Repair of Bigfoot coil for XLT - any ideas, gotchas etc ...

[Polymer]

My bigfoot coil does not work anymore.

I would like to bounce my ideas of going about a repair here, maybe someone has better ideas.

Have cut the cable leaving 4 inches for measuring and removed the pigtail.
The main cable with plug to detector is ok.

The main defect is that of an open circuit on the TX line, BUT - when carefully moving the TX coax wire I can get the following three states:

1.) Open circuit
2.) Short circuit at 0.2 Ohms
3.) Nominal TX coil resistance of 1.6 Ohms

I have looked at some pictures of the bigfoot inside from the internet and suspect the problem lies within there.


There are some bare wires to be seen and they look like they have only been taped down.
Maybe the tape glue has become old and "unstuck" itself, giving the opportunity for movement causing short circuits and such.
The open circuit scenario maybe a "gone bad" loose connection solder joint.

I believe the bigfoot housing is made of ABS. Is this correct?

Opening the coil shell only seems viable with cutting, since it appears to have been solvent welded around the sides.

My best idea so far would be to very carefully cut the coil housing at the bottom on three sides with a width of say 4 Inches where
these bare wires are located. The fourth side I would heat so I could bent it open like a flap.

Assuming I hit the right spot I could see whats going on and maybe just have to resolder a wire without moving the wires around.
Maybe only a re-taping is necessary.

Once I have managed that I could re-heat the flap, close it and re-weld the 3 open seams.

Any better ideas, hints, tips out there before I begin?

Hell, I am really anxious about messing up the alignment of the feedback? balance? wire bits.
Seems like it was a dark art getting this done correctly.

Hoping for ingenious inspiration, Polymer

NOTE: This bigfoot appears to be different than other descriptions I have found, in that the whole cable itself has NO shielding.
Only 4 Pins of the plug are connected. There are only two coax wires inside.

[Carl-NC]

The Bigfoot is easy to open. The welded seam is an overlap and you can run a utility knife into the seam to split the whole thing apart.

It could be that something inside has come loose, more likely that the coax has broken and needs resoldering. Since you've cut the cable, you'll have to replace it anyway. The cable is a standard White's cable, 2 internal coax with no overall shield. I don't recall a cable with the extra shield. You might be able to contact White's and get a new cable, or a used one. They throw away a lot of used ones.

[Polymer]

Thank you so much Carl! I am very glad I asked.

It really was very easy to split the coil housing open.

My coil ist working again - better than before.

I have re-used the old cable, it's a tad short, but workable for now.
I live in Europe, so I am not sure if I could source a longer one with White's in Scotland as compared to White's USA. Found none on their website.

I took many photos and some notes along the way and will be posting them here once I have processed them.

Ah yes, the extra shielding: I read somewhere that the "outside" cable shielding was causing problems on the bigfoot over time.

Here is one of the pictures that came with it:

Hmmm, I am wondering - truth, partial truth or bollocks ...

[Carl-NC]

I live in Europe, so I am not sure if I could source a longer one with White's in Scotland as compared to White's USA. Found none on their website.

It wouldn't be on their web site, you'll need to call and ask for the Service Dept. They should have cables since they make their own coils.

Ah yes, the extra shielding: I read somewhere that the "outside" cable shielding was causing problems on the bigfoot over time.

OK, I remember now. Yes, the foil shield is very poor quality and causes noise as it flexes.

[Gwil]

Foil shielding is ok for cables that don't get moved about much (e.g. inside equipment) but outside in the real world it breaks up and causes intermittent problems. Best avoided.

[moorejl57]

Foil shielding is ok for cables that don't get moved about much (e.g. inside equipment) but outside in the real world it breaks up and causes intermittent problems. Best avoided.

We had a large axis robot at work where we bought special cable that had multiple foil and mesh shielding. It faulted all the time and was too stiff. We then bought a much more flexible unshielded cable, put a large ferrite donut on the end of it and it worked like a champ.

