I have owned several F75 series detectors.. I have read the manual many, many times......... But I guess it was selective reading 
From day one I have had deep Older pull tabs ring in high on the F75... Outside of setting disc up to Zinc pennies, and then have the machine average up the full Aluminum Tab and pull ring
into the Silver ranges I have lived with it. Low disc, and digging plenty of Tabs so as to not miss any Good targets... Always getting Silver tone and ID on many of those OLD OLD Tabs...
After reading the manual again... Straight from the Engineers clearly saying ... AT First Use ......... In the Notch section...
So Listen up all you F75 users....
Reset your detector to factory setting...Machine off........ Hold red menu button in, hold toggle switch forward..........and turn on while holding both in their required positions for this maneuver... Release menu and toggle switch.. While on the discrimination side of operation...............
Press Menu ( RED BUTTON) switch 4 times..... Takes you to notch...... OK so far... Now while in Notch rotate the knob clockwise till you reach the number 40 showing on the screen.....
Press menu ( Red Button) once.. A line will hightlight above TABS.... Now go ahead and change your sens and anything else you want to set up................ Many of the pull tabs I and I bet you have been digging with good tones and Id will no longer bother you while hunting...... In fact Half of the pull tabs and pieces will be gone...
How did I miss this very clear setting from the guys who invented this machine............ I'll tell you.......Like many, Me, and I bet a lot of you too...we didn't read the notch section....
Give it a try.Let me know what you think..It sure made a difference for me.............. Not to mention it's suggested from the designers of the machine...... ( Engineers)

From day one I have had deep Older pull tabs ring in high on the F75... Outside of setting disc up to Zinc pennies, and then have the machine average up the full Aluminum Tab and pull ring
into the Silver ranges I have lived with it. Low disc, and digging plenty of Tabs so as to not miss any Good targets... Always getting Silver tone and ID on many of those OLD OLD Tabs...

So Listen up all you F75 users....

Reset your detector to factory setting...Machine off........ Hold red menu button in, hold toggle switch forward..........and turn on while holding both in their required positions for this maneuver... Release menu and toggle switch.. While on the discrimination side of operation...............
Press Menu ( RED BUTTON) switch 4 times..... Takes you to notch...... OK so far... Now while in Notch rotate the knob clockwise till you reach the number 40 showing on the screen.....
Press menu ( Red Button) once.. A line will hightlight above TABS.... Now go ahead and change your sens and anything else you want to set up................ Many of the pull tabs I and I bet you have been digging with good tones and Id will no longer bother you while hunting...... In fact Half of the pull tabs and pieces will be gone...
How did I miss this very clear setting from the guys who invented this machine............ I'll tell you.......Like many, Me, and I bet a lot of you too...we didn't read the notch section....
Give it a try.Let me know what you think..It sure made a difference for me.............. Not to mention it's suggested from the designers of the machine...... ( Engineers)