Building the Geotech Barracuda Rev A (from silverdog)

[homefire]

Even a $20.00 bangood special is better then no scope. http://www.banggood.com/buy/oscilloscope-dso.html

[azigta]

Well, today I went to the electronics store and they didn't have the 2N5485 but I bought round female headers, and I managed to solder them and hold them steady on the board. One trace was broken but I managed to solder a copper wire from the other side of the board to the resistor it was connected to. It was difficult, but now it's stable. When I have the transistors I will plug them in and don't ever get them out xD Just in case I break the traces... I think I will end up buying the transistors from silverdog because I these transistors aren't available in Spain, I'll wait to see what the electronics store guy tells me.

[chevesne]

2N5485 equivalents
2SK148
2SK154
2SK168D
2SK193MF
2SK195J
2SK199
2SK19Y
2SK23A-8
2SK315F
2SK41F

[azigta]

thanks for the equivalent list. I got the 2N5485 today, so I didn't have to use any equivalent.

I'm lucky, The board did survive the operation that I had to do because I destroyed the transistor pads and a trace. I soldered female headers and I putted the transistors on, and It now does exactly the same as before.

I'm going to write as much as I can about the board behaviour, maybe that way someone can think some component is bad, this is everything I've tested and done, and how the board is working right now:

1- I have and pin 2 shorted to pin 1 because in the step 7 it says that you have to short pin 2 to either 1 or 3. (in PL2)
2- When I apply 12v with the battery pack, the coil emmits a high pitch audible tone that gets louder when I place a big metal object near it.
3- When I touch the positive terminal of my multimeter on U4 pin 1, that pitch tone goes down, not the volume, the tone. I noticed that today (Having the black probe connected to the battery black wire, gnd) (Is this normal? should I even hear a tone comming from the coil? maybe U4 is bad?)
4- I have this values on U7 when U7 is not inserted in the socket:

1-> goes from 1.87v to 2.65v when a big metalic box is far away or very close to it
2-> 1.83v
3-> 0v
4-> -5v
5-> 0v
6-> 0v
7-> 0v
8-> 5v

When it's inserted in the socket:

1->0v (no matter what I do with the metallic object)
2-> 0v
3->0v
4->-5v
5->0.21v
6->0.33v
7->-3.71
8->+5.12

5- I checked the 2N5485 pins and I have -5v on the gates, 0v on the source and drain.

6-I desoldered and tested R14 and it does change its values to the desired values corresponding to the resistor value.

For now it is all I have done. Sorry I'm no good at troubleshooting... Please if anyone can tell me what to test or some clue on why I get 0v on U7 pins (and therefore I can't adjust R14 to give -2.1v measuring PIN 1) or if anyone sees weird measurings and knows what could be the problem, I would really appreciate some guidance. Thanks

[azigta]

I measured pin 1 on U7 and it now doesn't measure what it measured the last time I checked it on february 20th, so I think I broke something else. I guess I can't make it. I abandon de project. Thanks to all those who tried to help me. I will buy a new one, I just wanted to have fun building it but I'm not having it and it's depressing me too much. I've read that this year there are going to be great metal detectors , so I will wait until garret, fisher etc. start selling the new 2017 detectors. I was going to buy the garrett at pro, but I've read about Depar DPR 600 and it's a much better machine for almost the same price, so I will wait to see what else comes up and compare.

Thanks

[ionut_mtb]

voltage at pin 1 u7 can be different. i attached pcb, check all components, you have to short pin 1 and 2 like in picture beacuse pin 3 is not connected to pcb. post a picture with your board top and bottom, and picture with your coil please.

" I can't adjust R14 to give -2.1v measuring PIN 1) " in my baracuda i could not adjust for this value. minim value i had like 1.2v, so maybe is ok.


Al least you can try to solder all the components to see whats happen...

[azigta]

voltage at pin 1 u7 can be different. i attached pcb, check all components, you have to short pin 1 and 2 like in picture beacuse pin 3 is not connected to pcb. post a picture with your board top and bottom, and picture with your coil please.

