Charles (Upstate NY)
Well-known member
A forum member asked me about a bad coil and I ended up writing more than I planned to, here's a copy for anyone who might find this useful...
Lets talk coils so you are armed with information and understand why a coil might be bad from the factory, how to test one, even fix one or build one.
Inside an Explorer DD coil there are two windings of copper wire, that's all its not complicated. The transmit winding TX transmits the signal inducing signals into targets, the receive winding RX listens for or receives the signal back from the targets.
When you build a DD coil the two windings of copper wire have to be precisely positioned to be in "balance". This is where the term balanced coil comes from, the positioning of or balancing of these two windings of copper wire. You balance a coil using an oscilloscope. You connect one channel of the scope to TX to monitor the signal the detector is transmitting, in the case of the Explorer a square wave. And you connect the other channel of the scope to the RX receive winding, you don't want to see any signal on the RX receive winding, it should be a flat line.
Now if the two windings of copper wire are not positioned correctly, RX will not be a flat line. You will see a small amount of the square wave being transmitted out the TX winding on RX, not good. The further out of position or balance the two windings of copper wire are, the stronger the signal will be on RX, screwing up the coil. With signal on RX that should not be there I think this hurts depth and tends to swamp small target signals.
So here's the thing, this balancing or positioning of the two windings TX and RX, its touchy as freaking hell. Just a tiny nudge and whammo the coil is out of balance. The factory has to get the windings balanced, then try to hold them in balance while the epoxy cures. Just the epoxy curing can pull the damn thing out of balance so yes its entirely possible for the factory to build a bad coil.
The factory probably has a range from perfectly balance to slightly out of balance that they consider good enough to ship but ideally you want a coil that's balanced perfectly for best depth on the deepest targets. This is why owning an oscilloscope comes in handy, its pretty easy to test coils. More on testing coils below.
The other thing that can screw up a coil, and is more frequently the problem imo is shielding. The coil needs to be wrapped in a blanket of conductive shielding, completely encased with no gaps or pin holes. Typically they use carbon black shielding paint. This paint is electrically conductive, but not so conductive that it can be detected by the detector. Warning: There are shielding paints on the market that are too conductive for building a coil, stick to carbon black. This shielding routes static (and some other complicated capacitive stuff I won't get into) off to ground so its not detected on RX and creating false signals.
This shielding layer then itself must be blanketed in a non-conductive layer, the coils plastic outer shell. If there is a gap in the shielding paint, even a pin hole the coil may false, if there is a gap in the outer plastic shell, even a pin hole again the coil may false. Especially in wet grass or on a wet beach. Water makes its way through the gap or pin hole creating an electrical path from grass, wet seaweed, etc. and will cause the coil to false like crazy. Original Explorer coils were susceptible to this problem when you got them wet. If you are building a coil and want to test your shielding, grab a tuft of about 20 blades of grass, gently rub them all over the coil, if there's a gap in your shielding or its too thin in some area you will get a false signal on the detector.
Finally there needs to be a minimum gap or distance between the shielding paint and the windings or again the coil can false. I think this was the problem with the early Explorer SE slimline coils which were noisy and falsed, some worse than others. You can't for example just spray the shielding paint on the copper windings, its not a huge gap but there needs to be some gap. What happens when during coil construction a loop of copper wire comes up to the surface of the epoxy very close to the shielding paint, bingo a coil that falses and that's what I found on those early slimline coils. This type of false signal is small, such that most users might assume its just the site conditions, bad soil, mineralization, and turn down their sensitivity when in reality the coil is the problem.
Suffices to say that while the components of an Explorer coil are simple, two windings of copper wire and shielding paint, construction of the coil is quite difficult, a sandwich of materials you have to get just right or the performance of the coil will be off.
TESTING A COIL - Its fairly simple, you will need a 2 channel oscilloscope, you will need to construct a test fixture e.g. a short bridge between the coil connector on the detector and the coil's connector so you have some bare wire to connect the scope leads. I purchased a male and female coil connector for my test fixture, soldered them together with about 3/4 inch of bare wire. Do NOT cross wire your test fixture, going from female to male to female to male its easy for your brain to screw up, draw it out on paper to make sure each of the 4 coil wires are passing straight through and not crossing over due to your brain getting flipped flopped. I'm speaking from my own DOH here.
