Need advice on Minelab 11" coil repair and design

[bklein]

I was going through coils I bought off ebay last year and came across an intermittant in this 11" (really measures 10.5").
I opened it up and found a coil wire broken off at the cable junction. But I also found an intermittant ground connection due to the crimp fitting at the input - so the cable needs to be replaced or shortened. It is so stiff and coiled that I fear it will just break internally again if I use it and here's my chance to test using a USB cable in its place.

Some things to document:
The Minelab stock cable consists of two coax lines; one bigger than the other. The smaller one has black insulation on the center wire, the larger one has clear insulation on the center wire.
Smaller: shield to pin 1, black center wire to pin 2. My understanding is that this is the RX.
Larger: shield to pin 5, clear center wire to pin 4. Jumper wire from pin 4 to 3. My understanding is this is the TX.
Jumper wire pin 5 to 1 shields.
So a question I have is what is this "resistor" for? The reason I have the quotes is that it measures 510 ohms but also measures 237uH with my LC102 LCR meter. Without this the RX coil measures 427uH, with it in parallel it measures 289uH. So what is this part for? The other 510 ohm resistor shown in the photo I took from my bin and measured inductance and it too was 237uh. So should we be considering the resistor inductance in our designs and what is it for anyway on a RX coil?
As far as using a USB cable as a replacement, I have access to USB2.0 as well as 3.0 cables. I was going to use a 3.0 as perhaps it has less capacitance. Then I was going to use the Vbus red/black as the TX/gnd and one data pair as the RX. Rx ground ties to the coil shield (paper) and resistor/coil junction as before.
Note that not fully shown in the photo (wires at 5 o'clock angle at bottom right) but the RX has a gathering of wire jutting out towards the ID of the coil. Is this extra or a TX signal nulling technique?

Barry

[bklein]

I redid the coil measurements as they didn't make sense. The resistor does have an effect but not as much:
No resistor, the coil measures 427uH. With the resistor it measures 404uH. The resistor by itself measures 233uH. The TX coil measures 274uH. My LC102 can actually measure cable capacitance so I am measuring the original vs alternates and will post values later.

[Mechanic]

HI Barry,

The resistor is there for a damping resistor. In the ML machines there is only a damping resistor on the tx side of things. Without the damping resistor on the RX windings the rx coil would ring and possibly damage the front end. The extra bit of Rx winding bunched up to the side is for the fine nulling.

Also check the batterys in your lcr meter, they start to do funny things when the battery starts to get low! Also that resistor could be a wire wound resistor, hence why it is showing up with inductance. Have you tried measuring other resistors to see what happens?

With the usb cable, you need to make sure first that you cannot detect the cable, so try that with one of your good coils. Then you need to make sure there are enough conductors in the cable in order to reach the required peak current before tx turn off. Then you need to make sure the capacitance is low enough. I normally would just cut the cable an inch or so above the break and re connect it to the coil innards as it was before.

Cheers Mick

[Bill512]

I redid the coil measurements as they didn't make sense. The resistor does have an effect but not as much:
No resistor, the coil measures 427uH. With the resistor it measures 404uH. The resistor by itself measures 233uH. The TX coil measures 274uH. My LC102 can actually measure cable capacitance so I am measuring the original vs alternates and will post values later.

Hi bklein,
can you take some photos of the coil windings and the two coaxials? I'm curious to see the cable on Tx and Rx, Linz or not, insulation etc.
thanks

[bklein]

Hi Mick,
I'm a bit confused by your second sentence - but I guess you are saying that the resistor is needed on the RX coil for damping the TX signal imposed upon it.
The LCR meter can be either AC or battery powered and I was using it AC. I think I just got readings confused as I was in the middle of measuring all my coils when I came across this one's problem. I'm pretty confident in the last measurements - I have a BK LCR as a backup and it read pretty close.
The resistor shows up as inductance and resistance. Weird that it measures inductance in our typical coil value ballpark... Others I have in my parts bin measure similarly. I could try and find some carbon comps and see if it changes the game - could be a measurement artifact of the meter itself too.
Yes, I plan to test the cable for detection - kind of a catch22 if you want to make a fast coil and you don't have one yet. I'm thinking most of my cables will fail this test as the ML one does not have the nickel or whatever braid or aluminum/mylar foil shields. I probably do have enough cable left to re-attach the original. I may as I don't want to make a huge project out of it. The capacitance measurements are kind of interesting - I didn't realize the LC102 did these until I read its quick user guide where it documents doing so.
It was a worthwhile exercise just to learn of the RX damping resistor. I don't plan on making DD coils though - mainly smaller hopefully fast 8" or eliptical monos to see the small stuff.

Barry

[bklein]

Hi bklein,
can you take some photos of the coil windings and the two coaxials? I'm curious to see the cable on Tx and Rx, Linz or not, insulation etc.
thanks

The photo in my first post (wires, foam, resistor) shows both. The TX is thick Litz and the RX looks to be thin Litz. I'll have to look at the RX with a magnifier to verify though.
I can measure each with calipers too. I didn't take the shield cover fully off as it is hot-melt glued to the foam and it tears when I try and pull it back. I've seen another photo of the cable cross-section here on this site but I can get a shot of it.

[Bill512]

Thanks bklein.
By the way, I have a 11" DD Commander,do you want to check the inductance for Tx and Rx?

[Bill512]

[Bill512]

	Rx(uH)	Tx(uH)	Rx +Tx (uH)
coil 1: commander 11"DD	915	315	1190
coil 2: commander 11"DD	930	320	1210

[Bill512]

	Rx(uH)	Tx(uH)	Rx +Tx (uH)
coil 1: commander 11"DD	915	315	1190
coil 2: commander 11"DD	930	320	1210

with fresh batteries on meter, the values seems a little more accurate (based on Tx inductance, which is close to 300uH)

	Rx(uH)	Tx(uH)	Rx +Tx (uH)
coil 1: commander 11"DD	880	296	1160

[bklein]

Wow, your RX coil is double my value. Why would that be?
I'm sorry - I put my coil back together with the original cable as others were going to turn into a project and I realized I don't have the time. I did take a couple extra photos before putting it back together if that is of any interest to you.

[Bill512]

it's ok,
but if it's not a big trouble ,I would like to see some extra photos.
Actually I have two 11" DD coils ,and I get about the same readings.
As for the "inductance" of these resistors ,probably is nothing more than "false" readings.
Using a potentiometer, i managed to get a "pseudo-reading" of 15mH.
I can't imagine a carbon resistor with a inductance of 330uH ,or even a carbon pot with 15mH!!! (some wire-wound metal film resistors,may have just a few uH).

[bklein]

I put the housings back together but before I did I took this photo of where the two coils cross.

[Bill512]

thanks!