[kt315]

Foil shielding is ok for cables that don't get moved about much (e.g. inside equipment) but outside in the real world it breaks up and causes intermittent problems. Best avoided.

you fogot naked wire inside the cable that touched the foiled film. so there is not fatal problem if the film is not ragged on whole length.
also, people always confuse EMI vs ESF ununderstanding electric physics. for ESF even total ragged foiled film is not still big problem.

[kt315]

[bklein]

Bill Lahr repaired my BF a year or two back, don’t know if he still works on them but knows them pretty well.

[Mahally]

Eventhough I'm not the one who asked the question, I've learned a lot by reading all the answers of my fellow members. Thanks for sharing.

[Polymer]

[ATTACH]46743[/ATTACH]

Hmmm, this is not the cable my bigfoot has. Maybe Whites and/or Jim Karbowski used the type your picture shows in earlier production runs.

[Polymer]

Taken time to put up the pictures and comments.

One BIG question arises from this for me:

When do you know when a coil is not optimal until it really breaks, like in my case?
I had the occasional chatiness and sometimes instabilty.
That I thought was either funny soil or buried cables or other odd EMI. Changing frequency did help strangely enough.

Anyway, I have tested my bigfoot outside for quite a few hours now. I am amazed at how super quiet and super stable everything is now.
Often I thought its not working anymore - tested with dropped object, no problem. Found more stuff than usual along the way.

The freshly opened bigfoot:


Center from the side and 45 degrees - note the singular hanging coil windings on the left side. The are glued/epoxied like the others, they do not flop around.


Here the carbon shield top side - measured resistance in 4 quarters lengthwise - 2.1k, 3.4k, 4.3k 4.8k Ohms.


Here's the shield contact. The black foam inside is NOT conductive, the outside U-shaped thick tape is. I somehow like the way this has been done. Cleaned it with IPA.


Can't post more than 10 pictures in one post - see next post

[Polymer]

... continued

last picture of shield contact.


Closeups of the problem zone. The Tx capacitor is 33nF and the white box presumably in parallel I couldn't decipher.
Didn't want to dig it out to see the other side. The Rx capacitor has 47nF.


Here's everything resoldered:


Another note:
I unstuck the masking tape with the help of a heat gun - it surprisingly was still sticking extremely well.
The foamy translucent stuff around the hot melt stuff does not like that heat at all. The volume of that stuff disappears quickly, so careful with that.

Thanks for all your comments - I have also learned from them too, Cheers

[dbanner]

Sasquatch coil.
https://www.ebay.com/sch/i.html?_fro...w=Bigfoot+coil

[Polymer]

It wouldn't be on their web site, you'll need to call and ask for the Service Dept. They should have cables since they make their own coils.



OK, I remember now. Yes, the foil shield is very poor quality and causes noise as it flexes.

Thanks Carl!
I wrote to Whites UK in August asking about the cables and other things like rods for shipping to Germany where I was visiting & staying with friends at the time.
No luck there. They directed me to a german metal detector dealer who might be able to help!? Being a traveller and often in "out of the way" places seems to
clash with service modalities of detector companies. That's why I appreciate geotech1 so much. It helps me being more independent of physical location and
I also learn a helluva lot of stuff too ... Thanks again and cheers.

I have a second Bigfoot coil -brand new- as a spare. This one has the Belden M 8723 CM cable on it. Yep, it has foil shielding and is thinner than the other cable
that I have worked on. This cable I will want to change too. The skinny cable I also don't like because it "curls up" "knots up" too easily ... for lack of better words.

I am still on the lookout for good double coax in round cable and have tested a few dozen S-video cables which also coincides with my PI home build adventures.

Is there such a thing as a "good" foil shielding for IB detector cables that does not break down over time?
I am asking this as it appears that most current production cables of this type all have foils and/or semiconductive layers inside.
Older cables usually have braided or spiral shielding.