Take care not to accidently short something during testing, I find it best to tape everything down. You are going to have coil cable and scope test leads laying around, tape them down so that if you bump something or get tangled up in a wire you don't yank on this tangle and short something.
Next you need to setup your test space so that there's no metal near the coil. Figure 2 feet in all directions, no metal above, below, or to the sides. Note large metal like a metal pipe or sheet metal from an appliance or workbench can be detected from several feet away. I would secure my detector to a bench with the coil and lower rod sticking out in mid air and that work well. You can test indoors but you need to turn your sensitivity way down due to AC electrical interference from your house.
With everything connected go ahead and turn on the detector, you should see a flat line on the RX receive winding if your coil is in good shape. What if its not? What if its out of balance slightly can you fix that? Some people have had luck rebalancing a coil by using a small piece of ferrite, you can move this ferrite around the coil watching the balance, and when the coil is balanced to a flat line again, glue the ferrite in place. In some factory coil construction one technique they use is, they leave a small secondary winding of RX free, just a few loops maybe 1 inch in diameter, they let the first pour of epoxy cure, then recheck the balance, they can move around this small second winding to rebalance the coil if the epoxy curing has knocked it out of balance.
What else can I do with a scope? Well you can gather a bunch of targets and watch how RX reacts to different metals, depths, target orientation, swing speed very important in learning how swing speed effects depth. With the Explorer its possible to swing too slow and too fast. You can play around with co-located targets, a rusty bottle cap trying to hide a silver coin. Move a large rusty nail or some other iron near the coil, note how distorted the signal is vs copper or silver coins or gold, iron is butt ugly on a scope. Move some metal near the coil and hold it steady, not moving. While the Explorer is a motion machine you will see that metal even if the coil is not moving, is distorting and swamping the signal. If its pouring rain outside or its winter and the ground is frozen solid spend some in your lab time testing with your detector and oscilloscope.
BUILDING COILS - I have just barely touched on the subject of building coils above. Building your own coils can be a fun 2nd hobby, and it can be maddening its one of those things where the devil is in the details. But if people are interested maybe we'll start a discussion thread on this subject.
Lets talk coils so you are armed with information and understand why a coil might be bad from the factory, how to test one, even fix one or build one.
Inside an Explorer DD coil there are two windings of copper wire, that's all its not complicated. The transmit winding TX transmits the signal inducing signals into targets, the receive winding RX listens for or receives the signal back from the targets.
When you build a DD coil the two windings of copper wire have to be precisely positioned to be in "balance". This is where the term balanced coil comes from, the positioning of or balancing of these two windings of copper wire. You balance a coil using an oscilloscope. You connect one channel of the scope to TX to monitor the signal the detector is transmitting, in the case of the Explorer a square wave. And you connect the other channel of the scope to the RX receive winding, you don't want to see any signal on the RX receive winding, it should be a flat line.
Now if the two windings of copper wire are not positioned correctly, RX will not be a flat line. You will see a small amount of the square wave being transmitted out the TX winding on RX, not good. The further out of position or balance the two windings of copper wire are, the stronger the signal will be on RX, screwing up the coil. With signal on RX that should not be there I think this hurts depth and tends to swamp small target signals.
So here's the thing, this balancing or positioning of the two windings TX and RX, its touchy as freaking hell. Just a tiny nudge and whammo the coil is out of balance. The factory has to get the windings balanced, then try to hold them in balance while the epoxy cures. Just the epoxy curing can pull the damn thing out of balance so yes its entirely possible for the factory to build a bad coil.
The factory probably has a range from perfectly balance to slightly out of balance that they consider good enough to ship but ideally you want a coil that's balanced perfectly for best depth on the deepest targets. This is why owning an oscilloscope comes in handy, its pretty easy to test coils. More on testing coils below.
The other thing that can screw up a coil, and is more frequently the problem imo is shielding. The coil needs to be wrapped in a blanket of conductive shielding, completely encased with no gaps or pin holes. Typically they use carbon black shielding paint. This paint is electrically conductive, but not so conductive that it can be detected by the detector. Warning: There are shielding paints on the market that are too conductive for building a coil, stick to carbon black. This shielding routes static (and some other complicated capacitive stuff I won't get into) off to ground so its not detected on RX and creating false signals.
