I guess I'll bury another nugget here.
You know the F5 has an audio boost feature for weak signals, right? It is located in the Threshold control. Settings of +1 to +9 boost the audio response of weak signals, making them more robust. It is a great feature.
Now to use the Audio Boost feature, you have to get your threshold up to 0 (zero), which means the detector is wide open to the smallest of signals, which include electrical interference. Moving the setting past 0 (zero) and activating the audio boost feature also increased the audio response of the interference. This often results in lower Gain settings due to the noise, which in turn made the Gain and Threshold control settings a balancing act, which can sometimes be hard to learn (but well worth it).
There is another feature to this audio boost feature that has been discussed in the past, that I thought must be a bug or limitation of a potentiometer. But I now I treat it as it is a part of the audio boost program. That is the +9 threshold setting. The +9 audio boost setting is a two part setting. If you just barely rotate into the +9 setting, it works just like the +1 through +8 settings. But if you rotate the dial into the hard stop, like you were going to a +10 setting, you invoke a different setting. This setting still performs the audio boost function, but it raises the audio threshold response point to where electrical interference type signals are too small to cross over. The end result is that you get the audio boost benefits without the electrical interference chatter - which translates to higher usable Gain settings and a quieter, more stable detector.
I wonder if any one from First Texas has commented on this special mode in the F5?
It is easy to set up. Raise your Gain to 75. Raise your threshold to +5. You will most likely hear electrical interference. Now rotate your threshold control into the hard stop at +9 setting. The electrical interference chatter should be gone. If not, rotate the Threshold control counter clockwise a little and then back clockwise into the hard stop. Once you get it activated, you can just leave it there and it will still be there. Even if you turn the unit off, or remove the batteries for storage, once you turn the unit back on, it will still be in that mode. Now you can raise your Gain, often max it out in place and get the ultimate performance out your F5 for coin hunting (ground mineralization permitting, of course).
If you are a coin hunter, you should be in this mode as it will make a significant difference stability and increase the depth potential of the F5.
HH
Mike