BFO Coil Works For PI Detector?

[GB_Amateur]

Thanks to all for the responses.  I have a friend who is a top notch electronics tech and he'll have an inductance meter so I can do a check of both coils before trying to hook anything up.  I figured there was some kind of dead weight solution (e.g. sand bag or the Anderson disk) but the latter apparently only fits the large coil and the former just seems so inelegant!  (I really prefer small coils around here since we have, oh, 200 years of trash spread everywhere, including the waterways.) 

And regarding the warranty, I likely already voided that when I changed out the (faulty) 5-pin male panel mount on the front of the control box -- the coil's cable attach point.  Further, I've been strongly considering adding the conductivity switch (covered on this forum in another thread) which for sure would void the warranty.  Lastly the S-rod is too long to fit into my Pelican hard shell (airline proof) travel case so I need to shorten it before traveling to the Colorado Rockies in June.  Some people just can't keep their clumsy mits and dull tools off a pretty device.    If the inductances match (or are close) I'll probably try the retrofit.  I didn't mention that the lower rod of the Garrett mates perfectly with the upper rod of the White's -- as if they were designed to be compatible (very unlikely, but maybe both companies use the same source for their components??).  If I do that I'll post here the outcome, assuming the resulting fire doesn't also burn up my computer.

[Geotech]

I also looked at this many years ago when I did the Hammerhead design, I wanted a dirt-cheap coil that's easy to get, and there are a billion of those old Garrett BFOs still around. From my notes, the Garrett BFO coils have about 120uH inductance and 3 ohms of resistance. The TDI prefers 300uH and less resistance, maybe an ohm or so. I decided not to use the Garrett coil for Hammerhead but don't remember why, it woulda/coulda worked.

It also would probably work on the TDI, in that the likelihood of damage is pretty nil, but I have no idea how it would work. Certainly the damping resistor would need modification. Lower inductance will increase the TX field but weaken the RX sensitivity, and higher R will weaken the TX field.

Addendum: As I think about this some more, it seems like I may have stuck that huge 24" square Garrett BFO coil on a TDI. I was looking for a way to find my silver test cache. I must have been disappointed in the results because I wound my own 24" coil.

[Guest Paul (Ca)]

Back in late ninties, Once was in a pinch and soldered on a BFO coil to an Eric Foster Aquasport Pi had picked up on  trade with a defective coil, Can't remember if it was the 1B or 2B model just remember it was black. 

 

The BFO coil worked but was not near the sensitivity or depth of the original 8" coil in which picked one up later. Can't remember if the pi was 30us or 40us, the pi was designed for deep diving at great depths and was not very sensitive.

Even if you're able to get the bfo coil to work, it won't be as sensitive as my experience with the Eric foster aquasport pi. Only because the TDI operates at or near 10us, you may have better performance adjusting the delay because of the bfo coil.

Wish I would have kept that pi, was behind the times with today's Pi's but was a beautiful well built detector.

Good luck,

Paul

 

[GB_Amateur]

UPDATE.  I finally got an inductance meter (my electronics tech expert has responded but as yet has had no time to drop by) and I measured the inductance of three of my TDI coils, all around 300 microHenries -- calling that uH from now on -- as Roughwater said they should be.  I actually measured closer to 350 uH, but as always this could be a calibration issue of the meter.  I then measured the Garrett BFO coils and they came in around 180 uH, about 50% higher than Geotech said he had measured.  This also could be calibration or just variance between actual coils.  And he did say maybe he measured a very large (24 inch) coil, but I wouldn't think that should matter.  If the detector wants a certain inductance then shouldn't all stock coils be built with that inductance, or something close?

It's interesting to note that the Garrett coils actually have two windings, in some ways similar to White's "dual field" coils, except that only one is used at at time on the Garrett.  There is a switch on the detector unit that allows you to select between the small and large coils.  I was hoping that if I put the two coils in series and then measured their combined inductance that it would simply add the two and I would get close to the desired 300-350 uH.  That didn't happen.  I got around 200-210 uH when the coils were in series.  This may just be due to the orientation/geometry of the coils.  (I need to review my physics texts.)  Note the Garrett BFO coils are not concentric but rather co-planar, non-concentric.

So where do I go from here?  I'm still counting on my friend to provide more insight.  I'm wondering if adding an external choke (coil) in series, well away from the searchcoil, would work.  Of course it would have to be well shielded, otherwise it would respond to varying background metal just like a standard searchcoil does.  And even if kept quiet it might rob the intended coil of its sensitivity.  I started out thinking this project was a longshot and the odds are getting even longer.  But, you know the old saying, "the next best thing to a positive result is a negative one."    Stay tuned.

[RickUK]

@GB_Amateur,I used 2 Garrett SeaHunter coils on my original Crossbow PI last year,these are the 8'' Mono and 14x10'' Mono which if i recall correctly did have a unusual 2 winding setup,the coil connectors had be taken off and George from Geotech put new ones one for me and also added a dampening resistor  to each coil so that they worked fine,if i recall the Crossbow is around 500uH ,but these coils work a treat,not the fastest coils going but the Crossbow was designed on the Hammerhead design for real deep hoard hunting,and with the 14x10' Mono coil it pretty deep and does everything that i want from it.

