XL500 sample timing

[homefire]

Cool on your 500! Yes the threshold tone is most annoying. Never measured it but it sounds like 700hz modulated with something like 15 or 20hz. Mine works best if I can just barley make it out.

My ears are about wasted anyways!

What is that resistor tied to? .5 ????

[Micheal]

today I tried out my xl 500 on the beach after doing the 5k resistor mod. With very light threshold on it was erratic and chattery when I used in the water.however with no threshold on it was ok.I was using the large 33cm coil( found 2 coins ).
I dont feel these factory coils are very good at all .Has any body got plans for a coil for this machine that I can have .I feel that this machine has a lot more potential.I believe that more depth can be extracted out of it ,has anybody body got ideas that will help.

[homefire]

Cool! Use should have been able to loose some of the chatter by adjusting the Elimination pot a bit.


Here is the best info I know of on coil construction!

http://www.geotech1.com/cgi-bin/page...oils/index.dat

Here is the Coil Parameters from this site.


Garrett
Name (ohms)

XL500 7" 149.5uH 1.2

XL500 12.5" 140.8uH 0.746

When time permits, I think I will give it a go and build a shield coil for it.

I would love to make a Etched coil for it but clueless on how to construct one of the correct inductance.

[porkluvr]

Hello Homefire, everybody.
For an etched coil, a flat spiral wound coil calculator should put you in the ballpark so that you're not completely in the dark.
http://deepfriedneon.com/tesla_f_calcspiral.html
http://www.csgnetwork.com/spiralcoilinduccalc.html

So far as chattering goes when trying to reel in your primary sample time - you are going to run into two obstacles that I can think of.

1) Your receiver opamp has a finite overload recovery time. It can be reduced by either reducing the stage gain, or changing to a faster opamp. IMHO neither option should be attempted unless you have an oscilloscope because you may need to re-adjust your offset voltage pot, and you want to see where your sample pulse occurs in relation to your opamp's overload recovery. That's just me - some brave souls would hack their detector with nothing more than a Radio Shack multimeter.

2) If you're in the surf then the salt water will act like coil capacitance, and lengthen the flyback pulse duration. You should wait for that noise to subside before taking your sample. So... I have read that you must wait a minimum 10us because of the salt water but I suspect that number could be even bigger depending on other factors.
Just some thoughts.

[homefire]

Cool Info thank you!

As For the Scope! Your Right. I don't have one and It would be most cool to see what to hell is going on.

I took the stab and it worked within reason. How would pulling in the sample time effect the Amp off set? It should be centered or not I would think.

Off Set did not seem to be a major issue but I'm sure it could clean up part of the mess. I question it was optimized to start with.

Hell, the way they put these things together I wonder how any two act the same.!!!

Any mods to the Transmit pulse would require modding the Damping for sure.

1) Your receiver opamp has a finite overload recovery time. It can be reduced by either reducing the stage gain, or changing to a faster opamp. IMHO neither option should be attempted unless you have an oscilloscope because you may need to re-adjust your offset voltage pot, and you want to see where your sample pulse occurs in relation to your opamp's overload recovery. That's just me - some brave souls would hack their detector with nothing more than a Radio Shack multimeter.

How would pulling in the sample time effect the overload issues on the amp?

If it was set up right, it would not reach overload. Recovery time could be a issue. Could something be done to bias the recovery time?

Hence the chatter? The 500 had the problem, but could be corrected by a slight adjustment of the elimination pot pulling the sample time back out a bit.

I attributed this to the fact the coil and cable are unshielded after modding the 200 that has a shield cable and coil worked just fine.




2) If you're in the surf then the salt water will act like coil capacitance, and lengthen the flyback pulse duration. You should wait for that noise to subside before taking your sample. So... I have read that you must wait a minimum 10us because of the salt water but I suspect that number could be even bigger depending on other factors.
Just some thoughts.

Splash I heard it called. The salts in the water expands the return.

My goal has nothing to do with Salt water or even water.

I seek the small Gold nuggets in creveses and within or under black sands.

Mission Impossible!

PI are most good at seeing through nasty mineralized ground. Once you have them humming, they see the little differences within.


Good thing the Bumble Bee's can't Read! They would Never Fly!

[porkluvr]

Good thing the Bumble Bee's can't Read! They would Never Fly!

You're wrong! You can even make a deer fly (if you hit it at the right angle of inflection with a big enough truck.)

Changing timing would NOT affect offset, but if you decided to play with amplifier gain, that might, and if you replaced the opamp, then it is an absolute certainty. It's not something you should attempt without a good deal of thought and preparation (and taking before and after scope shots).

There are some really good opamps available nowadays but only through methodical testing would somebody be able to say if replacing your 709 be worthwhile.