" I can't adjust R14 to give -2.1v measuring PIN 1) " in my baracuda i could not adjust for this value. minim value i had like 1.2v, so maybe is ok.


Al least you can try to solder all the components to see whats happen...

[ATTACH]38968[/ATTACH][ATTACH]38969[/ATTACH]

Hi, I will solder all the components, but I don't think it's going to work. The last thing I noticed was a short on R14 and y desoldered it and destroyed the downside pads because apparently there was no other way to prevent the short. The pcb had some scratchings very close to the pad so it was impossible to not make a short there. I soldered some wires to other places of the board where r14 goes and I soldered a 20k potentiometer there because I didn't have any other cermet resistor, which died in the process of desoldering, of course... *facepalm*

In a few days when I have soldered every component on the board I will post here if It worked or not, but I don't have hopes on it.

I shorted pin 1 and 2, in fact, pin 3 is connected to pin 1 so that It doesn't matter which way to connect the potentiometer. With the potentiometer in r14, max value is like 0.22v , which is new because with the cermet installed it was always 0. So it's something but very far from even 1v... Like I said, I will post more when I solder the rest of the components, but I think I screwed up the board somehow.

Thanks for the reply

[azigta]

I putted all the components on the board, obviously it still doesn't work. But I give more details just in case someone can give me a clue of what's going on, how to test it etc.

Now I don't notice any change on U7 pin 1 when I place a metallic object near the coil
In step 8, it says to set the dc level to 0v adjusting the threshold pot while measuring U7 pin 7. Voltage on U7 pin 7 is 4.44v, threshold pot doesn't change anything.
Speaker works, although the tone coming from the coil Is way much louder than what I can hear through a 0.5w 8ohm speaker (It doesn't have volume control, so I have to get my ear close to the speaker to hear something). What I hear is the same tone as in the coil, but very weak volume, I guess that's normal since I don't have a volume control. By the way, the volume from the tone comming from the coil (Still don't know if that's normal) Does get louder. Weird...

So well, something has changed when I destroyed the transistors and pads and resoldered some female pins and putted new transistors there. Before the destrucion, I could notice a voltage level change on U7 pin 1, 1.87v to 2.65v when I placed a big metallic object near it. It now doesn't change, either when the IC is on, or not.

I can do tests, if someone can tell me what to do, Because at this point, I don't have any idea. I will have to abandon the project if no one tells me what to test, because I can't figure it out by myself.

I now have an LCR meter and I tested the coil and it's 294uH, very close to the minimum 300uH required. The dumping resistor that I'm using is 470 ohm resistor, I was told to use that by Qiaozhi, since I don't have an oscilloscope to get the dumping resistor value.

Well, I can't do anything else. I don't see any shorts, maybe some component is bad, but I don't know what could it be. I can give voltage measurings on every point of the board if someone thinks that could help troubleshooting... Thanks

[kt315]

You shouldn't have to randomly desolder components for testing, or replace them by guesswork. In this situation you really need to get access to an oscilloscope, and debug the circuit in the proper manner.

for tng.

all resistors are checked by measuring WITHOUT desoldering from board. if you use a modern digital tester.
an exception from this rule is for low resistance circuit part, but it will rare case.

[azigta]

I think I know why the threshold pot doesn't work, I didn't connect the third leg. I will connect it tomorrow, now is late, almost 2am and I was looking at the schematic and I saw that. No wonder why it didn't work! Now, the other problems, no idea how to solve them. I have another 2N2222 and I think Is the only thing I didn't test, and i socketed it so tomorrow I will simply change it, and see what it does. I have no hopes on this, but I have at least solved the r14 problem because now the voltage on U7 pin 1 does vary when I turn it. The voltage doesn't increase when I place a metal object in the coil, although the tone coming from it gets louder hm I don't know much about electronics but if the coil tone gets louder and the voltage doesn't increase on pin1, then it just be something between the coil and U7, and the only transistor that I didn't test or replace is that 2n2222 and Is close to the coil and part of the transmit tx circuit. It could be shorted or damaged... I'll take a look and post here. I think I have done bad damage to the board And that I won't be able to repair it but at least I'm going keep trying things from time to time until I buy a new detector.

