Problem with stability of HH

[Satel]

Hello!

Some days ago I built HH Rev1.0-25.10.2005.It working, but not sability.If SW2 opened, the speaker gives always signal. If I removed pot R23 and ( out of IC 6, pin 6 disconnected) SW2 open, then speaker gives signal.I measured voltage in pin 7 IC8b and saw, that it is not zero but "swimming" from one side to other side. Please tell me, what is the problem?

Thanks

[Satel]

Now my HH works, 25 mm coin it find in 18cm with unshielded coil. But if I shield the coil (with Al tape), then MD lost the sensitivity.Who know, what is wrong?

[bbsailor]

Satel,

What is the approximate main delay that you are using? AT low delays the aluminum tape will become a target. Try this. Use an unshielded coil and see if you can detect a 1 inch square piece of foil that you are using as the shield. Adjust the delay over the whole range of adjustment and let us know what distance the foil can be detected at each end of the delay range.

I suspect you need a shield material that is less conductive, like lead tape or an unwound capacitor that contains mylar strips acting as the plates. Make sure that there is a gap in the shield where the coil wires exit the coil, otherwise you may have similar problems that you report. The shield should only be connected to the ground of the coax cable on one side of the shield gap.

These tips should help. Let us know?

bbsailor

[Satel]

Hello bbsailor and Thank You for answer!
I don`t know what is my main delay value because I don`t have two channel osilloscope.If I used unshielded coil, then the main delay pot R42 regulated in one side.When I used shielded coil, then I must regulate R42 to the end of second side and then the coil "see" 25mm coin in 2cm distance.
I think, that You right, the shield material is too conductive. In the weekend I try to do test with pieces of my shield, that You recommended.
Yes, I had a gap in my shield and it connected to the ground on one side of the shield gap. I used two wires microphone cable (stereo cable).
Thank You, Your answer help me go ahead!


bbsailor[/QUOTE]

[Satel]

now I have two channel osilloscope and I measured sample delays: main sample delay I can regulate between 15-75uS and sec delay between 75-300uS.
The coil shield I make new, from kitchen folium, and sensitivity was stay normal (25 mm coin 18 cm). One problem I have still-when I turn or move the coil, then it takes signal.Looks like it react to vibration, but it is not vibration.Who know, what`s wrong?

[Satel]

It is my HH.

[Carl-NC]

One problem I have still-when I turn or move the coil, then it takes signal.Looks like it react to vibration, but it is not vibration.Who know, what`s wrong?

If you're testing inside, this can be interference.

- Carl

[Satel]

Thanks Carl!

Yes, I tested my HH inside. I watch carefully, that in test time there were no metal objects near my coil. Can You explain me shortly, why inside metal detector react to interferents, but outdoor not?

Thanks
Satel