Vallon VMH3CS Mine Detector

[kingswood]

Interesting about your grasses, here in Victoria at a place called stringy-bark creek if you touch the ferns there they send my VLF machines nuts, have not tried a PI, its a gold bearing creek, the whole area is a goldfield but more concentrated along the creeks, panned a few nice specks from the creeks, lots of small gem stones to, I dont know but could the ferns be sucking up some of the mineralisation ? , running the vlf machines on the bare ground around the mullock heaps is no problem , but get near to touching the ferns and its no go, very puzzling.

Given that they have found gold in the leaves of Australian trees, I think your thing about sucking up excess and likely concentrating the minerals may be right on the money

[Ferric Toes]

The grass is wild grass that grows on our gold tenement....I sure know that if the coil hits a bit of this, it false alarms....both the coils do it, but worse today with the 30cm Makes a signal noise as if you have just hit a rock or branch. Its not "regular" grass, more like this stuff: [ATTACH]39243[/ATTACH]
(The stuff in the middle, not the big bushes )

There is a possibility that it is a static discharge. I have experienced this in Australia with another detector when you get a long hot dry spell, particularly if it is windy. A static charge builds up on the plastic of the search head, which when discharged by touching vegetation, gives a sharp audio pulse. Any coil shield will not stop this, as it is 'grounded' only to an electronic reference and not to a true ground. Try it again when you get a damp, cool, and rainy day.

Eric.

[kingswood]

There is a possibility that it is a static discharge. I have experienced this in Australia with another detector when you get a long hot dry spell, particularly if it is windy. A static charge builds up on the plastic of the search head, which when discharged by touching vegetation, gives a sharp audio pulse. Any coil shield will not stop this, as it is 'grounded' only to an electronic reference and not to a true ground. Try it again when you get a damp, cool, and rainy day.

Eric.

Yes I will try that...Added bonus of the Vallon is been able to use it in the rain
In regards sweep speed, did you find with the bigger coil you made that you had to have a faster or slower sweep??
I am not sure now if I was sweeping to fast or to slow with the 30cm coil!! I started with the same speed I use with the standard head and it made all sorts of false signals!....I then slowed right down and the alarms mostly stopped....what I didnt try was speeding up! Looking at the report I posted earlier, they said the Vallon UXO needs to be faster than 900mm a second....I was probably doing less than half that speed Oh well, more playing with it!!
EDIT: I looked at the promo video on the Vallon website and they were sweeping the 30cm coil twice as fast as I was...will have to try that

[kingswood]

Now I have been thinking about this more....It was the first time I have taken my mobile with me...Could that have caused the 30cm to appear to be unstable?? (Phone was in my shorts pocket!)

[Ferric Toes]

Now I have been thinking about this more....It was the first time I have taken my mobile with me...Could that have caused the 30cm to appear to be unstable?? (Phone was in my shorts pocket!)

Yes, that is definitely a possibility and I have seen it happen with other detectors, both here and in Australia. I don't know much about mobile phone technology but it appears that they communicate periodically with their network when switched on, even if they are not actively being used. Although the rf frequencies are way outside the passband of the detector, they get demodulated and appear as a sporadic offset at the preamp output which results in a short burst of audio.

Eric.

[kingswood]

Yes, that is definitely a possibility and I have seen it happen with other detectors, both here and in Australia. I don't know much about mobile phone technology but it appears that they communicate periodically with their network when switched on, even if they are not actively being used. Although the rf frequencies are way outside the passband of the detector, they get demodulated and appear as a sporadic offset at the preamp output which results in a short burst of audio.

Eric.

I am hoping that it was the phone!!!
Minelab document says: "Some mobile phones (but not all) will cause interference if less than about 2 metres (6 feet) away from the sensitive PI units. Beyond 2 metres, there is no effect."
My phone was literally 80cm away

[Ferric Toes]

I am hoping that it was the phone!!!
Minelab document says: "Some mobile phones (but not all) will cause interference if less than about 2 metres (6 feet) away from the sensitive PI units. Beyond 2 metres, there is no effect."
My phone was literally 80cm away

I think that this is an important point to bear in mind for all users of PI detectors, and maybe other types too. Also to be included are VHF/UHF handheld tranceivers. I had a lot of trouble on a large industrial site where workers communicated with each other by these latter devices and caused interference with underbelt conveyor PI detectors when transmitting within 20ft or so.

With the popularity of mobile phones it is so easy to have it on one's person in a standby state and not relate it to interference on the detector. If you can, switch it off while detecting, I will do some tests with my mobile phone to see if it upsets the Vallon

Eric.

[Ferric Toes]

Current Draw and Channel Setting

Now that all the channel settings have been sorted out, I thought it would be interesting to see if the current drawn off the batteries changed significantly when changing channel (pulse repetition rate). I would expect that, all else being equal, the current would change if the prf changed and that is the case with the Vallon. Running off a bench power supply set to 4.5V the current draw is 350ma when set to the highest frequency channel (13,01. prf 2040). On the lowest frequency channel (12,10. prf 1650) the current drops to 296mA. All the other channels sit at current values in between these two limits.

A 50mA difference can be quite significant in terms of battery life so it would be more economical to select a channel that is in the top 16 of KingJL's 'By Freq' chart on post 728. That would be from the top and down to 14,06. prf 1790, current 315mA, where you are saving 35mA compared to 13,01.

If you can't find a noise free channel in the top 16, then obviously try further down the chart but try and avoid 19xx - 20xx pulse frequencies.

Eric.

[Chet]

I have found some two way pocket radios that interfere during received messages. A Pin-Pointer can interfere even when off. A spare coil inside a backpack or a VLF detector on the backpack can also be detected as you swing from side to side or bend over. Orientation of a spare coil to a perpendicular position will increase the pickup distance.


Try to test each electronic device or metal object to determine how much distance is needed to prevent interference or detection.


Have a good day,
Chet

[kingswood]

A Pin-Pointer can interfere even when off. Chet

Really?!!...Oh dear, first time I had one of these with me too!...That was in my hip pouch
Okay so this may all be good news though!...I really struggled to stabilize the bigger coil and now I am a bit more reassured that it may well have been due to EMI rather than anything else
Eric, on the battery draw: I havent changed any channels, but I am still using the same batteries that I started the Vallon with....I would estimate about 20 hours of use at the moment. Pretty good. "Energizer" brand.

[Altra]

I havent changed any channels

Maybe this is why you cannot get the large coil to stabilize? I was amazed at how quiet the detector became when I found a good channel. Out of the box 3 or 4 leds constantly chattering, now smooth and sensitive.

[kingswood]

Maybe this is why you cannot get the large coil to stabilize? I was amazed at how quiet the detector became when I found a good channel. Out of the box 3 or 4 leds constantly chattering, now smooth and sensitive.

I will try channel selection if it is still unstable when I try it next...without the phone and the pin-pointer

[6666]

After some testing of my 2010 machine by pushing + and - together it appears to only have manual channel select, very time consuming

Interestingly , as you change channel, if you listen carefully to the internal audio tone that comes from the case
the frequency of the tone changes with the channel.

[6666]

Okay so this may all be good news though!...I really struggled to stabilize the bigger coil and now I am a bit more reassured that it may well have been due to EMI rather than anything else

Of course you were not wearing steel capped boots, or have metal shoe lace eyelets in your shoes or have metal ends on your shoe laces

[kingswood]

Of course you were not wearing steel capped boots, or have metal shoe lace eyelets in your shoes or have metal ends on your shoe laces

ha ha...No actually got rid of my steel caps recently after making that mistake!