Finished mpp rev-e, issue

[Hrthomas]

Hi, I have finished 2 mpp rev-e boards now, every step is ok, but now when testing it will not detect anything more than some cm from the coil (20cm coil, 409uh, 2,2ohm)
And it will not detect small items at all.. anyone with some tips? Please..

[Hrthomas]

I have tried to shield the coil and control box, but that does not help..anybody with some tips on adjustment etc?? Im using a mono coil..

[kt315]

again mylar and EMI protect? we need more info with pics to answer you right.

[waltr]

Hi, I have finished 2 mpp rev-e boards now, every step is ok, but now when testing it will not detect anything more than some cm from the coil (20cm coil, 409uh, 2,2ohm)
And it will not detect small items at all.. anyone with some tips? Please..

Very little info to go by but check the following:
With NO target:
First sample delay & sample (TP6 ?) verse when the pre-amp (TP3) recovers from over Voltage. Look at both with scope. Adjust delay.
If the delay to not long enough then sampling does not get where the greatest change is. Same if delay is too long.

Can you see a change in the decay curve at TP3 when a target is brought near the coil? It will not be large.
Ensure the first sample happen during the period of most change.

Look at TP4, integrator output. This should be close to center of the op-amp's rail Voltage (Zero V).
Same with SAT out (TP1). Does TP1 Voltage change when target brought near coil?
Ensure there is NO METAL near the coil. This is easily over looked and a single nail or screw near the coil can cause issues.

The Audio pot (PL3) adjusts the offset of U4a (TP1) and when Q5 turns on the 555 (U. Try adjusting Audio pot, turn util you hear sounds then back off until sound just goes away.

[Qiaozhi]

Also, did you leave a gap in the coil shield?
If not, it will act like a shorted turn and the sensitivity will be heavily compromised.

[Hrthomas]

Hi, yes i have a gap in thr shield. And all the messurements are ok.. i could take some pictures of the messurement if that helps?