Vallon VMH3CS Mine Detector

[kt315]

http://www.head-fi.org/t/316937/fake...n-the-wild/165
post #177 is there a way to check genuine vs fake -?

[Phiphi]

I have bought a Vallon VMH3CS on eBay on a UK site for 185 £ + 25 £ shipping in France.
I received it today and it's seems to work but I hadn't time to use it very much.
The serial plate is missing but there is a punched tag with 6 and 10 removed so it must be June 2010.
It has pinpointing as Eric's one.
I have measured the coil inductance and found the same value than Eric : 1.5 mH and 3.7 ohms resistance. I'll try to measure soon the self resonance frequency.
I don't have a complete manual but just two plasticised papers with very basic instructions to use it.

What are the tests to do to be sure it works fine ?

[greylourie]

Does anyone have pinout information for the "serial interface", for connection to a laptop ?

[Phiphi]

I'll try to measure soon the self resonance frequency.

I have measured 208 kHz as the SRF so it gives a 390 pF capacitor.
I have made the measurement on the connector between the shaft and the body.

[mschmahl]

Here is the pinout information

[Ferric Toes]

[ATTACH]38150[/ATTACH]
Here is the pinout information

The pinout is a bit puzzling as you would think pin B is the live audio connection. However, it appears that pin D is the one to choose (marked Tone). To mute the internal piezo speaker you link B to F, which can be checked with a test meter on the connector on the phone supplied with the detector. Some suppliers do not include a phone which is a one-ear job and doesn't look the part.
So far I have only tried piezo headphones such as Gray Ghost waterproof and Whites Surfmaster phones. The Gray Ghost are fine as they are, but Whites were much too loud and I had to make an in-line attenuator. The audio Volume can of course be adjusted on the detector by switching to the speaker symbol on the control knob and the pressing the + or - buttons to increase or reduce the volume. This is fine except that when the detector is switched off, the new setting is not stored. A sensible precaution for mine detection when you want every detector to have a default setting on switching back on, but a nuisance for hobby use. I have not tried low impedance phones, but I would be surprised if they worked without a matching transformer. Piezo phones do not appear to be readily available except for the high priced models so far tried.

Eric.

[greylourie]

Thanks Michael and Eric, for the info. I'm curious about any potential for recording ground/archaeological data.

Jon.

[Phiphi]

I had time today to make measurement by connecting a scope probe on the coil of my Vallon VMH3CS like that :



First a view of the two successive pairs of pulse



As you can see the two pairs are 610 us time spaced with a +/- 250V amplitude.
Now the first pair



The first negative pulse is unusual as a sawtooth waveform but the second one is more common. They are 48.6 us separated.
And now the second pair



The second pair is as the first one but amplitude reversed.

The first pulse of the first pair about 9us long:



The second pulse of the first pair about 8.2us long :



The first pulse of the second pair about 9us long:



The second pulse of the second pair about 8us long :



Now some more amplitude details. First, the end of the negative sawtooth signal



It finishs at -6V.
And finally, the end of positive pulse. There is an undershoot of about 2 V. It doesn't seem to be well damped.



What do you think of all that stuff ?

[Phiphi]

There wasn't enough room in the previous post for two pictures of my lab with the Vallon on the bench !

[Ferric Toes]

I have bought a Vallon VMH3CS on eBay on a UK site for 185 £ + 25 £ shipping in France.
I received it today and it's seems to work but I hadn't time to use it very much.
The serial plate is missing but there is a punched tag with 6 and 10 removed so it must be June 2010.
It has pinpointing as Eric's one.
I have measured the coil inductance and found the same value than Eric : 1.5 mH and 3.7 ohms resistance. I'll try to measure soon the self resonance frequency.
I don't have a complete manual but just two plasticised papers with very basic instructions to use it.

What are the tests to do to be sure it works fine ?

