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705 Variable Sensitivity

R

RandyF

New member
I searched the forum to see if this was already addressed and didn't find anything exactly related.
Recently I was hunting just fine at an old schoolhouse using a 6"HF DD coil on my 705 with the sensitivity set at 20. After about 1.5 hours for seemingly no reason I started getting a lot of noise. So much noise that I needed to dial down the sensitivity to 8. I checked the battery level and even changed to the stock 9" coil to no avail. I also tried doing a factory reset. Nothing I did seemed to help. I needed to keep the sensitivity down low for at least a half hour after which I checked again and then I was able to increase it to about 20 again. I was in an area that was far from power lines and other potential sources of noise.

A few questions come out of this:
Has anyone else experienced this type of behavior?
What are the potential causes?
Have there ever been any studies/tests done to determine the loss of depth per sensitivity step (I know there are a lot of factors/variables here)?
Could some type of atmospheric disturbance (i.e. "sun spots") cause this? (It was a calm, sunny day in Minnesota).

Thanks in advance for your thoughts.
Randy
 
Well, first of all.....Welcome to the forum!

Now on to your question(s).
Currently there are no active sunspots, nor is there any noteworthy coronal holes spewing cosmic wind our way. So the chances that there is any atmospheric or astronomic reason are zilch.

The most common reasons are a variety of EMI sources that can include (but not limited to) invisible fence (used for containing dogs primarily), above ground electric fences, data transmission lines, Wi-Fi systems, cell phones, mowers or other vehicles with bad plug wires or without resistor plugs, electrical storms, and HAM/CB radios or other similar personal communication devices. I've even experienced sections of woven wire or cyclone fence accumulating a charge that is randomly discharged when there are storms in the vicinity. Power lines and transformers are also often a problem, but you already said that there weren't any nearby.

My suggestions for these types of interference are simple. First thing to always try is re-noise cancelling your detector. If that doesn't work, leave the area and come back to it later. You can even try a different day, but do so at a different time of day. And if it still persists, it might just be one of those places that you can't detect. But don't feel bad, because nobody else can detect there either. In general Minelab machines are capable of handling EMI better than most if not all others out there. I frequently hunt near power lines, and do pretty well there because there is so little competition due to the interference that other detectors can't deal with.

I hope that this helps. GL & HH!
 
I'm had a detector to do that once. It had a piece of trash floating around in control box. Once that was removed and cleaned I never had the problem again.
 
Longhair and Christian,
Thank you both for your greetings and kind suggestions. I've been detecting off and on since the early 80s but I haven't contributed much to the forums although I have come to the forums for insight into the use of the 705. Diggers instructions I have found extremely helpful. My comments about atmospheric conditions may have been "grasping at straws" a little bit, but the sudden increase in noise (same location) made my mind start to go down that path. I think Christian may be on to something though and will need to open up the control box to see if there is something loose or floating around. The noise started after I set the 705 down to dig a hole and picked it up again. My 705 is approximately four years old so it's possible something could have come loose. I'll let you know when I find the issue.
Thanks again.
 
Well did you find the problem. I hate suspense !!
 
Pulling the cover off the main unit and checking the connections didn't reveal any "smoking gun". The symptoms have happened to me on 3 occasions. Because the problem is intermittent it is difficult to identify the cause. Once I have it identified I will post the solution. I found out that I purchased the 705 in May of 2010 so the detector is older than I originally thought (time flies) so there could be some age related issues going on. I can eliminate the issue being a coil because it happened with different coils attached.
 
check the prongs on the unit for tightness after you unscrew the coil wire.
 
Ronnie,
Thanks for the suggestion. I'll double check the coil connector when I get home and let you know.
 
The coil connector on both ends looked good. I hunted two days this past weekend without an issue. I may never know what caused the issue but as long as it doesn't come back I'm ok with it.
 
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