[6666]

I think I know why the threshold pot doesn't work, I didn't connect the third leg. I will connect it tomorrow, now is late, almost 2am and I was looking at the schematic and I saw that. No wonder why it didn't work! Now, the other problems, no idea how to solve them. I have another 2N2222 and I think Is the only thing I didn't test, and i socketed it so tomorrow I will simply change it, and see what it does. I have no hopes on this, but I have at least solved the r14 problem because now the voltage on U7 pin 1 does vary when I turn it. The voltage doesn't increase when I place a metal object in the coil, although the tone coming from it gets louder hm I don't know much about electronics but if the coil tone gets louder and the voltage doesn't increase on pin1, then it just be something between the coil and U7, and the only transistor that I didn't test or replace is that 2n2222 and Is close to the coil and part of the transmit tx circuit. It could be shorted or damaged... I'll take a look and post here. I think I have done bad damage to the board And that I won't be able to repair it but at least I'm going keep trying things from time to time until I buy a new detector.

Yep, fig 11 of build document, it needs THREE wires on threshold pot.
check the legs on the 2222 very carefully, there are many configurations.

[azigta]

IT WORKS?!?!

WOW! Today I connected the third leg of the 50k threshold pot, and WOW! It works! I don't know if it works as It should, but it beeps very hard when I swing a magnet over the coil from a decent distance, and that surprised me a lot. Also it starts beeping when I turn the threshold enough. And if I turn it to max, it beeps harder. I think that is what it's supposed to do. I turn it until It beeps and then I adjust it so that is at the point where is about to beep but it doesn't because it's not turned enough (Do I explain myself? xD). Then I tried swinging a small coin over the coil, and it beeps, very weak volume, you have to pay attention, but sure enough, it beeps when I swing a small coin over it. Also bigger coins obviously, and bigger metal objects, not just the magnet. I need an amplifier circuit for the speaker, I have to find a low power consumption amplifier circuit, simple as possible. Does anyone know?

The 2N2222 was in good conditions, I had another one to compare the measurings and they measured the same so no problem with that.

R14, I adjusted it by ear , if you can call it like that. I don't know exactly what does this variable resistor do, but it's called "min delay" So I guess it controls the "speed" at which it can detect, because I tried swinging the coin over the coil fast and it didn't beep multiple times, so I adjusted R14 until it beeped as fast as possible. Now I will have to do it again because I soldered a potentiometer there and It might move while I do things with the pcb and so on. I will try to change it for a cermet and solder it with some cables to the same points on the pcb, That will be more stable.

I can't believe it .__.

I will give more details If I make any improvements.

[azigta]

It can't detect much deep. A 2€ coin (biggest coin in european union) Detecion is about 8cm. Something must be wrong. Maybe the damping resistor value? I'm using a 470ohm reistor, my coil is made out of insulated copper wire, 20 turns on a 10" diameter former, insulated with aluminum foil, it measures about 294uH. Someone knows why only 8cm for a big coin?

Also the mosfet gets hot (even with the heatsink and with thermal plaste (it's been like that from the first day)) Is not burning hot, but it is a bit hot, is that normal? Thanks

[ionut_mtb]

make a picture of your coil and post here, i want to see also the wire you use. i think 20 turn is less than what you need, maybe your uH metter not showing good. you can play more with the delay. for now i don't have power potentiometer to find the best value for dumping resistor, but i bought few resistors 200, 240,280....400, 440 ohm and i just test to see best result.

[jladre]

Azigta you are very near to finish it. Just adjust your delay trimmer until it hardly pick up the magnet and pick a decent depth on coins. Wave a coin under the coil while adjusting the delay.