I often wonder why some of these PI detectors use the relatively noisy 709 opamp when there are much less noisy (and fast) amps available, and I can only guess that 709, with its custom compensation scheme, may have better overload recovery than the competition. So, then the design engineer would have weighed overload recovery heavily in comparison to lower noise!. But, that's only a guess. Also remember, XL500 is an old design (I think) so 709 may have been a top choice at the time.

Because of the fact that spice testing is not real life, (and also because I have no scope at this time for real life tests) I can't test my hypothesis. On top of that, all the 709 manufacturers I know of did not bother to include the custom compensation pins on their 709 spice models, so I'm really screwed (and really irritated)! The models are incomplete, err, like it's no big deal???? Well, I couldn't do any better so I shouldn't throw stones.

"How would pulling in the sample time effect the overload issues on the amp?"

Before you sample you need to wait until the amplifier's output accurately reflects its input or else you are sampling noise.
See this page: http://www.geotech1.com/forums/showt...7835#post87835 (I wish I knew how to direct you to the precise pane of interest but I haven't figured that out). I will quote Carl Moreland here, from his Hammerhead thread: (where is Carl when you need him?)

"... Some opamps that otherwise look great have poor overvoltage recovery.
... the BW spec determines how much gain you can run. This spec is actually a gain-bandwidth product, so an NE5534 (GBW=11MHz) with a gain=1000 (HH) will have a BW=11kHz. This results in a response time constant (tau) of 14.5us. If you want to sample at 15us, then you really need a faster response, so either a higher GBW or a lower gain (for some reason I thought the NE5534 had a higher GBW).
Ferinstance, let's say that at 15us we want to be at 3*tau settling on the opamp, so that means tau = 5us, and BW = 1/(2*PI*tau) = 31.8kHz. For GBW = 11MHz, this limits the gain to 345. You can run a higher gain (as with HH) but the opamp settling won't be out of the way and can degrade sensitivity. So HH really could use a reduction in preamp gain, which Reg Sniff has pointed out previously. Or a better opamp."

This is not easy reading so go over it three or four times if necessary.
Cheers (and watch out for flying deer, they can be very dangerous)!

[homefire]

Come to think about it I have Launch a deer before!

Cool stuff!

I need access to a scope.

Living in a two and half horse town , it's hard to obtain.

The 709 has been history for a while. I have some stashed. Metal cans and all.

[Unregistered]

Hi guys and thanx for all the information Im not really very knowledgeable with electronics,however I have read all the information on making a coil and today I gave it a go...25cm diameter 21 turns of #25 pvc insulated strand wire (blue color )after doing this I continued winding all around the 21 turn bundle aswell which I saw on the forum by aziz I then used the scotch 24 shield material to stop interference (and boy it really works).Once the outside shielding of the coax cable was connected to the the scotch 24 sheilding material Presto no noise...Anyway I did some qiuck air tests with a Australian 20cent coin...with no threshold the best I got was 30cm and with threshold at barely audible minimum 35cm....I thought that was fantastic ...I think it can be tweaked a bit more....I would really like to somehow change the tone to a mono tone Rather than a pulsating tone and somehow how add a VCO to this machine, I think it has real potential...Do any of you guys know I do these mods because at at the moment the tone problem is a real drawback...a single tone will make it easer to distinguise when an object is passed by the coil...I think if I can do this mod with some help from you guys It should be able to hear the coin from 40cm...any help is welcome and appreciated(please boys dont write in such a complicated way it takes me hours to translate things so I cant understand even 20% percent of what you guys say)...I will do some test and post results here in while ...

[Micheal]

Here are the results of some quick air tests for xl 500...
small factory coil ... 33cm factory coil...... My home made coil 25cm

Coke can-------- 62cm ---- 67cm ------------ 70cm
small 14kt ring--- 12cm ---- 16cm ------------ 24cm
20cent coin--------- ---------------- --------- 30cm
$1 dollar coin(similar to euro dollar)---------------------------- 32cm


Althuogh these depths arent great compared to the new state of the art PI machines...I think that they are ok for a machine that is made in the 80's...i will keep tinkering with the xl 500 and try making a slighty larger coil because I think it has a lot potential and perhaps for a cheap machine and very simple machine it is a good machine to try mods on and any ideas is welcome and unlike some poeple I am not scared to fiddle with the machine and to see what results are going happen ...I think a vco would be a great feature if anyone has ideas and a single tone rather than the annoying 2 tone pulsating tone will make a difference....any ideas I will give it a go

[porkluvr]

Micheal,

You don't say whether or not you have an oscilloscope to watch your flyback voltage. Of course you know that the factory installed damping resistor is optimal only for a relatively small range of coil parameters and that if your homebuilt coils vary much from the factory spec then the damper would probably best be changed.

