VLF Magnum Build

[baht7]

Regarding the Support Shaft from the Search Head to the Control Box. How far, safe distance does my Support Shaft have to be from the Search Head if use an aluminium shaft?
I found a cheap chinese Floor mop that is collapsible and was thinking it would ease my transportion area.

I know this will be a lame question for you guys, but bare with me.

In the Magum build project, is that 4 inch Pick Up coil on top or underneath the Search Coil...hahahahahahahah.
I warned you.

Gary N Bangkok

[WM6]

Regarding the Support Shaft from the Search Head to the Control Box. How far, safe distance does my Support Shaft have to be from the Search Head if use an aluminium shaft?

Try moving your shaft (left-right) near to working coil to determine position at which you cannot detect your shaft any-more. Then this is the right position/distance.

[mikebg]

Try moving your shaft (left-right) near to working coil to determine position at which you cannot detect your shaft any-more. Then this is the right position/distance.

Gary, this is not true because Magnum is motion detector and the shaft not moves relative to coil. However the RX preamp sees a signal from metal shaft and you should reduce the gain to avoid its saturation. To avoid reduction of sensitivity, make the clevis tubing from wooden rod long about 250 mm.

[Qiaozhi]

Try moving your shaft (left-right) near to working coil to determine position at which you cannot detect your shaft any-more. Then this is the right position/distance.

Gary, this is not true because Magnum is motion detector and the shaft not moves relative to coil. However the RX preamp sees a signal from metal shaft and you should reduce the gain to avoid its saturation. To avoid reduction of sensitivity, make the clevis tubing from wooden rod long about 250 mm.

Actually, you are both correct.

For a motion detector, the shaft does not move relative to the search coil. Therefore it will not be detected when in use. But, it will negatively affect the sensitivity of the detector. Also, if the angle between the coil and shaft is altered in any way, the balance of the coil will be upset.
Now ... having said that ... read the following text from Carl:

"The Magnum is a VLF design with ground balance and a TR discriminate mode, but lacks a motion ground balance/discriminate mode. Thus far it is the most advanced hobbyist metal detector project I've come across, amazing considering the date of publication. In 1980 the Magnum would have been a fairly advanced metal detector, just one step behind the motion discriminators that were introduced only 2 years prior."

As you can from the text, and by simply looking at the schematic, the Magnum is not a motion detector.

[mikebg]

As you can from the text, and by simply looking at the schematic, the Magnum is not a motion detector.

As you can see from shematic, the Magnum is motion metal detector, but there is a TUNE HOLD key to set nonmotion mode for pinpointing

[baht7]

Thxs a bunch guys.

This is my first dealings with Metal Detectors, so I just naturally went hmmmmmm when I was viewing the assembly with metal parts in the Search Head especially near a coil in a balanced circuit.
I concur with motion being a factor in this type of unit.
I also desire to keep the sensitivity as precise and stable as possible also.
The 250mm distance "safe zone" will be applied.

Placement of the 4 inch Pick-Up coil, Since Ive slept and now reread the construction process, on top of the Search Coil appears to be obvious location.

Gary N Bangkok

[Qiaozhi]

As you can see from shematic, the Magnum is motion metal detector, but there is a TUNE HOLD key to set nonmotion mode for pinpointing

Clearly you do not understand how a motion detector works. Motion detectors (such as the TGSL) react to the rate-of-change of the receive signal, and not the absolute amplitude itself. Look again at the schematic for the Magnum. Where do you see a differentiating stage? ... [pause] ... There isn't one!

The Magnum is a non-motion detector with auto-tune, which is not the same as a motion detector. The TUNE HOLD button you mentioned turns off the auto-tune function to allow more accurate pinpointing. Incidentally, this detector consists of only one channel, which can be used either in GEB mode or in DISC mode, but not both at the same time. You would need two channels to do this, such as in the TGSL.

So I repeat ... the Magnum is a non-motion detector with auto-tune, NOT a motion detector.

[mikebg]

Clearly you do not understand how a motion detector works. Motion detectors (such as the TGSL) react to the rate-of-change of the receive signal, and not the absolute amplitude itself. Look again at the schematic for the Magnum. Where do you see a differentiating stage? ... [pause] ... There isn't one!

The Magnum is a non-motion detector with auto-tune, which is not the same as a motion detector. The TUNE HOLD button you mentioned turns off the auto-tune function to allow more accurate pinpointing. Incidentally, this detector consists of only one channel, which can be used either in GEB mode or in DISC mode, but not both at the same time. You would need two channels to do this, such as in the TGSL.

So I repeat ... the Magnum is a non-motion detector with auto-tune, NOT a motion detector.

"Motion metal detectors require the search head to be kept moving in order for the target to be detected. Motion metal detectors are more sophisticated than non-motion and do not require regular retuning as they are constantly retuning automatically. This type of metal detector generally offers better analysis of the target and is less affected by ground mineralisation."
Source:
http://www.abmetaldetectors.com/acatalog/glossary.html

[Qiaozhi]

"Motion metal detectors require the search head to be kept moving in order for the target to be detected. Motion metal detectors are more sophisticated than non-motion and do not require regular retuning as they are constantly retuning automatically. This type of metal detector generally offers better analysis of the target and is less affected by ground mineralisation."
Source:
http://www.abmetaldetectors.com/acatalog/glossary.html

There is a subtle difference between a true motion detector, and a non-motion detector with auto-tune. This difference is not covered by the information in the link you provided. In fact, the link you seem to be relying on for correct information is actually incorrect when it states: "... and do not require regular retuning as they are constantly retuning automatically" in reference to a motion detector. Motion detectors do not need to be re-tuned.

