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F75 emi, interference, all metal question.
Posted by: Greg (E.Tn)
Date: March 25, 2017 11:41AM
It seems like every time I take my F75 out, regardless of the location, I get some sort of interference that causes my machine to emit a cacophony of short beeps and chirps, accompanied by the target ID numbers jumping around. It lasts from 10-30 seconds in most cases, then stops for various periods of time, then repeats, almost as if the machine is picking up transmitted data or signals from a unknown source. It happens regardless of sensitivity settings. It's nearly impossible to detect while the machine is under this onslaught of interference, because I can't tell the false sounds and numbers from actual detected targets.

I don't carry a cellphone or electronics when I detect, and the interference occurs when I am detecting alone. At Petersburg on a organized hunt, others with F75s experienced the same thing.

The last two occasions this occurred were in two different states--near the site of the Petersburg CW battle in Virginia, and near my house in Tennessee. This doesn't mean it has only happened on two occasions--it happens nearly every time I use the machine, regardless of the location

The Virginia site was a rural timber covered site where the nearest power lines were probably a quarter to a half mile away, and no houses within a half mile.

The Tennessee site was probably a hundred yards from power lines, and the same distance to the nearest houses. * This location is about five miles from a sizeable airport.

Settings were BP/Sensitivity at 85, but lowering the sensitivity had no effect on the interference, or changing from BP to JE to DE and so forth.

What's odd about this is, when it occurs, I change to All Metal Motion, it disappears completely. That helps, of course, but I am not sure my machine gets better or worse depth in All Metal Motion.

Having the interference disappear when I switch to All Metal makes me think that EMI is not the culprit--it is something other, somewhere transmitting a signal in bursts which my machine picks up.

One thing I HAVEN'T done yet is switch frequencies to see if that makes a difference.

Question: Is All Metal Motion as deep as, say BP? When I use it, the machine is set in BP on the disc side, but when I switch to All Metal Motion, do I lose depth as opposed to BP?

Secondly, I don't think this is a quirk with my machine only. At Petersburg, there were others using the F75, and all were experiencing the same interference.

Thanks in advance for any advice, opinions, and assistance.

Re: F75 emi, interference, all metal question.
Posted by: markg
Date: March 26, 2017 06:51AM
BP carries over to the all metal mode.
You will gain depth while in the all metal mode.

Can you induce the machine to make this noise by shaking the coil in the air?
Do you have another coil to try?

The next time you have this problem set sensitivity to 75 and raise the discrimination to 15 and see if it makes a difference.

Re: F75 emi, interference, all metal question.
Posted by: WV62
Date: March 26, 2017 09:16AM
This one stumps me, and I have been running the F75 since 2010. :shrug: Now if it was just you I would say detector problem.

I have had 4 of the F75's, 2 of the black Ltds and 2 of the std gold models. None had any problems like you are having.

Ron in WV

Worked and lived in the Huntington West Virginia area all my life, retired 2010
US Air Force Vietnam Veteran (68-69 & 70-71)
Started getting into metal detecting around 1975

Re: F75 emi, interference, all metal question.
Posted by: MI-AuAg
Date: March 26, 2017 09:16AM
Hello Greg,

You said that you haven't yet tried a frequency shift.

In my cases when my F75 gold (DST) accasionally gets the yips, doing a frequency shift is the FIRST thing I do. In most instances it will do the trick, without lower sensitivity.

It doesn't hurt to re-check ground balance occasionally also.

Posted by: Greg (E.Tn)
Date: March 26, 2017 09:06PM
No, I have tried shaking the coil, manipulating the coil cable to see if that is the culprit--

When this occurs, I will hold the detector horizontal to the ground, and hear the blips and yelps, watch the numbers jump back and forth, then after several seconds, it will stop on its own accord. After awhile it will start again, run for a few seconds, then stop.

Thanks for the input.

Posted by: Greg (E.Tn)
Date: March 26, 2017 09:08PM
Next time I will try the frequency shift recommendation to make sure. I usually run in the default freq.

I periodically ground balance, especially in all metal--doesn't seem to have any effect.

Thank you for your suggestions.

Thanks Ron.
Posted by: Greg (E.Tn)
Date: March 26, 2017 09:11PM
The one thing that stumps me is, near Petersburg when this happened, a buddy of mine was running a F75 (not close to me) and he later commented that his was doing the same thing.

Later that same day, I talked to another F75 user who came walking up, and he commented the same.

