XL500 sample timing

[Micheal]

Ok porkluvr thank you ,this is great,I am starting to see things more clearer because I was way off i was looking in other places the way you are describing things I am making progress I am starting to understand ... the description is much better now porkluvr .I have downloaded the datasheets like you said and I am going through your directions at the moment ...and I feel I can decipher things easier now...(If I cant work it out I will email you hi res pictures)...its looking good I,ll let you know how I go .

[porkluvr]

.If you think you know locations please mark the locations on the photo I have provided earlier this will be much easier for me to understand.

Simply put, I can't easily follow your fotos and trace out your board when I don't have an accurate schematic. I would prefer the board in front of me; those pictures wont do. I would need higher resolution pics.

I pointed out on the schematic where to make connections. I will try to explain.

Start with ground. Ground would probably be a fat trace running around the edge of the board. The (+) side of the large electrolytic should connect to it. Look on Homefire's schematic and find it on your board. Ground is where you would connect the black probe of a voltmeter to make voltage measurements.

(+5V) Look at the schematic and see where +5V is. Find the LM741 pin 7 and CA3140-7 (also LM358-8 and LT1037-7) and use a voltmeter to confirm +5V on those pins. Then look at the bottom of the board and trace back to the voltage regulator LM78L05. You should be able to read the part number on the regulator. LM78L05 Pin-1 is +5V regulator output.

(-5V) Find LM741 pin-4 and CA3140 pin-4 (also LM358-4 and LT1037-4) and use a voltmeter to confirm -5V on those pins. Then look at the bottom of the board and trace back to the voltage regulator LM79L05. LM79L05 pin-3 is -5V regulator output.

I make mistakes so you should download LM78L05 and LM79L05 datasheets and double check what I said. Actually, saying pin-1 or pin-3 is meaningless unless you know where to start counting from - so please get the datasheets and take a look.

So we have three of the five wires, right? Try to find connection points for those three wires as close to the regulators and their output filter caps as possible. After you locate a good tap-in point you should drill a small hole, (make sure you don't drill into a trace) insert wire from the top of board and then solder underneath. This affords some strain relief. Or, maybe you can find jumper wires on top of the board to solder to.

For the VCO input locate your CA3140-6 (the output) follow that trace and the resistors until you come to the LM741-3. You should probably solder in the mid-point, to the junction of the two resistors and capacitor between CA3140-6 and LM741-3. They may have changed your circuit so keep your eyes open. I doubt it, but maybe.

VCO output should be at the end of the 47k resistor that connects to the LM358-7. You would need to disconnect the LM358-7 because that WAS your sound oscillator output but is no longer.

Sorry, but I had trouble following your fotos. If you need more help, I need a clearer pic. Send to me directly by email.

[Micheal]

I tried the VCO again ...and I just cant get it to work...Can be kind enough to mark where the 5 wires have to go on the photo of the board I have provided please ...I have gone through the VCO board and it seems to be as your diagram.Without faults.But just can get anything to happen.I feel that I am not connecting the wires to the correct locations on the board.If you think you know locations please mark the locations on the photo I have provided earlier this will be much easier for me to understand.

Also the photo that I have provided previously above that has the remove indications on them are the parts I have removed so that the xl 500 has mono tone and it is working as such perfectly(thanks to you porkluvr).


By the way I am learning and making a little progress.(i can now mesure resistors with my multimeter).

[porkluvr]

Micheal, since Garrett has replaced the MJE3055 with an FET they would have almost certainly changed the value of damping resistor unless they have changed the coil specs, which seems ludicrous. (Changing coil specs to match a damper? Naaah. I don't think so, anyway.)

You should be able to trace the drain circuit and find the damping resistor.

But wait. You already have an FET. What possible benefit can be gained from replacing it with a different FET unless you have the equipment and know-how retune your system? Take the time you would spend doing that counter-productive action and get to know what you already have.
If you hack your PI's coil and driver (the FET) without having the basic tools AND WITHOUT BEING FAMILIAR WITH THE CIRCUIT you are wasting time. It's foolish, so leave the FET alone.

Search through the forum on topics of interest and do some browsing.

And remember: you should buy an LCR meter if you don't already have one, and a dual trace oscilloscope. It doesn't need to be a 500Mhz quad trace Tektronics. 35Mhz or so should be plenty good. Those are two basic and instruments. They are indespensible for metal detector hacking.

edit: Please wash the solder flux residue off your board if you haven't done so already.

[porkluvr]

...the one on my machine is RFP8N1NL and it is in the same way as the first drawing you show ,that is the heatsink is on the boards edge side .

Is that a typo? It looks like RFP8N18L in the foto. I think they have modified your board layout compared to Homefire's. They turned your part around! Check it out. The tab IS wrong in the parts placement diagram but on your model's board they have reversed the pads and that component's orientation! If we were looking up from the bottom of the board then the tab would be correct, but we're not.
http://www.datasheetcatalog.com/data...MJE3055T.shtml
No big deal, except that I was looking at that PPD picture and comparing with IRF640 pinout when I made the incorrect assertion that IRF and MJE pinouts were mirror images. (Stupid me.)

Moving right along. Yeah, please try to get the VCO working. I really believe it should be compatible with the XL500 but I may have "wound" the component values too tight because I tried to eliminate unused potentiometer travel. You should get something out of it if it's wired correctly, i think.

