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F70 Threshold Confusion - Help

W

WTnFred

Member
I would appreciate some real world advise/understanding on the threshold adjustment of the F70.

I hunt inner city yards/lots with my F70 and F75 that have civil war relics, garbage and EMI. I believe the threshold adjustment on the F70 might make it more stable and deeper in the inner city than the traditional F75(I have a LTD ordered).

My initial thought of the threshold was that a lower setting/negative would reduce the smallest(relative) items from being detected. My interpretation of the manual is that you should lower the threshold if you are encountering EMI. Andy says if the ground is heavily mineralized you should turn the threshold up and sensitivity down.

I normally run DE, 2F or 3H, sensitivity anywhere from 30 to 75.

Your opinion would be appreciated.

HH
Fred
 
- - You have two gain controls on the F70.

1. What we call "sensitivity."
This is the common receiver input gain we are used to seeing as a "depth" or "SENS" control.

2. What Fisher calls THRESHOLD.
This is really a preamp gain, which amplifies the signal from the coil before it goes to the receiver.
Why they called it that, I don't know. Fisher would do well to just call these things what they are.

The thing to do is start with factory presets (SENS 60, THRESH -3) and work from there. There is only one Andreas Meyer, so apply what he suggests in your hunt environment, but don't try to duplicate it. Always keep this in mind.

I find that for general hunting in urbanscapes, you are better off leaving the THRESH alone. Work your sensitivity as primary, like any other detector. If you max out the SENS (highly unlikely) and are still stable then consider boosting the THRESH. But don't waste the effort if you are not on old ground with the chance of deep keepers.

Once you are out in clear fields or woods then you can bump both. Under these conditions, I often can get the SENS up to 80+ and the THRESH well above zero, into the positive numbers. This is heady stuff, mind you, and you go wear even angels fear to tread when you do this. Approach with caution.

Of course where EMI is a problem, you can go the other way. From preset, reduce the THRESH a level or two, and work the SENS as needed. Remember, the coil is an antenna and you don't want to boost the signal from it if it includes EMI.
 
I've typically set my threshold to -9 to knock out the little pesky stuff, but recently was told I'm loosing a LOT of depth by running the threshold so far in the negative.

I'll have to play around with it, try setting it at zero and adjust the sensitivity instead, perhaps it'll get more depth :shrug:
 
Cal_Cobra said:
I've typically set my threshold to -9 to knock out the little pesky stuff, but recently was told I'm loosing a LOT of depth by running the threshold so far in the negative.

I'll have to play around with it, try setting it at zero and adjust the sensitivity instead, perhaps it'll get more depth :shrug:
Click to expand...
Yes, there has to be a balance with a gain arrangement as seen on the F series.

That was part of the design, i.e., allowing you, the user, to strike that balance. I spoke with Mike Scott at Fisher about this last season.
His opinion is that Fisher did this on purpose, knowing that most people who choose such instruments have the experience to take advantage of it.
Such flexibility in the settings is even more pronounced on the F75 and T-2 line of detectors. Through no small coincidence, these two happpen to be the detectors of choice among serious, expert relic hunters. There has to be something that can be gleaned from that.

I do believe that a preamp gain of -9 is too low for almost all circumstances. I find that at most sites I can manage something between -3 and 0 as a starting point for the THRESH. Just remember - it better to start low and work up, rather than the other way 'round.
 
David,

A question I have asked in the past; does the threshold adjustment on the F75 only effect all metal? I consider it an improvement by Fisher to allow the threshold adjustment in Disc on the F70.

Your opinion please.
HH
Fred
 
Im afraid I cannot answer that, as I have no practical experience with the F75 myself.
That won't happen until the turn of the year, when I have scheduled to get one.
 
WTnFred said:
David,

A question I have asked in the past; does the threshold adjustment on the F75 only effect all metal? I consider it an improvement by Fisher to allow the threshold adjustment in Disc on the F70.

Your opinion please.
HH
Fred
Click to expand...

Oddly the F75 only has a threshold adjustment in AM mode.

I always wondered why the second fiddle F70 had threshold in disc, yet not the flagship F75 :confused:

It sounds like the F75 is then easier to setup if there's no threshold to mess around with in disc mode :shrug:
 
Since the F70 was built on the F75, maybe the threshold adjustment on the F70 was an improvement allowing more stable operation through adjustment of the threshold when operating in disc mode.

Another question; Since higher disc reduces depth, what if we run with 0 disc and then notch out the lower numbers, are we stilling losing depth? It would be great if somebody had an answer.

HH
Fred
 
WTnFred said:
Since the F70 was built on the F75, maybe the threshold adjustment on the F70 was an improvement allowing more stable operation through adjustment of the threshold when operating in disc mode.

Another question; Since higher disc reduces depth, what if we run with 0 disc and then notch out the lower numbers, are we stilling losing depth? It would be great if somebody had an answer.

HH
Fred
Click to expand...
I don't believe we see the depth loss we are led to believe occurs with the new units. That strikes me as one of those urban detecting legend things.
It was understood to happen on the older, progressive discrimination circuits of a few decades ago. But today's high tech digital processor models don't exhibit it, since they work differently. At least it hasn't been obvious to me, anyway.
 
A couple of years ago someone did a bunch of air testing and it showed a 2" decrease in depth going from 0 disc. to 5 disc. When I relic hunt, I almost always use 0 disc. Dont' want to take any chances.

Instead of notching out the lower numbers with 0 disc., just run in 2 tones. All the good stuff will show as high tone, and all the nails and small iron will be a low grunt. Just a thought.

keep on diggin'

jimmyk in Missouri
 
jimmyk said:
A couple of years ago someone did a bunch of air testing and it showed a 2" decrease in depth going from 0 disc. to 5 disc. When I relic hunt, I almost always use 0 disc.
Click to expand...

I thought that was on the F75, was there a batch of F70's with the same issue?
 
jimmyk said:
A couple of years ago someone did a bunch of air testing and it showed a 2" decrease in depth going from 0 disc. to 5 disc. When I relic hunt, I almost always use 0 disc.
Click to expand...

I thought that was on the F75, was there a batch of F70's with the same issue?
 
You are correct. It was the F 75. I am not aware of such a test with the F 70. Might be interesting to do.

jimmyk in Missouri.
 
On my F-70, with the 11" DD coil, I get about 2" less on a air test of a .69 mini when I set it at 5 on disc than at 0 disc..........MO
 
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