If the detector runs through its own test sequence when you first switch on, then after a few seconds settles down and detects metal, then you can be sure that the electronics is working fine. Did you get a test piece with the detector? This consists of an acrylic rod, 90mm x 15mm, with a small aluminium cylinder, 10mm x 2.5mm embedded in one end. The rod is on a cord so that you can swing it over the search coil, the metal cylinder being at the end away from the coil. If you swing it such that non-metal end is at a 10mm clearance above the coil then the sensitivity is on spec.

The only electronic malfunction I have found so far on one of the units I purchased (10+) is a stuck tactile switch on the indicator panel. This appeared to be due to the plastic overlay having hardened in the Afghan sun and losing its required flexibility.

The coil swivel on most units is very stiff and squeaks when you try and adjust the angle. This is due to fine dust that gets in the swivel and the O rings running dry. The end of the swivel shaft where the clamping bolt is, can be freed up with WD40. It is best to remove the bolt and both clean and lubricate this also. From the other side of the shaft you can remove the cover plate and squirt some WD40 in that side too. You might be inclined to make or obtain a special key to remove this as sometimes they used silicone sealant on the threads. I don't know why this was done as there is a perfectly good O ring under this cover and if it was clean and greased with silicon grease, that should be ample. There are two O rings on this end of the shaft but WD40 seems to penetrate past the two and free them up, but I prefer to dismantle the whole swivel to get the dust out and grease them with the waterproof silicon grease. This is best if you are going to used the Vallon on the beach and in water.

Eric.

[Ferric Toes]

Hi Phiphi,

The duration of both TX pulses should be 50uS. I suggest that you may be looking at the back emf of the front edge and the back edge, and your 48.6uS is the duration of one pulse. This would also account for the difference in your pulse shapes.

Eric.

[Qiaozhi]

What do you think of all that stuff ?

Very interesting.
It also seems like you picked up a good example of the Vallon VMH3CS for the price you paid. The ears on the arm cup are usually broken off, with the rest of it looking like it's been run over by a truck.

The unusual thing about this detector is the high inductance coil, since it has the ability to detect targets of low conductivity. These two facts tend to be mutually exclusive.

[Phiphi]

Hi Phiphi,

The duration of both TX pulses should be 50uS. I suggest that you may be looking at the back emf of the front edge and the back edge, and your 48.6uS is the duration of one pulse. This would also account for the difference in your pulse shapes.

Eric.

Hi Eric

Thank you for your answer.
I'll make a new measurement tomorrow with the detector far from metal because on the bench, there was the first 4 led lighted. But I doubt it can explain the waveforms !
I have the test acrylic cylinder and it is detected when I swing it over the coil.

Good night

Philippe

[techo_bob]

Hi Phiphi

Looks in fact that your pulse duration is 48 uS
They are recycling a portion of the previous flyback and kick stating the next but in the opposite polarity hence the 1/4 sine capacitor
discharge cycle at the start of the pulse, then the cycle is repeated in the opposite polarity

Cheers Techo_bob..

[Phiphi]

Hi Phiphi,

The duration of both TX pulses should be 50uS. I suggest that you may be looking at the back emf of the front edge and the back edge, and your 48.6uS is the duration of one pulse. This would also account for the difference in your pulse shapes.

Eric.

Hi Eric

I had a bad and a good night : a bad night because I didn't sleep well but also a good one because I think I found why I have so strange waveforms !
I have measured the voltage across the coil and everybody knows that V = L di/dt so di/dt = V/L.
You have written that the final current is 1 A. I see that the last 40us of the pulse are at 6V so it means 4mA/us (6/0.0015) or a 160mA variation in 40 us.
The pulse is 48.6 us long so after 8.6 us the current is 1000 - 160 = 840 mA. So in the first part we have about 100 mA / us average slope or 150V average voltage.
We have a sawtooth at about 300V decreasing in 8 us to 6V : that's about 150 V average or 100mA /us.
In the first 8 us, the current as a parabolic shape with a slope decreasing from 200mA / us to 4mA / us and the last 40 us a linear variation from 840 mA to 1A.

Maybe they do that by polarizing the Mosfet with a decreasing voltage from 300V to 6V by discharging a capacitor or something similar ...

Philippe