We talked about it, and both wondered if it was radio transmission from aircraft in a landing pattern, communicating back and forth with the tower.

I figured with the F75 being supersensitive and all, that maybe that might be the culprit.

Thanks for your observations and input.

Re: F75 emi, interference, all metal question.
Posted by: James/Texas
Date: March 27, 2017 09:46AM
Greg, Is your F75 the original one? If so, has it been upgraded to DST? If it is the original F75, then maybe I can help. The original F75 has a feature that some do not know about. The F75 had a 'high gain' setting and this may be what you are experiencing. I did not write the information below, but saved it for myself from somewhere and dug it up on my external hard drive. I do not remember where I got it or who wrote it to give credit. Anyway, this may be helpful:

The number one problem seems to be that the F75 can go crazy and beeps constantly and the VDI numbers jump all over the place.
Well....that happened to me and my F75 too.
Nothing was wrong with the detector. You just have to know that the F75 picks up sources of interference in the high gain modes.

Next you need to know what are the high gain modes.

1. High gain in DE mode kicks in if you have the disc lower than 5 or higher than 19.

2. The JE mode is a total high gain mode no matter what disc setting you use.

In a perfect world the JE mode with a high sensitivity setting would get us the best depth. Or the DE mode with the disc lower than 5 or higher than 19.

Now how do you find out if you have interference sources nearby ?
Turn on the F75 with a sensitivity setting of about 70 and lift the coil to waist level. Now select JE mode, or DE mode with the disc lower than 5. If the F75 goes crazy an interference source is picked up by the F75.

Now how do you get the F75 stable ?
1. Select DE mode AND disc between 5 and 19. Sensitivity as high as possible without falsing.

Posted by: Greg (E.Tn)
Date: March 27, 2017 06:51PM
First of all, James, many thanks for your information and advice.

Mine is one of the camouflage F75's. When I turn it on, the numbers 12-14-14 come up--which I THINK means that is the date of my last upgrade, when FA and DST was added to it.

The modes it currently has are DE, JE, PF, BP, FA, BC, and CL.

When I set the machine up as you suggest, I pick up a STEADY interference source from external interference. I am familiar with that.

What I am talking about specifically is separate than this type of interference--it is a "burst" of interference that lasts a few seconds up to several seconds, then stops, for varying periods of time--almost as if two people were talking back and forth on a radio, and every time they key their microphone to talk, my detector picks up the interference. When they un-key the microphone, the detector returns to normal.

And to add to that--I have detected remote woodlands for hours in BP mode with discrimination set below 10, detected the same general area for, say, two hours with little or no interference, then start getting the "burst" interference.

I will try to troubleshoot it a bit more, based on your suggestions and see if that has any effect.

Also, I am going to try to find some way of recording this on video/audio, and posting it here.

Again, thanks for the advice and assistance.

Re: James/Texas
Posted by: jim tn
Date: March 27, 2017 07:50PM
Greg, by chance have you contacted F T to see what they might have to say? With any of the three F 75's I've had/ have, I never experienced exactly what you describe. HH jim tn

Hello Jim.
Posted by: Greg (E.Tn)
Date: March 28, 2017 10:12AM
No, I have not.

I know I probably need to send it in with a detailed letter explaining the problem, and what I have done to troubleshoot things, but I have had this happen in situations where two other F75 operators (both detecting in the same general area as I was) encountered the same interference, so I don't think it is an issue isolated to my machine only.

If I can get a good audio/video recording of this as it happens, then I'll probably send it along with the machine to First Texas.

Thanks for the advice, Jim.

Re: Hello Jim.
Posted by: jim tn
Date: March 28, 2017 12:27PM
Greg, I hunt two spots that are near airports and frequently very near directly under flight paths. One, the planes are up in the air several hundred feet and making their decent, or take off. Only on a rare occasion does my detector began to chatter and then just briefly. On the other site Military, the Military jets are lower and my 75 goes off like crazy. But again just briefly. So, depending on how close the planes are does make a difference and you just may have it pegged. Good luck! HH jim tn

Thanks again, Jim.
Posted by: Greg (E.Tn)
Date: March 28, 2017 06:33PM
I'm going to try to record this anomaly with my video sunglasses that I never use. This will give me an excuse to use them.

Re: Thanks again, Jim.
Posted by: doc holiday232
Date: March 29, 2017 11:15AM
I'm betting on EMI--we are so over loaded with microwaves--look at all the towers--that we probably need a personal lead shield. Keep us posted as to your findings.

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