Firstly, look at the schematic for the Magnum, and read the text of the article where it says: "The output from the switch is passed through a 3rd order low-pass active filter with a cut-off point set at 40Hz. which removes practically all of the 15kHz signal, leaving only the average d.c. level."

This is the subtle difference. The filter in the Magnum is a low-pass filter with an fc of 40Hz.

Now look at the schematic for the TGSL, where you will note that the filters are band-pass filters having an fc of 15Hz and an fb of 7Hz. There is no dc amplification, which means that only ac (target) signals are allowed through. The coils must be kept in motion in order to detect anything. If you hover over a target you will receive a constant receive signal, and constant (dc) signals are blocked.

In the case of the Magnum, all frequencies from 40Hz downwards are amplified, including DC (0Hz). This makes the Magnum a non-motion detector. However, the designer has removed the usual Reset/Tune button, so often present in designs from this era, and replaced it with an auto-tune circuit. Pinpointing is achieved by turning the auto-tune feature off. The Bandido is also a motion detector, with a third non-motion channel specifically for pinpointing. This is required because it is not possible to turn off the motion mode. As I said before, your reference is incorrect. There is no constant retuning going on in a true motion detector. Hopefully you can now understand the difference.

So I repeat for clarity ... the Magnum is a non-motion detector with auto-tune, NOT a motion detector.

[Gwil]

The output of the 3rd order low pass filter extends down to DC, but it seems to me that the stage based around IC9 is a band pass filter with a response that falls to zero at DC. This is what creates its "auto-tune" behaviour. It also means that if the search head is held motionless over a target the initial signal will fade away to zero (if you wait long enough). I don't know whether this makes it a "motion" detector, but "non-motion" doesn't seem to describe it.

[mikebg]

COMDUCTIVE SHIELDING

My inscription "nonmotion" (in post # 5 near to TUNE HOLD key) is not enough correct. The stage with IC9 remains operating as an AC amplifier even when the TUNE HOLD key is pressed. In this mode, the stray conductance of PCB and C17 form a very low cutoff frequency of AC amplifier. To diminish stray conductance in pinpoint mode, the author uses TR3 and S2 instead a normal closed switch because at open position the real swich has significant stray coductance.

To diminish the role of stray conductance, the PCB should contain "conductive shielding" - an additional track surrounding R31 and JFET. The conductive shielding should be connected to noninverting input of IC9. This is not made in Magnum project. Note that the left lead of R31 appears near to negative supply rail (pin 4 of IC9) showed as blue track in the attached image. Despite this, in practice the project operates excellent in pinpoint mode.

The improvement is shown as red track. It is preferable to use MKP type capacitor for C17.

[smity]

Who has a PCB "Magnum" two channels? Share please.

[kt315]

Who has a PCB "Magnum" two channels? Share please.

take look on MD-5008. it is perfect non-motion one, i see it at work alltime. do not make Magnum.

[Qiaozhi]

The output of the 3rd order low pass filter extends down to DC, but it seems to me that the stage based around IC9 is a band pass filter with a response that falls to zero at DC. This is what creates its "auto-tune" behaviour. It also means that if the search head is held motionless over a target the initial signal will fade away to zero (if you wait long enough). I don't know whether this makes it a "motion" detector, but "non-motion" doesn't seem to describe it.

In the text it states, "IC9 is a d.c. amplifier and also the auto-tune stage."

I agree that the search head of the Magnum must be kept in motion in order to detect a target, but this does not make it a true motion detector. It is still a non-motion detector with auto-tune.

Motion detectors were invented to better cope with the situation where ground mineralisation changes relatively slowly (when compared to a metal target) in a given area. Any slow changes (usually less than 7Hz in frequency) are simply ignored. With an auto-tuned non-motion detector you would need to have a very fast auto-tune circuit to cope with these ground conditions, which would adversely affect the sensitivity. Or, alternatively, you would have to sweep very slowly. If you keep the search coil motionless over a metal target, while monitoring the output of the filter stage with a scope, a non-motion detector will still display a change in the dc amplitude. In a motion detector, the output of the filter stage will remain at zero. As you can see, there is a subtle difference between the two.

In practice, the non-motion detector can be swept more slowly, but cannot cope so well with changing ground conditions. The other difference, that becomes obvious when in use, is (as you stated yourself), "the initial signal will fade away to zero (if you wait long enough)". In other words, there is a big difference in the response and recovery speed.

[baht7]

mikebg,

Regarding suspected or probable conductance problems, a faster surer method is to clean the area and apply a silicone based shielding conductance material suck as PAN-HCl; Silicone rubber.

There are spray coatings on the market also, but a tooth brush and alcohol are my 1st choice.

I did note your drawing, perhaps moving the resistor to opposite side of the board or a shielded jumper wire.

Little things like the JFET/resistor laying across an exposed trace is asking for trouble.

I havent checked yet, but this circuit is on Magnum VLF?


Gary N Thailand