Do you have instruments to measure coil L and R? I used a coil calculator and plugged in your 25mm and 21t and got an inductance number that was significantly higher than what I believe is the factory spec for an XL500 coil. Considering that the diameter of your wire (including insulation) is unknown (and I don't believe the coil calculator's results ANYWAY), of course my calculation is not accurate and may not even be close. (Hmmm, so what am I doing here anyway?)

By the way, saying "#25" is incomplete information because there are different wire gauge standards. AWG, SWG, BWG (Babylonian Wire Gauge(?)).

If you have a choice, try to use teflon insulated wire because I believe you will wind up with less interwinding capacitance than with PVC. How much less, I don't know. And a larger gauge wire would not hurt. Your coil is a limiting factor on how small of items you can find. You could make all the other fine adjustments in the world but if your coil slows the flyback, well then you're not going to see much improvement anyway.

Heck, from the air-test data that you listed, you must be doing something right.

[homefire]

I'm starting to think the 500 was build loose so they could come out with a new finagled version later showing all sorts of improvement.

The more you look, the more you see that can be done to make it hotter, Faster, and just plain better.

[porkluvr]

Homefire, if you don't like the modulated sound in your XL500 I think you can remove the 1N4148 diode or the 1MΩ resistor (or both) between the LM358 sections, and have an unmodulated target tone instead of the stock modulated signal.

I've seen two different "XL500", schematics, so this is depending on whether or not what I have here represents your unit.

I don't fully understand the audio circuit, and I'm sure there are some minor mistakes in the schematic, and possibly some other errors that I haven't identified that are hanging me up - so this is all I can offer for now.

I've got a big smelly fish to fry, so bye for now.

[Micheal]

Thank you porkluvr ( big hug for you )I have removed 1N4148 diode and the 1MΩ resistor and you are absolutley spot on ...I have now mono tone "perfect"definitely more clearer with faint signals(i am excited).Here some results ....(xl 500 battery fully charged) using 12.5 factory coil...


coke can..............................................7 5(80cmvery faint).. (very very faint at 90cm)
14kt goldring same ring as above test............26cm(very very faint 40)
20cent australian coin.................................37cm(very very faint 40)
$1 australian coin.......................................35cm(ve ry very faint 39)


To get these results I have done the 5k resistor mod and removed the above diode and resistor thanx to porkluvr (another big hug to him)and I am not using the headphones because I have discovered that the headphone volume is terrible and the faint signals I hear with amp are not hearable with the headphones and belive me the difference is huge (I am using a marshall mini MS-2 pocket amp to amplify the audio and it works just beutifuly and it is very cheap with plenty of volume).....with the threshold on the xl 500 turned up slighty so the tone is very lightly hearable.

I am really happy with results now...It has surpassed my expectations.

By the way I have tried the irf 740 and even irf 540 mod and it is no good...big loss of depth and even with my home made coil which is shielded .

I really would like to add VCO to the xl 500 if it is possible I need help with it
I think the biggest achievemant to make this machine match the big guns of today would be an Auto Threshold .Thats all that is need really all the rest of the high tech stuff is not needed ...simple is and useable is best....Are these 2 mods possible guys

[Micheal]

just a correction I will add is the ring was supposed to be (very very faint at 30cm)

[porkluvr]

Micheal, so why don't you register?

I'll tell you just like I tell everybody. I don't want this to read like a lecture - but if you want to do any serious modification to your machine you must have an oscilloscope. You're brave to attempt an FET mod without an oscilloscope, but it is unrealistic to expect success. It is not a "drop in and go" modification. I hope I didn't give the impression it was, because it's NOT.

By changing to a FET you would need to change the value of the damping resistor and it is best to use a scope to find a good value. There is no way around that unless you have a set of components that have been previously verified to work together. You don't.

Also, pulse width would need to be be adjusted and you need to be able to see that. And, scope the FET driver to make sure that's OK.

The transmitter is a system and you cannot go and rev it up without looking at it as a whole. With an IRF740 you also need a higher voltage damping resistor. Which reminds me:

If you do get a scope make sure you get a high voltage probe or two. Most probes are not rated for the 400V that IRF740 would subject them to. I have a source for cheap HV probes somewhere and I'll try to dig it up if you need it. (I'll need it, soon - I hope.)
**************************
I think you could remove the .1 capacitor from section A of the 358 to disable that unused oscillator. Definately leave the 100k to ground intact.

If you can make simple PCB you could maybe build a VCO on an outboard and wire it in. But putting it on the existing real estate - I just don't know about that. Then, there's the veroboard option. That could be quick and easy. BUT, no scope, no hope. (Hey, I made a rhyme.) (Sorry)