F75 SE - EMI fun

Hello all,

I have spent the last couple of days reading as much of this forum as possible, really great info!
I am writing from sunny Australia.

I have been using the F75 SE for 6 months or more now, having a little Bounty Hunter previously.

It has generally been good but I have found it very confusing at times.
I have struggled with interference on many occasions.

Yesterday I found that with the 11" coil attached I had to reduce sensitivity to 40 before the VDI stopped jumping randomly and the tones going crazy.
I tried the frequency shift to no effect at all.
I switched between DE and BP for little effect.
Setting Disc to 5 helped, any lower and EMI was severe.

Fitting the 5" coil helped a bit, I was able to increase the Sens to around 55.

Is it normal to have to go so low with the sensitivity?

Powerlines were distant, I had no phone and my partners detector had no effect. (interference present whether it was off or on)

Thanks,
Ash

Sorry just some additional info which may be relevant.
I have read previously that the control unit itself is not very susceptible to EMI, would this mean that it should be silent with the coil disconnected?

I switched the unit on this morning, without the coil plugged in, and it went nuts - much like I experienced in the field yesterday.
To be fair I was inside my house, but I have switched the on inside before WITH coil connected and not had this issue.

I guess im wondering if the unit could be faulty...

I grabbed mine real quick and disconnected the coil and turned it on. It looked like a slot machine. I'm not having any unusual EMI problems while hunting with it.

beastdig Wrote:
-------------------------------------------------------
> I grabbed mine real quick and disconnected the
> coil and turned it on. It looked like a slot
> machine. I'm not having any unusual EMI problems
> while hunting with it.

Good to know, thanks!

I've just been reading a previous thread discussing use of very low sensitivities, it seems like this is not such an issue after all.
I may have been trying to push the machine too hard all along.

40 in BP mode is still very deep, and deeper than most detectors on the market. Heck, I run in CL mode with SENS=10! As deep and quiet as my Etrac running auto+3.

I often wish Fisher took the sensitivity number off the display.

Something does not sound right...... if you are still encountering EMI without a coil connected. Sounds like a problem.

NASA-Tom Wrote:
-------------------------------------------------------
> Something does not sound right...... if you are
> still encountering EMI without a coil connected.
> Sounds like a problem.

Hmm..no good. (I thought that might be the case but hoped not..)
Would you be able to confirm with your machine?

Already have!

So the display should be still with no coil connected, even inside my house? I know I'm not having EMI issues out in the field. I tried(coil disconneted) with my sensitivity at 50 and disc at 4 and it would chatter badly, but if I turned the disc to 5 the unit became stable until I turned the sensitivity up into the high 70's. My Omega with the coil disconnected can be turned up to max settings with no EMI problems.


Tom, was your unit stable at max settings? I just want to make sure before calling Fisher and possibly sending my unit in for repairs.

Ash did you try a factory reset. I find boost mode to be as quiet as any other.
I only have problems near power lines.
I run boost with sens of 80 or higher and Disc 6 or lower most of the time with no problem.
2 tone can quiet things down vs 3 or 4 tone also

Your detector sounds like a problem I had with my first camo edition F75. Boost mode was useless with it...falsed like crazy until I cut the sensitivity down below 50. I thought it was nature of the beast and sold it. Later on I was hunting with a buddy who had a black edition F75. His was smooth as could be even in BP mode. I later got a used camo T2 SE and it also ran smooth.

There should be very little/none EMI ..... with coil disconnected....... even at high-gain settings.

Damn, it looks like ill send it back for repair!
Thanks everyone.

Hello all,

New member from Sweden.

I have a F75ltd and tested to disconnect the coil and it went crazy. I think it works good out in the field but getting a little worried when I saw Tom´s message that it should be little or none EMI when the coil is disconnected.
I did the test inside the house and it could be interference from wifi and cordless telephone.
Still learning the F75 and it´s so much more powerful than the F2 I upgraded from.
Is there some other tests that I can do to see if there is something wrong with the machine or should I just let it go?

Regards

Dan

The detector isn't designed to work with the coil disconnected, so what it does under these conditions is irrelevant. If you seriously want to see if the control unit is picking up EMI, you need to short the two receive-coil pins together on the control unit, or maybe use a 43 Ohm resistor to simulate the coil resistance.

ashhhhh...

Thanks for starting this thread/post. Sorry my reply was misleading. I had no idea my unit was faulty. I wonder if my units performance is subpar, being that I never noticed anything unusual about it.

If the unit is fine out in the field........... but you incur EMI in the house (with or without coil)............ I suspect some form of heavy indoor EMI issue.

That must be it. I live in an old apartment building with a bunch of seperate breaker units and multiple wifi channels. I find it strange though, because I can max run my omega with the coil unplugged with no problems, and it is more prone to EMI than my F75 ltd. Guess I'll just keep an eye on it...

I'm a little worried about turning on your machines without a coil attached. When I used to mess around with CB radios, one thing you never did was transmit without an antenna attached. Your transmit transistor would burn out quickly if you did. Now a detector is basically a radio with tx and rx transistors and the coil is the antenna. I fear you risk burning out the tx transistor running these without a coil. You guys with electronics experience could chime in here.

ashhhh,

Have you tried the unit in areas where you are far from wireless routers, power lines ect?

I had a camo LTD years ago and it was horrible, and never stopped making noise, and only eased up when I took it in the woods. I sold it to some guy who lives down south in the country and loves it.

Make sure the coil jack REAL finger tight!

Aaron

As Aaron says if you dont get the connector good and tight it will go bonkers sometime's...

Dont use pliers or such but really get a good seat between the plug and the connector...it helps immensely on the f75 and the Omega and t2..

Theyre great to run when the noise is out of them to some extent..

I still think the T2 is one of the best machine's made in the last decade....

Thats applies to the F75 also..

Keith

Tom... my unit continues with heavy EMI interference noise in the house in JE or DE disc modes at zero disc with or without the coil attached. Away from EMI sources it runs quite stable even in JE mode... a few clicks here and there. In town using... I run the iron discrimination higher for EMI control and / or lower the sens a bit as deemed necessary for acceptable stability.

A personal note Tom. Some decisions are troubling and not easily made. I think you've made a very good management decision just lately. This forum has always been about intelligent, respectful discussion to advance technical education. I'm very pleased to see things back to normal, firmly under your guidance. This is why the forum has been such an enviable success over the years.

Jim.

Jim, due to the Sens levels that can be achieved with the F75 platform.............. even with the coil disconnected............ the F75 (especially with 0 Disc) is highly susceptible to medium (or higher) levels of EMI. I speculate circuit board tempest shielding would help to a certain extent. When no EMI is encountered, the F75 (especially in boost proces) is a remarkable platform.

Jim..... thank you. Freedom does indeed come with a price. The fragile angelic balance of harmony is a challenge to ascertain/maintain. Let freedom reign.