MPP revE need help

[Albdelafar]

If you have no signal from the secondary small coil it would seem that your detector is not transmitting a signal via its coil.

I agree with you but my signal change when a target is near the coil so I do not know what to believe.

The TX circuit is pretty straightforward. You've got the drive pulse to the gate of the MOSFET and then you've got your diode and coil and damping resistor.That's it. So not much to check by way of troubleshooting.So you can recheck everything, make sure the mur460 diode is good and correct orientation.Recheck the mosfet is good, etc.
Check the board connections are good.

I checked the board many times the components are in the right place I took a new picture . I checked conductivity of TX and RX pin to ground an i have no short. R10 is 3k3 ohm R7 is 10ohms. I noticed a 1M ohm resistance between the MUR460 in the otherside there are infinity resistance as expected.

Please post good pictures of:
- Solder side of your board
- new picture of component side - is the 3.3 Ohm resistor to coil really 3.3 Ohms - cant tell in the photo.
- how are the wires connected from pcb to coil - orange,grey,red,black,white - photo or drawing

in reference to this photo https://www.geotech1.com/forums/atta...8&d=1588671083

Hi you can see the solder side . The copper layer is a bit unstick on TX pin but i checked connectivity with R10 and R4 and it's good. No connectivity between TX and ground too.
For my coil connection TX and RX pins (midle pin of PL1 an PL2) are shorted on the coil plug and the other side of the coil is connected to ground on PL1 pin 1.

For the coil it is a 0.33mH 0.7ohm i added a serial 10 ohm resistor to up TC to 1/3 of the pulse width (100us).

[Polymer]

Hello,
So this step is good with the shunt between TX and R1 I think the tx pad was damaged when I changed the pin. What ever now my TP3 signal is good noisy but looks like normal !!
[ATTACH]50139[/ATTACH]
Next I adjusted R37 to have a minimum sample delay of 10us as you can see on TP6.
[ATTACH]50133[/ATTACH]
On the next step something differ from what is expected.
-EFE delay pulse width is expected to be approximately 100us but it is 118us as you can see here.[ATTACH]50135[/ATTACH]. I'm no worry about it but it is close to 20% difeerence.
-Pulse width at both TP9 (sample pulse width) and TP10 (EFE pulse width) is expected to be approximately 10us but I measure 17.6us that i think is more anoying because it's 76% more than expected....
[ATTACH]50136[/ATTACH][ATTACH]50137[/ATTACH]

Do you think my CMOS could be the problem ? I bought CD14538BE instead of MC14538BCP who was not available. I used 1% resistors and 5% polypropylene capacitors (oversized 2.2 nF_330 V ; 10 nF_630 V) and a 10% ceramic capacitor for C13 (220 pF_500V).
I join a picture of my PCB with components [ATTACH]50138[/ATTACH]

Please post good pictures of:
- Solder side of your board
- new picture of component side - is the 3.3 Ohm resistor to coil really 3.3 Ohms - cant tell in the photo.
- how are the wires connected from pcb to coil - orange,grey,red,black,white - photo or drawing

in reference to this photo https://www.geotech1.com/forums/atta...8&d=1588671083

[dbanner]

The TX circuit is pretty straightforward. You've got the drive pulse to the gate of the MOSFET and then you've got your diode and coil and damping resistor.That's it. So not much to check by way of troubleshooting.So you can recheck everything, make sure the mur460 diode is good and correct orientation.Recheck the mosfet is good, etc.
Check the board connections are good.

[surfdetector]

I did some tests :

With a litle coil under the tx coil there is nothing too(probe connected to a side and ground of the probe on the other side).

If you have no signal from the secondary small coil it would seem that your detector is not transmitting a signal via its coil.

[Albdelafar]

I did some tests :
With the coplanar coil :

https://drive.google.com/file/d/1k2r2Rb3FrH1MbPvqgblwIPy42wvLeOr3/view?usp=sharing

With a multistrand electric wire 0.200mh 0.7ohm nothing :
With a litle coil under the tx coil there is nothing too(probe connected to a side and ground of the probe on the other side).
I get this on the damping resistor tool when no coil is connected.

[surfdetector]

you need to connect your large coil to the metal detector and then the small coil leads are connected to the oscilloscope probe and ground clip. You put the small coil near the large coil and turn your detector on. If your large coil is working then you will see on the oscilloscope the induced signal from the small coil.

[Albdelafar]

The TX pulse and flyback can be checked without a connection to the pre-amp.

I disconected R1 on this tests so pre-amp was not connected.

Another way to test the coil is wind a small diameter coil, 2cm, of about 10 turns. Connect a scope probe to this small coil, both probe and its ground. Then place this small coil very near the TX coil, inside the TX coil's field.
If the TX coil is being energized and discharged then the small coil will see the changing coil current since this is a transformer.

I will try this. Do the coil need to be close or open ?
Thanks

[Albdelafar]

This coil work because I see voltage variation with a target near the coil.
I made 2 coils the first classic mono coil and the other a coplanar coil the coils was tested with my new composant testor as you can see on post #80 I get a 0.33/0.34mH for 0.7/0.8ohm coil resistance. I made them with the same 1mm coper enameled wire.
Today I took multistrand wire from electric wire i will try to do a coil with it.

[dbanner]

You probably have a degraded coil, not quite shorted, but there is leakage between the windings. Try another coil.
Otherwise how could you possibly have a DC response at tp4 with an open coil? Not possible.

[waltr]

Possibly the coil is shorted. That would give the TX pulse shape at TP3 but NO fly-back at the MOSFET Drain.

Still think the coil is open and what you see is completely due to the damping resistor, R4.

The TX pulse and flyback can be checked without a connection to the pre-amp.

Another way to test the coil is wind a small diameter coil, 2cm, of about 10 turns. Connect a scope probe to this small coil, both probe and its ground. Then place this small coil very near the TX coil, inside the TX coil's field.
If the TX coil is being energized and discharged then the small coil will see the changing coil current since this is a transformer.

[Albdelafar]

Hello waltr I checked many times my coil connections and it's like on the manual schematic and my TP3 signal looks like what is expected. So my IRF could be broken if I did not have signal like expected ? How the signal is supposed to be ?

I was thinking to another point this night did my problem can come from a bad coil wire insulation who not support the flyback voltage peak ?

[waltr]

The scope pic of IRF840 Drain looks like the Coil is NOT connected.
If coil is not connected or has an open or is connected to wrong place then there will be no flyback.

Carefully recheck coil connections.

[Albdelafar]

IRF840 pins scope measurement here :

I took several videos on R10 with analog and digital scopes on MUR460 too.

https://drive.google.com/file/d/1dadwGnh9zKqWHypsTd19C91UEbPncaUf/view?usp=sharing

https://drive.google.com/file/d/1Lcvahtqn2YkCzyHwqW3PIhg012KaSfiB/view?usp=sharing

https://drive.google.com/file/d/1SoaZgcXPq47sQ9nfgZJP22M5Clgpy3sO/view?usp=sharing

https://drive.google.com/file/d/1abcNikrJqv4rw-8sVKxhSjsS7xGvvmYP/view?usp=sharing

The only flyback voltage I can see is a more or less 400mv peak.......

[dbanner]

Oh, I forgot the revE is with mur460 diode.Try the probe on the mur460 cathode and mosfet.Maybe your scope will not be able to to see this point.Surely the analog scopes with 100x probe can do this job.

[dbanner]

There has got to be a flyback voltage, otherwise there are little green men walking around on the PCB.