I'll share the switching regulator -uhh soon. But after our success with the ultra simple VCO I have a troubled conscience. I roughed out the switcher PCB layout but it needs checked for errors. (Now, that's going to be a trick, checking my own work AGAIN.) I may etch that PCB after I get my Looboo PCB layout completed so I can do them both at same time. I haven't seen Looboo for a couple of days, so I'd better get busy. It's almost finished but there a subcircuit I still haven't quite de-bugged.

edit: Ohhh, about your damper. Look for a 1K resistor connected between the coil output and ground. Maybe your coil is different so that they may have changed the value, but it should be near the coil (unless it's IN the coil!!?) or maybe mounted on the connector.

[Micheal]

Hi porkluvr...Thank you for your patience and thank you overall....I totally agree with the way you feel about the Auto threshold mod...I have not got enough knowledge to proceed with this and I dont know anybody around here that does know anything about these..
I have had a look at the mje3055 and its not the same as what is on my machine....the one on my machine is RFP8N1NL and it is in the same way as the first drawing you show ,that is the heatsink is on the boards edge side . I will try this mod again becuase nothing bad will happen to my machine ...But what I am going to try this time is the IRF 540 and I will see what happens and I will try it both ways and trust me xl 500 can handle my abuse .
The other question is which one is the damping resistor (I hear a lot about this and cannot locate it )?is that the 100k resistor directly on the right of the mje 3055
I know you dont really want to educate me on the topic but please hang in there ....who else is there that can help us newbies and just think of all the xl 500 owners waiting to see the results.
And I am also going to retry the VCO and go right through it again I am just wondering if I do this a different way. I really think that if I take the source signals from the signals that go to the speakers (headphones)I am going to have better luck at it ,because the source signal voltage at the speaker(headphone)increases as the target is closer to coil ...hence isnt this what drives the vco and make it do what it is meant to do,the higher voltage signal the higher the pitch in the tone...So I will try to incorporate the existing vco unit that you showed me to somehow work in this way... Like the guitar effects pedal I have shown in previously....and it all then goes into a separate amplifier box rather than using the headphones ....

I,ll give all of this a go and let you know how it goes ...cheers

[porkluvr]

Possible mistake in parts placement diagram

Micheal, I have compared the picture of your board against Homefire's parts placement diagram. I think that the FET in the picture was installed backwards. Here's why:

I made an incorrect statement very early in this thread; that if an IRFxxx were to replace MJE3055 then it must be reversed 180 degrees. That was incorrect. I appologize.

Another issue is that the parts placement diagram shows the MJE3055 power tab towards the edge of the board. After looking at a MJE3055 datasheet it appears to me that the parts placement diagram is wrong in this respect. The power tab should be shown towards the inside of the board. This may have been partly the cause of my getting the pinout relationship wrong... but I should have know better. The power tab is shown backwards, RIGHT?*

I would like to point out that I still believe that any coil driver replacement should not be attempted without having a scope handy. Installing an FET in place of the MJE3055 will result in a different pulse width driving the coil (hence, coil current will be different) and will also require the damping resistor value changed. Also, drive requirements for BJTs and FETs are somewhat different so more parts besides just the MJE would need replacing.

*Feedback would be appreciated. I don't have a XL500 to look at.

[porkluvr]

Micheal. Your detector PCB looks like it is about 80% the same circuitry as the first circuit and PCB that Homefire posted in this thread. So it would be a good idea, and probably not to difficult for you to trace out the remaining 20%. That way you would know where the original circuit leaves off and your model begins.

I realize that what I just said may sound like a daunting task, but without a good idea of exactly what is on your PCB, instead of just guessing, I don't think it is wise to go hacking a power supply modification.

The VCO modification that I came up with before was simple and only involved a small portion of the entire circuit, and so things could fairly easily be put back the way they were if it did not work - as was the case. That was a bad omen.

Adding a switching regulator power supply is something of a different scale which would affect the entire detector. It would be irresponsible of me to try and walk you through such a task without my having more knowledge of your exact model, AND, your having a good deal of skill, knowledge and also, test equipment.

By your guessing at what components do on your PCB suggests to me that you need to gain more skill and knowledge or else have somebody do the work for you, or else forget about a power supply modification.

Between us, I don't think we have what is required to do the job succssfully, and I don't need a failure of such magnitude as this may turn out to be. I do not have time to teach you basic electronics and try and walk you through a tricky modification on a detector that I don't have a layout and schematic for. I'm sorry to have to tell you that.

Maybe if you got a new battery you wouldn't have trouble with the threshold fading so much

[porkluvr]

I have attached a picture of which ones to remove and add jumpers to...please let me know your opinion and the other question ..is it safe for the ciruit to run on direct 9volts from the battery without the voltage being dropped down by the regulators .

Huh?

NO.

I do not think you can safely run from 9V without having some way to adjust the TX pulse width intelligently.

You are going too fast for me. Do not remove the regulators, that would be very bad. (OW!!!)

Step back and take at look at what you're doing (you are scaring me). Without an oscilloscope and some more technical knowledge you are in very dangerous territory messing with the power supply.

Without having a schematic and PCB layout for YOUR model there is high likelyhood for error unless proceeding VERY carefully. Good communication is important.

I'll have some information tomorrow. It's too late today. I need to crash before getting too technical and making a mistake .

SLOW DOWN. THINK things through. Don't get over-confident.

I've been working on this all day and I need a break. (I'll be back. Tomorrow.)

edit: For what purpose were you asking about removing components? You gave no explanation. And remember, we have no documentation for your model so that means GO SLOW. Isn't your board different from Homefire's? How can I know what you are pointing at - a bunch of wires in the way... I'm lost!

[Micheal]

I have attached a picture of which ones to remove and add jumpers to...please let me know your opinion and the other question ..is it safe for the ciruit to run on direct 9volts from the battery without the voltage being dropped down by the regulators .