Vallon VMH3CS Mine Detector

[garyq]

Coin Tests

Have you tested it yet on a 5cent and 2 dollar coin ?

Hopefully kingswood can find time to test, even an air test result, using both coins for any difference between his stock Vallon coil and his newly acquired 30cm coil.

[kingswood]

The 2010 edition of the VMH3CS Operators manual:

3.6.1. Working with the automatic frequency search
The automatic frequency scanning is used to detect recurring interference signals which are
received through the search head, and suppress. It can be used to minimize interference eg
power lines or from neighboring devices of the same type. Sporadic or constantly changing noise
can not be suppressed.

1. Set the program selector to the volume control
2. + and - keys simultaneously press briefly. A deep, chopped tone (500 Hz) is then heard and

the automatic frequency search is enabled

During the frequency scan continues to sound a deeper tone chopped (about 500 Hz) and

alternately in the LED display will flash all odd or even LEDs.

The automatic frequency search may take up to 2 minutes. If it ends, the volume setting

(high tone) automatically, the detector switches back again.

The identified frequency is stored in the detector permanently and retained even after turning off

the detector. The next time it is set automatically.

But, IT DOESN'T WORK... at least on mine!

Essentially the same as the VMC1 then ......My manual is 2007 and doesnt even mention automatic selection! In fact, mine only mentions 2 channels........To scared to try your 2010 instructions in case I break something ha ha!!

[KingJL]

My manual doesnt even mention automatic channel selection.....
I dont know if this helps (You may already have this info), but the VMC1 auto instructions are: Turn to volume position, press and hold both the up and down volume buttons simultaneous. Hold these until the alerts happen (audio/visual and/or vibration). Release the buttons. The selection process takes about 3 minutes, during which the odd and even LED numbers will alternately light up and the alerts continue.

The 2010 edition of the VMH3CS Operators manual:

3.6.1. Working with the automatic frequency search
The automatic frequency scanning is used to detect recurring interference signals which are
received through the search head, and suppress. It can be used to minimize interference eg
power lines or from neighboring devices of the same type. Sporadic or constantly changing noise
can not be suppressed.

1. Set the program selector to the volume control
2. + and - keys simultaneously press briefly. A deep, chopped tone (500 Hz) is then heard and

the automatic frequency search is enabled

During the frequency scan continues to sound a deeper tone chopped (about 500 Hz) and

alternately in the LED display will flash all odd or even LEDs.

The automatic frequency search may take up to 2 minutes. If it ends, the volume setting

(high tone) automatically, the detector switches back again.

The identified frequency is stored in the detector permanently and retained even after turning off

the detector. The next time it is set automatically.

But, IT DOESN'T WORK... at least on mine!

[kingswood]

Strange that the manual does not give more detailed information. Manually going through 40 channels to find a quiet spot is a bit tedious, particularly as you have to switch back to 'normal', or 'mineral' each time to check for noise.Eric.

My manual doesnt even mention automatic channel selection.....
I dont know if this helps (You may already have this info), but the VMC1 auto instructions are: Turn to volume position, press and hold both the up and down volume buttons simultaneously. Hold these until the alerts happen (audio/visual and/or vibration). Release the buttons. The selection process takes about 3 minutes, during which the odd and even LED numbers will alternately light up and the alerts continue.

[Ferric Toes]

I found it interesting though... that the freq meter reading seemed to be exactly 1.5 times the actual frequency (the actual frequency being 0.666666667 the meter reading). Coincidences like that intrigue me!

I re-measured all 40 channels and all frequences were within 2 pulses of your measurements when using the clipping circuit.

It is interesting that the highest and lowest frequencies are just one click from each other, right in the middle of the channel table i.e. 12.10 is lowest, 1650 on my unit, and 13.01 is highest, 2040 on my unit. The next low/high pair are 13.10 and 14.01 and the final 11.10 and 12.01. I suppose in a noisy EM situation you could start in either of those three positions and one click would give you a good idea of which way to go, and then fine tune from there. Strange that the manual does not give more detailed information. Manually going through 40 channels to find a quiet spot is a bit tedious, particularly as you have to switch back to 'normal', or 'mineral' each time to check for noise.

The frequency counter I have is a TFC2700L

Eric.

[KingJL]

My frequency meter did not like the spikes at the end of each pulse, but when I fixed a resistor and back to back diodes to clip the spikes I then got similar results to yours in post 728. The frequency meter has a 250V max. input and the loosely coupled 3in pickup coil must have the spikes well below the max. voltage. I haven't been able to do an automatic channel adjustment either.

