Stuck in step 3

[gonsaIo]

Hi. First of all, I am an inexperienced person in electronics, so yes, I know my solders are really bad. Yes, I know maybe I wasn't careful enough, and maybe I messed it up.The thing is that originally I reached step 4 but a sound in the headphones wouldn't appear when getting them near the coil (420 uH), and at this point I did again the speaker test in U4 pin 4 and 0V and it didn't work. That was when I took out the U4 IC, placed it again and it worked (the speaker test, not step 4 test). That's when the speaker test from step 3 worked, but the headphones test form step 4 still wouldn't work. That's when I tried to change the 9530N mosfet that I had put in the PCB with the one that came with the kit. I put another one because in the kit came the 9640, so I thought it was mistaken with the 9530. I just did this to see if my guess was right or not, but it didn't work either. But the next day, I accidentaly connected the R1 between the up pin of CONN1 and the up pin from R3 and it worked, and the buzzer that you can see in the picture sounded really loud (with the 9640 mosfet), but the mosfet was super hot, so I stopped it and replaced it by the 9530. The thing is that now neither connecting the R1 and coil in place nor doing that connection with R3 it works. I checked some PCB traces related to the mosfet and the coil and they work fine, even though they look like burnt in the picture (yes, I know the mosfet is terribly solded, but I'm not skilled in desoldering). Now the mosfet (9530) has a -+17.52 V in its left and right legsfrom U7 pin 4, and the -5 and +5 V power supply works, and the speaker test from step 3 works too. In conclusion, step 1, 2 and 3 are checked, but step 4 doesn't work. I hope you can help me and with the information I've given. If anything else is needed I will tell you. Pictures: there doesn't appear the damping resistor, but I used it in the tests.


PCB: (front)

https://drive.google.com/file/d/1xUc...ew?usp=sharing

https://drive.google.com/file/d/11ds...ew?usp=sharing

https://drive.google.com/file/d/1W85...ew?usp=sharing

https://drive.google.com/file/d/1jvE...ew?usp=sharing

https://drive.google.com/file/d/1KQe...ew?usp=sharing

https://drive.google.com/file/d/1FgR...ew?usp=sharing

(back)

https://drive.google.com/file/d/1CRu...ew?usp=sharing

PCB and coil:

https://drive.google.com/file/d/1405...ew?usp=sharing

[Qiaozhi]

Q1 should be an IRF9640. The 9530 only has a VDS of -100V.

The first thing I would suggest is that you tidy up the solder joints, and also cut the long wires on the rear of the PCB. It's difficult to see from the photo, but it looks like some of these long wires may be creating a short-circuit.

[gonsaIo]

Thanks for your help, it works now. I'm in step 5 now, and experiencing some problems which I'm trying to fix. If I have to ask for help, shall I make a new thread or continue in this one?

[Qiaozhi]

Thanks for your help, it works now. I'm in step 5 now, and experiencing some problems which I'm trying to fix. If I have to ask for help, shall I make a new thread or continue in this one?

Keeping this all in the same thread would be better than starting a new one.

[gonsaIo]

Hi again Luckily I didn't stuck in any other step. I have finally built my detector, the problem is that the detection depth is very poor. It detects a gold ring at 10cm maximum and a 2€ coin the same depth. I have no oscilloscope so I adjusted R6 so that it gives 0V in U2 pin 6 just like the manual says, and I adjusted R14 in step 7 instead of 6 because I have no oscilloscope and set the voltage level in U7 pin 1 at 2.3V previously shorting pin1 to p2 of PL2. The thing is that for those values, initially, for a 0.37mH (370 uH) 9" diameter coil, the detection is less than 5 cm for a gold ring and a little bit thin gold ring isn't even detected (perhaps it's too thin), and with a 0,45 mH (450 uH) 15 cm diameter coil, the thin gold ring is detected but really low depth and a gold ring at 10 cm as I said before. The delay is at maximum value. I don't know what to do for changing this so I am asking for help. I'm more interested in setting up the detector for gold rings rather than coins if it's possible. Thanks for the help.
edit: forgot to mention I am using the damping resistor that comes with the silverdog.co.uk kit.

