IDX-PRO+VDI

[jlsilicon]

Does anybody know the best Mods for increasing the Range noticeably ?

It seems that I had made a few Mods on the original that seems to give me a greater range than my new model without any Mods.

[salih]

I knew the cause of the problem (( 1-depth sensitivity in the ground is weak compared to the tests the air? ))
Reason for the weakness of the device in the ground is Resistor 4.7k Shown in the picture

[calebsone]

so what do you change it to a higher resistance lower or take it out.

[salih]

lower

[calebsone]

OK is it just something I have to play with to find the right resistance? thank you though because that alone should help.......I hope because its a lot easier than most mods

[jlsilicon]

Yes,
What do we change the 4.7K to ???
Oh .. I see, Variable Resistor - 10K Pot in later Schematics ...

[jlsilicon]

Sorry,
But, I dont see any effect.
Centered / CW / CCW - same range ...

[salih]

I changed R 4.7K to VR 4.7K to find the best value.

[golfnut]

If you make the fixed R into a trimpot and reduce its value to almost zero - in this state the unit may well air test a little better


What is happening, is the user is feeding more signal from the GEB CH thru the back end of the detector.

As the GEB CH does have higher gain ( due to when it samples ) you would be forgiven for Air test result


What you may not have spotted is, you will be getting LESS Disc action - so it will be harder or impossible to reject Iron or aluminium.


For example if you short out the 4k7 resistor, then wind the P7 Disc pot so the wiper goes to the bottom, in schematic, the back of the Detector is getting all of its signals from the higher gain GEB CH.

The GEB CH is there only to remove ground signals, everything else can come thrrough - because of this you may have had a better air test, to some alloys, but less Disc action (likely none).


If getting Iron is good for you, short out the 4k7 and wind the Disc pot to min. This may give a minor range range improvement to some alloys ( close to Iron phase response ) - but in the ground, in use, it wont be much.

You may lose sens to some alloys like silver which occur later - phase wise or may not see it at all.

Have a try

[salih]

golfnut You are correct But I am interested in the small things like gold nuggets, I'm not interested Disc ,I'm interested Stability of the device and good GBE to Find anything small. can you give me Tip and I Predestined

[golfnut]

If your looking for small items AND you made this machine as per the general instructions . i.e you made a ~6.5kHz Transmitter - Then, you wont find small objects like small nuggets.

You need to make the Tx work at a higher frequency. You can do this by using a smaller capacitor on the Tx coil and a few less turns on tx coil.

The Rx needs to come up too by the same amount.


For a squeak more sens you can move the Rx coil / cap pair closer to the Tx frequency ~ maybe as much as 80% of Tx freq - to do this you need a trouble free free ground which is uniform in its make up e.g. clean flat soil, or dry sand.

General purpose commercial machines known for finding smaller items cover the range ~17 to 25kHz

Specialist very small gold machines are higher - they are not DEEP - but can find very small items near the surface,

[calebsone]

I think I see so his mod lowers the disc of the machine making it find all metals easer like a permanent all metal.

[golfnut]

Yes its taking only the signals from the GEB CH, exactly. It will be all metal mode, No Disc action.

Essentially a single channel machine when used like this.



( The Disc Sampling time is preset on these machines, the user merely selects more or less output from the All metal/GEB CH or the Disc CH with Pot 7 )
( The Disc CH Sampler is preset to Disc out most metals you dont want say Iron thru Aluminum )

Other machines actually alter the time position WRT to Tx at which the Disc Channel samples and have a pot in the phase shift Rs and Cs

[jlsilicon]

If your looking for small items AND you made this machine as per the general instructions . i.e you made a ~6.5kHz Transmitter - Then, you wont find small objects like small nuggets.

You need to make the Tx work at a higher frequency. You can do this by using a smaller capacitor on the Tx coil and a few less turns on tx coil.

The Rx needs to come up too by the same amount.


For a squeak more sens you can move the Rx coil / cap pair closer to the Tx frequency ~ maybe as much as 80% of Tx freq - to do this you need a trouble free free ground which is uniform in its make up e.g. clean flat soil, or dry sand.

General purpose commercial machines known for finding smaller items cover the range ~17 to 25kHz

Specialist very small gold machines are higher - they are not DEEP - but can find very small items near the surface,

-

Golfnut, I am interested in your idea !

But, what do we need to adjust in the OpAmps ?
- Don't they Filter out other Frequencies outside of the 6.5KHz Band ?

... Also, doesn't this Frequency change at the same time - change the Coils Design / Inductance / Turns ?

[golfnut]

The signals in a Rx ,, superhet are say 100MHz IN, 110MHz L.O. so result in a 10MHz IF. So here you need 100 in for 10 out as you imply.



It doesnt hold like this on VLF IB dets,

The Tx and Rx are on the same frequency.
BUT The Rx tank, has to be detuned often between 50 and 75% of the Tx freq - this eliminates a dominant step change in phase of the Rx front end that would occur if Rx was co tuned in frequency with Tx. Some machines use PLL techniques etc to allow co tuned Tx Rx but simple kit like here does not.




if these machines are static, there are no a.c. signals fed to the op amps.

when you sweep a target the signal from the target/coil has 2 important properties, a phase the samplers can view! (due to when they sample) and an amplitude whose value changes rapidly - which is due to the movement over the target..


It is the rate of change of these signals that allow the opamps to amplify them. (As they form high gain differentiators ) So amplify only signals whose amplitude is changing. At a specific rate (linked to shaft length and arm sweep speed ) Results in signal energy in the op amps at approx 8Hz.

This is why machines are sweep rate dependant, go too slow and the rate of change of amplitude is too sluggish and the opamps dont have gain ( due to the open cct pole, or capacitor )

Too fast again the gain is limited due to the cap on the feedback.


So the op amp would not need filter poles shifting/retuning as the signals here are from shaft length and resultant rates of change of signal amplitude.