Nulling Tesoro Coils For Multifrequencies

[Silver Dollar]

So I tested the 11" Fisher 440 coil against the original brown Tesoro 8" with poor results!

Sometimes it works almost the same but mostly it is about 1" less distance in an airtest.
It seems to chatter a lot inside so I took it outside to test. It is figidity to ground balance
and breaks up a bit earlier than old browny. I'm not sure what is wrong with it I expected
a bit more depth as it's 4" larger. I did notice the null is quite a bit larger when the shield
is not connected to ground. Maybe I should null with the shield disconnected? I used one side
of the coil and grounded it to TX and RX to null expecting it would be close to operating
conditions.

It is running at 10 khz and no attempt was made to fine tune the detector. A renull could be
accomplished if I knew how to make it better. I could try different grounding schemes.....

[Silver Dollar]

Well I opened up the Bandito and probed the TX and RX and found the TX is a bit
smaller than with the factory coil (5V P/P vs 8V P/P). Also the null is 1V P/P vs 0.5V P/P.

The TX came out to 36 ohms where the original is 26. So I think I need to make a new
TX with thicker wire and null by looking at the preamp output. I didn't want to do that
as I hope to use this coil on more than one detector....

[Silver Dollar]

One other strange thing I noticed is that when you touch the Ground Balance knob
you get a false signal. This makes me think the shielding is bad somehow. I did see this
in the past playing with tuning the TX and RX on a Bandito so somehow it's related to
the coil and tuning....

I ran the TX and RX and Gnds seperatly with the shield tied to the graphite in the coil
and run to the connector. I shorted the shield and TX ground at the MD end in the cable.
I thought this was the low noise way to go...