Dual Field Calibration Help?

[bklein]

My DF has always been a disappointment but maybe it’s my fault - not knowing enough about the calibration process. I’m hoping one of you has some experience you can share. 

Is there supposed to be a damping resistor on the pcb? I had one tacked across the coil input at 390 ohms but just discovered it kills the outer coil.  With a typical damping resistor jig the duration of saturation lengthens as you tweak it. From 10uS and up. And this would not be a 10uS coil… You don’t see the typical random pulses if no resistor - which makes me wonder if it in the coil already. 
I ended up on settling on 600 ohms. But the coil will not see a foil 1/2” square.  It runs pretty noisy if knobs set to see a target. Both coils sensitive to nearby nickel or gold ring - I would expect a quieter machine and more depth. My pi pro outperforms it in depth and user experience. Photo shows how I have it set now.
 

[bklein]

I discovered there was a resistor already across the coil. That explains the trouble I was having.

[bklein]

To help others with interest in this, check my discussion at Geotech as well. I ended up adjusting the damping resistor to about 380 ohms which leaves an overshoot artifact - not a smooth decay. Then I sampled at 6.8 us! This resulted in a side of coil sensitivity of about 3” and it can sense a 1/2” square of foil 3” from coil center. Way better than before. If yours can’t do this you may want to play with damping resistor value and sample delay adjustment.  As the resistor was 360 ohms it may be ok as is.