Eric.

I found it interesting though... that the freq meter reading seemed to be exactly 1.5 times the actual frequency (the actual frequency being 0.666666667 the meter reading). Coincidences like that intrigue me!

[Altra]

Thanks for the great info TH'r, KingJL and Eric.

I was able to find a few quiet channels. Channel 5 bank 11 was quiet indoors in my location (60Hz envirnment), same as TH'r. Probably noisey in a 50Hz area. My unit is a 2008 model, which seems to have the same features and sensitivity as others have posted.

[TH'r]

I fired up the oscope and measured the prt for all channels. Calculated the freq from the measured prt... results: Low 1651.8 Hz (channel 12,10), High 2037.9 (channel 13,1).

Channel sorted by frequency
[ATTACH]39208[/ATTACH]

Channel sorted by channel
[ATTACH]39209[/ATTACH]

During my testing I attempted to select the operating channel automatically. Following the instructions in my manual (2010 edition), I could not get it to automatically select a channel. Any ideas?

Somewhere it is written that the British Military ordered these detectors without the automatic channel or surf adjustments.

[Ferric Toes]

My frequency meter did not like the spikes at the end of each pulse, but when I fixed a resistor and back to back diodes to clip the spikes I then got similar results to yours in post 728. The frequency meter has a 250V max. input and the loosely coupled 3in pickup coil must have the spikes well below the max. voltage. I haven't been able to do an automatic channel adjustment either.

Eric.

[Ferric Toes]

I need to pull out my scope and check mine... something doesn't compute... i.e 530us prt (1886.792 pps).

You are right. My frequency meter is giving erroneous readings compared to the scope. It seems not to like the waveform, so ignore the results I have posted. I will look into the problem and re-post later. I am using a pickup coil taped to the Vallon coil.

Eric.

[KingJL]

I measured all 40 channels on a precision frequency meter and found that the pulse rate on the VMH3CS varied between 3060pps and 2475pps. The setup is a bit strange as I would have expected one end (high or low) would be on CH11, LED1 with the other end being CH14 and LED10, but this is not the case. Low pps is CH12, LED10 at 2475pps and high pps is CH13, LED1 at 3060pps. One thing for certain is that I found a channel where I can run the Vallon quietly in my workshop, whereas before I had to go outside in the yard, so, many thanks to TH'r for investigating this channel feature.

I will post a full table of 40 channels tomorrow after I have checked another Vallon or two, to see if they are the same.

Eric.

I fired up the oscope and measured the prt for all channels. Calculated the freq from the measured prt... results: Low 1651.8 Hz (channel 12,10), High 2037.9 (channel 13,1).

Channel sorted by frequency
vmh3cs freq.pdf

Channel sorted by channel
vmh3cs channel.pdf

During my testing I attempted to select the operating channel automatically. Following the instructions in my manual (2010 edition), I could not get it to automatically select a channel. Any ideas?

[kingswood]

Hi Eric. Came across this this morning....not sure if it helps
"Another solution to mitigate the effect of low frequency fields is to alternate positive and negative useful pulses as in the Vallon ML1620 or the newer Vallon VMH3."
"The PRF may further be adapted to the prevailing EM background in a way that the dominant part of the background spectrum is not aliased in the critical frequency band. The Vallon VMH3 has this functionality. Several PRFs between 1.5 and 2kHz can be used. The selection of the optimal PRF can be performed automatically or manually"......Who would ever have thought Pulse induction metal detectors were so complicated??!! ha ha .www.sic.rma.ac.be/~pdruyts/publi/PhD_Druyts.pdf

[kingswood]

That is interesting . The manual mentions 40 channels and I wondered how they came to that number. I will have to set one up on a frequency counter to see the range of the TX prf. over the 40 channels.

Eric.

My manual doesnt mention 40 channels....But my manual for the VMC1 does...

[kingswood]

Righto...thats very interesting!....
Now this is where my lack of technical knowledge comes in ha ha....How does the pulse rate effect the delay time?...or does it??

[KingJL]

I measured all 40 channels on a precision frequency meter and found that the pulse rate on the VMH3CS varied between 3060pps and 2475pps...

I need to pull out my scope and check mine... something doesn't compute... i.e 530us prt (1886.792 pps).