the coil as a U-shaped shield with the open seam on the top of the coil. This keeps the wire mesh shield from forming a shorted cross section loop around some coils that may be detected at low delays. I used this U-shaped shield method along with Teflon insulated coil wire for an 11" 300 uH coil used on your CS6PI low power PI design that I modified to operate at 13.5 KHz at about a 7.5 uS delay. This gets me about 14" on a U.S. Nickel. The spiral wrap shield method, however, may not be an issue with this VMH3CS mine detector coil where low noise and not coil speed is the main issue.

Thanks for the update.

Joseph J. Rogowski

[daverave]

Hi Eric, Carl
I just tried a US Nickel with My White's TDI-SL with the 12" Dual Field Coil. It gets it solid sound at 9.5" and an iffy sound at 10.5". The VHM3CS has been in production for many years. Do your units have manufacturing dates on them? The date may be on an inspection seal. The one that Carl has may be an earlier model or an earlier software version.
Have a good day,
Chet

when you tested with US nickel was that with ground balance on ??? if so what is the depth on nickel with ground balance switched off ???

[Ferric Toes]

Hi Eric, Carl
The VHM3CS has been in production for many years. Do your units have manufacturing dates on them? The date may be on an inspection seal. The one that Carl has may be an earlier model or an earlier software version.
Have a good day,
Chet

Mine has the year 2010 on the serial no. plate. There is also a 5 and a 10 punched on a final inspection label, so could be May 2010.

[Chet]

when you tested with US nickel was that with ground balance on ??? if so what is the depth on nickel with ground balance switched off ???

Hi Daverave
The Ground Balance was off. The test was in my back yard with the coil lying on a 1.5” thick board lying on low mineralized ground. The 1.5” represents the average scan height over cobbles, pine needles, leaves and branches encountered in the California gold country. I just retested by ground balancing beside the board (at 7 on the control) then placed the coil back on the board. It surprised me that it only lost 0.5” in depth. I know that is loses much more when on high mineralized ground in the field.
Have a good day,
Chet