Only 13 of the original Crossbow had been made and i have No13 so its a rare beast and still in the condition as it was made by George the co-owner of Geotech.

I also had a 5'' Mono coil made up for the CB by a someone called Sven in Canada for the detector mainly for a specific use on a highly mineralized roman villa site that we only had access to when the crops came off,hence the small coil design to get in between the stiff stubble,this small coil has since been transferred onto a 'Mirage PI machine which is rated at at about 350Uh and once  again that works a treat as well.

The Mirage PI and the 5'' Mono is mainly used for fine jewellery hunting and does work well as it has a fine gold timing mode of 10us and if i recall he ordinary timing of 15us.

The latest new beast that i have is a new TDI Pro was out yesterday with it using the Coiltek 14x9 Mono from  GPX and the Jimmy Sierra 20'' Grand Madre Mono,the latter combination is a total beast depth wise,but that setup is not for nugget hunting just solely for deep hoard hunting on pasture sites to get below the plough line and it does that with easy

 

 

 

[GB_Amateur]

Thanks, Rick!  Sounds like there are more coils out there that potentially fit my need.  I wonder if the Garrett Sea Hunter coils can be modified to work on the TDI.  The 8-inch (open) looks promising for what I'd like to have.

Regarding Coiltek coils, they appear to be mostly built for Minelabs but I do see the TDI occasionally mentioned on their Ebay descriptions.  Did you have to get the 14x9 modified (e.g. change the connector) or is that a standard issue?

 

[RickUK]

5 hours ago, GB_Amateur said:

Thanks, Rick!  Sounds like there are more coils out there that potentially fit my need.  I wonder if the Garrett Sea Hunter coils can be modified to work on the TDI.  The 8-inch (open) looks promising for what I'd like to have.

Regarding Coiltek coils, they appear to be mostly built for Minelabs but I do see the TDI occasionally mentioned on their Ebay descriptions.  Did you have to get the 14x9 modified (e.g. change the connector) or is that a standard issue?

No the Coiltek coil was not modified in anyway,all Minelab GPX/GP coils will fit a TDI range and do work very well without any form of modification,but TDI coils 'will not work' on Minelab PI machines,i current use the Commander 15x12 Mono,Detech 18'' Mono and of course the Coiltek coils on the TDI straight out the box,the other coils that i use are all made for the TDI.

Cannot comment regarding the SeaHunter coils on the TDI,gut feeling tells me it could well be possible the 8'' Mono is a really beautiful coil to swing,deadly on jewellery for beach hunting.

I was lucky i bought some coils about 4-5 weeks back from the Miner John Ebay store,these coils are thye ones that he made brand new for Jimmy Sierra before he closed his outlet down for what ever treason i dont have a clue,but one of the bargains was the massive 20'' Mono that had been sold for nearly $500 but i was able to buy it and include shipping over to the UK brand new for $230 all in including postage and all import duties as well,and 2 other coils for also great discounts as well.

I dont use my TDI Pro for nugget hunting as yet i have not found them near London but i use all my large coils for deep hoard work,so i have to take the 'cost to reward' price into account,would certainly not pay top premium prices for coils to just sit around gathering dust just to be used a few times a year,but they do get used and currently working on 2 pots hoards,one celtic gold staters and the other a roman silver dinari hoard.Hence the need for a large coil to help locate the container itself.

[Geotech]

13 hours ago, GB_Amateur said:

So where do I go from here?  I'm still counting on my friend to provide more insight.  I'm wondering if adding an external choke (coil) in series, well away from the searchcoil, would work.

The TDI should be just fine driving 180uH. If you want to play it safe, put a few ohms in series with it to start with. And, as I said, it'll need a different damping resistor, the TDI has 680 ohms which will probably end up too low.

[GB_Amateur]

Thanks, George and John.  Thought of another question for you guys since you've been so generous with your knowledge:  I seem to recall reading that putting a non-TDI coil onto the TDI requires that (or at least works better if) the pulse delay is adjusted.  My TDI is the Sierra Super Pulse (SPP) which has the delay locked at 10 microseconds.  When you run your TDI with the non-Whites coils do you adjust the delay away from the minimum (10 us) setting?

[Geotech]

GB, on a whim I tried the Garrett BFO coil on my TDI. It's a dual 5+12, the 12" reads 120uH and the 5" reads 140uH. Each coil can be run individually as long as the idle coil is dampened to keep it from ringing. Sensitivity is poor, 4-5 inches on a nickel, but I did not re-dampen the active coil, and I'm sure it's overdamped and reducing the sensitivity. Furthermore, the ~3 ohms of coil resistance is limiting the turn-on current; the TDI has an internal 2 ohm series R that could be shorted to compensate for this, but the SL has no such resistor.

So the short of it is, yes, it will work. And with some tweaking, it may work OK. But those tweaks will likely make the stock coils not work, unless you add a way to switch the damping R. Also, running the Garrett coils in series could also work, but would require each coil to be dampened to get to an overall good decay. And then, the 6 ohms of coil resistance will be a problem you can't solve.