PimentoUK

If a rival has an orange pointer, known as the 'Carrot' , then can't N-M make a cream coloured pointer and name it the 'Parsnip' ? Parsnip Pointer, has a certain flow to it. Or alternatively, name it the 'Neep' , which is colloquial Scottish for turnip. It's not turnip-shaped, clearly, but naming pointers after root vegetables is fashionable currently. The Nokta Neep ...

Here's the amusing Scottish Wikipedia page on neeps:

https://sco.wikipedia.org/wiki/Neep

The two coins were 150 metres apart, yet someone decided they belong 'together'. Sure, they could have been dropped together, and one got stuck to the mud of a tractor tyre and taken for a ride up the field. But these details can have implications, affecting whether a find gets officially declared 'Treasure' or not ... with financial / legal procedures / paperwork to deal with.

A great story, and some hunts you'll remember.
But as a Brit, I have to correct you on your "KG3" identification. It's actually a William 3rd, it clearly reads GVLIELMVS TERTIVS ( Gulielmus tertius ), and based on the diameter it's a halfpenny. And so that dates it to approx 1700, considerably earlier than the George 3rd coins.
Tony Clayton's halfpenny page:

http://www.coins-of-the-uk.co.uk/pics/halfp.html#w3

A similar topic came up over on the Geotech1 forum. The guy was wanting to know if shallow-ish stainless steel tips from archery arrows would be detectable or not etc.
They turned out to be trickier than expected to detect, higher freq machines fared better. Stainless steel is a really poor electrical conductor, which pushes up the target frequency. And long thin items have their target freq determined by the small dimension ( eg. diameter of wire, fastening bolts ) so these tips also favoured high freq machines.
A modern Australian-made machine seemed to be capable of finding them OK.
 

I don't think there's much obvious profiteering in that UK dealer's Coiltek pricing. It matches well with the prices for the standard ML coils over here:
Example prices : Main ML dealer, Joan Allen:
ML 11 inch: 190 GBP ; 267 US Dollars
ML 15 inch: 215 GBP ; 303 US Dollars

https://joanallen.co.uk/catalogsearch/result/index/?p=1&q=Minelab+Equinox

That's what 9% import duty plus 20% Value-added tax ( VAT ) does. ( that's 31%, to save you the maths )

To be honest, I doubt UK folks are clamouring for these coils. Partly because of the price, partly because they don't seem much different to what's already available from ML.

A little bump to remind anyone trying this 'challenge' that the hunt has officially started. I don't have time to to sort out my old Whites machine, so I'll have to wait until the next such hunt. Maybe a winter hunt next, for those of us who find hot dry summers rather impractical, and softer wet ground more appealing.

The primary purpose of the diodes is to prevent a 'strong' pair of batteries charging-up a 'weak' pair. Primary batteries generally don't care for being recharged ( except in specific ways ) , so diodes block charging current flow. In practice, a strong battery-pair will be drained first, until the voltage starts to match that of the weak pair, then they discharge together in a balanced way, a convenient secondary effect.

It would technically be better to use Schottky diodes in this circuit. They behave much the same as regular p-n junction silicon types, but they have a lower forwards voltage drop, typically 0.40V , versus 0.70 V. This would allow the batteries to be discharged that little bit more, before the detector stopped functioning. A suitable common type would be the 1N5818.

Not quite right, Joe, a couple of diodes are missing. I was going to wait for DB1000 to reply before posting up some design details. Seems like I'd better do it now ... it would be useful to other folks, anyway.

Update1:

Here's the electrical wiring diagram for the 4-cell board. The two diodes are general-purpose 1 Amp silicon diodes, probably 1N4001 , you could substitute other types, including 1N4002 / 1N4003 / 1N4004.

If you go with the simpler "dual 9V batteries" arrangement, the diodes are not needed, see diagram attached.

Update2:

Here's a pic of the battery holder sold on eBay.

And a Pic showing the PCB tracking of the board. Simple enough that it could be copied by cutting blank copper-clad GRP board with a sharp knife and a metal ruler.

And a pic showing how a simple dual-PP3 board could be made by hand.

"That is expensive. Hoping to just find a writing diagram from someone"

It seems you're trying to make one of these devices, yet have no knowledge or skills to do so ?

There's already been enough tech info given on here for you to come up with a homebrew solution, even if it is a bit of a 'bodge/kluge/improvised' solution.

So to help us out here, what know-how etc do you have? Can you solder ? Can you obtain electronics parts? Do you understand electrical wiring diagrams ? Do you have mechanical engineering skills like sawing/filing/drilling?

Are you actually trying to make a faithful close equivalent to the original Fisher part, or do you just want to get the machine working with very minimal financial outlay?

It's hard, very shiny black resin, sometimes called "potting compound" in the manufacturing industry. And it's tough, so in most circumstances, the coil cover is not needed, saving weight --- important, as the coil is quite big and heavy, giving the overall detector a nose-heavy feel to it.

Back when the machine was designed, batteries were not so good. So it's quite likely that with good modern batteries, it would work OK with just two series-connected batteries, just shorter run-time. That would simplify a homebrew attempt.
Blank copper-clad fibreglass/GRP sheet , the raw material for PCB manufacture, should be available from eBay / electronics surplus dealers / ham radio suppliers. It can be hand cut with a sharp knife to create PCB tracks.
The PP3 terminals could be salvaged from dead batteries, or the flying-lead connectors of some obsolete gadget.

