ace 250 with Nel Hunter for a Tejon

[golfnut]

I have been using an IDx with a nel hunter (for Tejon) at 12kHx for approaching a year. My IDX is a bit tired and can be a little temperamental - mainly due to many mods/experiments, its a bit butchered.


I was given a couple of 250 s which had the same minor fault, post repair, I had a go with them but they were not as good as the IDX .. although fun



I wanted to try the hunter coil on a 250 --

I needed a higher frequency to see small items. So modded the Tx in the 250 to run the hunter with 16kHz Tx.



The phasing was off and had no ferrite reject and tones / display didnt match target types.


To crack, I used a cap switch box to increment the Rx coil tune cap, this adjusts not only the Rx amplitude but the phase too whilst viewing the preamp out and the Tx,

Im there now with ferrite reject, with some sens, average null. Im going to add a user GEB pot, rather than the internal trimmer.


My issue is I get the double beep on the pass. I clearly cant swap the Rx leads at the coil as its a cmmercial unit.

I cant swap the leads at the box, as the Rx cable is coax and the braid has the screen on it at the coil end.


So I need to flip the Rx signal 180deg in the box if poss.



If I add a unity gain inverting buffer after the preamp - Is this the same a flipping the Rx coil wires in the coil - will I get the single beep pass, rather than the current double one?

ps I run it with 2 lipos - rather than 4 AA alkaline

Steve

[Henrikras]

Steve why not make a plug to plug / female - male short connector.. There you can swap the RX.. You can even solder them together with Very short wires.

[golfnut]

If you have a twisted pair for Rx with a separate braid ( with e screen on other end ) then OK your right..

I know what your saying, but as my cable is coax, that would make the Rx preamps hot input, connect to a braid with the e screen on it, and the cold side of the Rx coil.

(I did try it and didnt function right, I also tried it on Tx but didnt do it either)




I just made up an inverter using a ne5534 with 2k2 resistors 100nF decouplers on the rails on a tiny bit of board wrapped in Cu tape and to my suprise its working.


Its a handy mod if you potted your coil and its good apart from double beep and you have no other way to modify..



It was all working but - the double beep. The signal coming out of the differentiators going down instead of up - the initial phase appeared correct on scope!!! i.e.preamp out ~ in phase with Tx volts. This was In line with the original coil - unmodified. Plus I had ferrite reject too. But double beep on the pass. Im running 16kHz on this.


The Rx invert was done by cutting the trace from the preamp out and feeding that to the inverter, the inv out to the mixers/detectors.


Nice single tone now per pass. Still rejects ferrite. Iron Low tone other two tones for bronze silver

I need fresh battery and maybe a project case and I will do more testing and post.

Im not saying hack your 250, just that you sometimes can bludgeon some of the bits you have into something new and a little more suitable for you.

Steve

[golfnut]

Had 5 mins on my test bed with the 16kHz garrett I set the sens one bar less than full scale.

I had preset the GEB trimmer to a ferrite ring in my lab.

No falsing on my lawn, so the soil and the grass no problem,



Targets


I have a 12.5mm dia silver blank at 6.5" depth below a lawn.

idx/hunter would register 90% of the time, mostlly one way - scraping the grass
250/hunter registers 100% both ways from 1" above the grass
250/original coil does not get this at all.

12.5mm lead balls

idx/hunter got the 6" often 1 way. Not the 7".
250/hunter gets the 6" ok, and the 7" mostly 1 way occasionally both ways
250/original coil 4 and 5" not the 6 onwards

uk 20p 4"

idx/hunter got this easily, from 3" above the grass
250/hunter gets it 4" above the grass
250/original coil gets from 2" above the grass


Im quite happy with it as it is - tests were with just the pcb in my hand and was all open and leads/coaxes hanging off it, battery swinging. Long cable like on goldmaxx power.

Worth persuing for a nice bit of kit - needs boxing up. I use hip mount.

One thing struck me is the sweep rate on the 250 - for a target to register at all the speed has to be in a slot... Slow or fast and nothing comes thru, just right and its what I got in the test results.

Steve

[golfnut]

Boxed up the 250 into an old hammond case.

16khz Tx


Put 2 lipos in and had an hour out earlier.


Pulled a couple of roman, the small one is 11mm acros , few bits of lead, copper alloy etc encouraging.