[waltr]

Thin, small and jewelry gold are low conductors with short TC (time constants) so you need to set delay to minimum not maximum to detect these targets.

Have you tried a Sliver coin which has a longer TC?

What about the Damping resistor value??
You really do need an O'scope to adjust the damping resistor value to obtain 'critical' damping.

[surfdetector]

Step 7
You should set the voltage to (minus) 2.1 volts

[gonsaIo]

Thin, small and jewelry gold are low conductors with short TC (time constants) so you need to set delay to minimum not maximum to detect these targets.

Have you tried a Sliver coin which has a longer TC?

What about the Damping resistor value??
You really do need an O'scope to adjust the damping resistor value to obtain 'critical' damping.

The thing is that if I set the delay to minimum, the thin gold ring doesn't even sound. Perhaps the pot is not welded in the correct position, even though I'm really sure it is (I don't have the PCB right now). Tomorrow I will check this and do a test with a Silver coin and post the results. About the damping resistor: how can I adjust it? Do I have to connect a 1k potentiometer in series with the damping resistor and check with the oscilloscope with diferent values of it (I don't know if there's a threat dedicated to this inquiry, but I couldn't find it)? Also, I would like to know what value and where I have to check, since I've never used an oscilloscope, but I'm willing to learn .

[gonsaIo]

Step 7
You should set the voltage to (minus) 2.1 volts

I don't understand, I'm looking at the guide ( [Doc. Ref. 20200822-A]) and in Step 7 it says Set the DC voltage level to around 2.3V.

[Qiaozhi]

Step 7
You should set the voltage to (minus) 2.1 volts

That's what it used to say in the old Build Document. Have a look at post #1 in the part on updates and corrections:
2020-08-24: Build Document updated to match kit components, and also to correct an error concerning the adjustment of R14 (minimum sample delay).

The value of 2.3V is correct, as it used to say -2.1V.
Check your copy of the Build Document. It should be labelled Ref. 20200822-A (shown in the footer on each page).

[gonsaIo]

Well, by build goes from bad to worse... I wanted to check the U7 pin 1 value just to see if it was correct, just to check, and it results voltage was below 2.3 V (it was at 0,4V). I took out the threshold pot (47K) and shorted p1 of PL2 to p2 and tried to adjust R14... nothing changed, it was still in 0.4 V. I took out the PL2 pot and shorted p1 and p2, but it still showed 0.4 V. What can have happened?? I didn't touch anything since it was working before.
edit: I checked it again and it's in -4.3 V now... -5 and +5V power supply is working. Now there's no variation of voltage when placing a target near the coil, it's stuck at -4.3V

[waltr]

edit: I checked it again and it's in -4.3 V now... -5 and +5V power supply is working. Now there's no variation of voltage when placing a target near the coil, it's stuck at -4.3V

-4.3V output from an op-amp powered by +-5V means the output is at the 'rail' and can not go further.
Re-check all your soldering for bad connections and solder bridges, shorts. Use a 10x magnifier and good lighting to see possible issues.

[gonsaIo]

OK, I'll do it as soon as possible. Thanks.

[gonsaIo]

I don't know if it was the original problem, but it's quite possible: the IRF9530 pin trace connected to R2 and R3 I think (the left one from this point of view) doesn't work, it's burnt. What can I do, connect the mosfet pin directly to R2 and R3?

[gonsaIo]

I already solved the problem. A trace of the mosfet was burnt. Now I'm adjusting the damping resistor. I have put a 10k pot in series with a 200R resistor, and a 2k resistor in parallel with both the pot and resistor (I read it from Qiaozhi in a thread). Does this include the damping resistor? I assume I have to connect this adjusting tool in series with the damping resistor, am I wrong?