If I was designing that U-shaped clevis part, I would have it fit outside the lower rod. There would be no weak-point where the diameter is reduced, the diameter would be larger hence stronger, and the plastic could be made much thicker, as there's no space constraint.

-------

Additional thoughts:

Have the end of the clevis tube 'wavy', to spread out stresses on the carbon rod, like seen on lugged bicycle frame joints.

Machine notches in the end of the carbon rod, to mate up with protrusions moulded into the clevis. This will help resist twisting forces.

Glue the clevis on to the rod in addition to mechanical fixings.

A bit of search engine work tells me it's this one:

https://en.numista.com/catalogue/exonumia71576.html

A nice bit of history, in decent condition.

Do Predator make custom versions of their tools, like short-handled ones ?

Here in the UK, Evolution Products will make 'short' versions of their spade, and their long-handled trowel has a plain shaft so could be shortened by yourself. Probably expensive to import, though.

https://www.metaldetectingauction.com/listing/evolution-pro-trowel-with-teeth-long-version/104855

This impartial, honest, battery test website is well worth a look:

https://lygte-info.dk/review/batteries2012/Common18650IndividualTest UK.html

Notice how tragically bad the 'xxxxx-Fire' ones, with ridiculous stated capacities perform:

https://lygte-info.dk/review/batteries2012/UltraFire CN18650 9800mAh (Yellow) UK.html

... they must be full of air or cardboard or something equally inert.

Two informative battery-related websites/forums are the CandlePower one for flashlight junkies, and SecondLifeStorage for recycling enthusiasts and off-grid / prepper types:

https://www.candlepowerforums.com/vb/forum.php

and

https://secondlifestorage.com/index.php

It's a serious problem, there's plenty to trap the unwary. It's easy to take any old cell ( old stock, salvaged from unused equipment, salvaged from USED equipment, small cell fitted into a larger package etc etc ) and then put a pretty new plastic sleeve on it and claim it's 12000 mAh.

"As 18650s improve we are seeing mAh's of 4,000-10,000"

I suggest treating any claims of over 4000 mAh as horse excrement, particularly those anonymous no-brand types.
If they were from a genuine top brand manufacturer, or re-wrapped ( usually protected ) ones from leading Vape brands or Flashlight/torch makers, then perhaps 4000 is now possible. But the generally-accepted best capacity ones are from Sanyo-Panasonic, coming in at 3500 mAh.
Development has been a bit slow on 18650's recently, the electric vehicle makers are favouring 21700 size now, and it may take a while for the tech to make it to 18650 size.

Are we to assume the '900' will have 9 modes, just one more than the '800' model ?
I would guess 'All-Metal' is the obvious missing one, and easiest to implement. That may disappoint those wanting a '1200' or '1600' model.
[ though the mighty power of mathematics also allows for a machine with 9 modes such as Park1/2/3; Field 1/2/3 ; Beach 1/2/3 ... but not that compatible with the existing LCD display, unless you 'illuminate' both Park1 AND Park2 for the third mode. ]

".... train the ears"

I think the important bit here is the S of ears. If the audio was seperated in a frequency-dependant way, there's likely to be more chance of picking out nuances. This could be done with the existing mono output and post-processing, though it would evidently be better if it was performed internally by the Eqx software.

"A rectifier diode of some sort? Not quite sure what that's doing in a simple 8 cell AA pack"

It's wired from the + of the charging socket to the + of the battery. It conducts ( forwards biased ) when the pack is charged ... but importantly : it prevents discharge via the charging socket, if something metallic gets in there accidentally. It's a pretty standard safety precaution.

I think it's internally very similar to these "USB Power Bank" devices, except probably slightly tweaked to give 6 Volts instead of 5 Volts output. Many power banks actually give out 5.3 - 5.5 V , probably to allow for a bit of drop on the cable, so it's possible it's running exactly like a power bank, same electronics.

I'm pretty sure the RNB pack has a voltage regulator in it, to set the output at 6 Volts. The actual battery is likely to be 3.7 Volt ( nominal ), a typical Lithium cell voltage. So the battery may be charging correctly and fully, but when you try and discharge it, via the regulator, it fails. Or the battery may be faulty, hence not charging or discharging correctly. Or the charger may be faulty, and it's indicating it's done it's job but in fact hasn't.

Presumably it's covered by some warranty/guarantee from the manufacturer or seller?

As Nok-Mak have a good reputation with regards to customer relations, it may be worth asking them about the scaling of the Ground Strength meters for the particular machines you're using.

To help understand what these 'ground strength' / Fe3O4 / mineralisation meters are showing, it's worth a read of a few posts on the "Equinox improvement ideas" thread, starting from this post:

Notice the scaling of the F75 bargraph is logarithmic, to handle the wide range of possible values, and observe that it's not that precise, it goes in roughly factors of 3 increments, so there could be a 50% discrepancy between two different machines, and you wouldn't see it. This makes comparisons harder.

The 'notch' feature on the F75 is a nuisance, and it is easy to accidentally set it wrong, and not realise. I recommend performing a 'Factory Reset' , see the Owner's Manual for details of how to do this. This will clear any peculiar settings you may have got.

I notice the central foam piece from the armcup is missing, that is unusual. Also, on the genuine machines I have seen, the 6 bolts that hold the upper armcup to the lower section are painted black, not left shiny silver. This makes me suspect the machine is counterfeit.