This shielding layer then itself must be blanketed in a non-conductive layer, the coils plastic outer shell. If there is a gap in the shielding paint, even a pin hole the coil may false, if there is a gap in the outer plastic shell, even a pin hole again the coil may false. Especially in wet grass or on a wet beach. Water makes its way through the gap or pin hole creating an electrical path from grass, wet seaweed, etc. and will cause the coil to false like crazy. Original Explorer coils were susceptible to this problem when you got them wet. If you are building a coil and want to test your shielding, grab a tuft of about 20 blades of grass, gently rub them all over the coil, if there's a gap in your shielding or its too thin in some area you will get a false signal on the detector.
Finally there needs to be a minimum gap or distance between the shielding paint and the windings or again the coil can false. I think this was the problem with the early Explorer SE slimline coils which were noisy and falsed, some worse than others. You can't for example just spray the shielding paint on the copper windings, its not a huge gap but there needs to be some gap. What happens when during coil construction a loop of copper wire comes up to the surface of the epoxy very close to the shielding paint, bingo a coil that falses and that's what I found on those early slimline coils. This type of false signal is small, such that most users might assume its just the site conditions, bad soil, mineralization, and turn down their sensitivity when in reality the coil is the problem.
Suffices to say that while the components of an Explorer coil are simple, two windings of copper wire and shielding paint, construction of the coil is quite difficult, a sandwich of materials you have to get just right or the performance of the coil will be off.
TESTING A COIL - Its fairly simple, you will need a 2 channel oscilloscope, you will need to construct a test fixture e.g. a short bridge between the coil connector on the detector and the coil's connector so you have some bare wire to connect the scope leads. I purchased a male and female coil connector for my test fixture, soldered them together with about 3/4 inch of bare wire. Do NOT cross wire your test fixture, going from female to male to female to male its easy for your brain to screw up, draw it out on paper to make sure each of the 4 coil wires are passing straight through and not crossing over due to your brain getting flipped flopped. I'm speaking from my own DOH here.
Take care not to accidently short something during testing, I find it best to tape everything down. You are going to have coil cable and scope test leads laying around, tape them down so that if you bump something or get tangled up in a wire you don't yank on this tangle and short something.
Next you need to setup your test space so that there's no metal near the coil. Figure 2 feet in all directions, no metal above, below, or to the sides. Note large metal like a metal pipe or sheet metal from an appliance or workbench can be detected from several feet away. I would secure my detector to a bench with the coil and lower rod sticking out in mid air and that work well. You can test indoors but you need to turn your sensitivity way down due to AC electrical interference from your house.
With everything connected go ahead and turn on the detector, you should see a flat line on the RX receive winding if your coil is in good shape. What if its not? What if its out of balance slightly can you fix that? Some people have had luck rebalancing a coil by using a small piece of ferrite, you can move this ferrite around the coil watching the balance, and when the coil is balanced to a flat line again, glue the ferrite in place. In some factory coil construction one technique they use is, they leave a small secondary winding of RX free, just a few loops maybe 1 inch in diameter, they let the first pour of epoxy cure, then recheck the balance, they can move around this small second winding to rebalance the coil if the epoxy curing has knocked it out of balance.
What else can I do with a scope? Well you can gather a bunch of targets and watch how RX reacts to different metals, depths, target orientation, swing speed very important in learning how swing speed effects depth. With the Explorer its possible to swing too slow and too fast. You can play around with co-located targets, a rusty bottle cap trying to hide a silver coin. Move a large rusty nail or some other iron near the coil, note how distorted the signal is vs copper or silver coins or gold, iron is butt ugly on a scope. Move some metal near the coil and hold it steady, not moving. While the Explorer is a motion machine you will see that metal even if the coil is not moving, is distorting and swamping the signal. If its pouring rain outside or its winter and the ground is frozen solid spend some in your lab time testing with your detector and oscilloscope.
BUILDING COILS - I have just barely touched on the subject of building coils above. Building your own coils can be a fun 2nd hobby, and it can be maddening its one of those things where the devil is in the details. But if people are interested maybe we'll start a discussion thread on this subject.
