Sniffer XR-71 help

[Carl-NC]

I know you haven’t worked directly on this circuit, but if you could roughly explain what the output of each IC should look like, I’ll try to fix them with effort and time

That's the problem. The RC clamp and the offset feedbacks alters the waveform responses so I don't know exactly how they should look. I can roughly guess, but it doesn't take much error in guessing to create an invalid answer. What you need are some correct waveform plots to guide you, and I don't have those.

[Jafar]

After D1, D2, and D3 start working, which ICs should be checked next?
If you list them one by one, I’ll inspect and troubleshoot them step by step to get them working.

[algan]

Solder a new board, starting with the power supply first, then check the frequency and so on, step by step, you're just wasting your time with this, you won't be able to fix it, learn step by step.This detector is not one of the best for deep searching, because it also gives false signals. As you dig, the signal disappears. The other thing is that it recovers slowly, there is no speed of recovery, you need to make corrections to the scheme, but for a hobby and to learn, it is fine.

[Jafar]

T2 and T3 are not for overload protection, they clamp the RC reset on D2A so that the timing delay begins when the preamp comes out of saturation, no matter what is loading the coil. I think there is something wrong, either with the transistors or with the RC.



No, your mains frequency doesn't matter so 346 Hz is fine.

Jafar, I have not built this circuit so I will not be much help. But I will say it is a more difficult circuit than other projects and, if this is your first attempt at building a metal detector, there are easier options. For now, it is possible to proceed without using the T2 clamp, just leave R13 out and increase the Signal pot to get the right timing. You may need to increase C4 to get enough delay.

Thanks, Carl. I’m currently in the city — tonight I’ll replace the transistors and share the results.
I’ll also test the RC circuit and let you know what I find.
I know you haven’t worked directly on this circuit, but if you could roughly explain what the output of each IC should look like, I’ll try to fix them with effort and time — or I’ll ask other friends for help.
Carl, I’m not using this device for coin detection — I chose it because I want deep detection capability.

[Carl-NC]

your circuit is overloading by DC component on preamp output. Overloading Protection circuit works, T2 is open.
in normal mode Overloading Protection circuit must be turn off.

T2 and T3 are not for overload protection, they clamp the RC reset on D2A so that the timing delay begins when the preamp comes out of saturation, no matter what is loading the coil. I think there is something wrong, either with the transistors or with the RC.

Our mains electricity is 50 Hz. My device is currently operating at 346 Hz — should I change it?"

No, your mains frequency doesn't matter so 346 Hz is fine.

Jafar, I have not built this circuit so I will not be much help. But I will say it is a more difficult circuit than other projects and, if this is your first attempt at building a metal detector, there are easier options. For now, it is possible to proceed without using the T2 clamp, just leave R13 out and increase the Signal pot to get the right timing. You may need to increase C4 to get enough delay.

[Jafar]

Our mains electricity is 50 Hz. My device is currently operating at 346 Hz — should I change it?"

[Jafar]

abnormal signal. huge. it is not limited by the diodes VD3 VD4. 'delta pulse' must be approx 2V P-P. not more.

I checked VD3 and VD4 on the board — they are functioning properly."

[Jafar]

D2 and D3 completely stop working.

your circuit is overloading by DC component on preamp output. Overloading Protection circuit works, T2 is open.
in normal mode Overloading Protection circuit must be turn off.

Do you have any recommendations for resolving this issue? Which parts should I inspect?

[kt315]

D2 and D3 completely stop working.

your circuit is overloading by DC component on preamp output. Overloading Protection circuit works, T2 is open.
in normal mode Overloading Protection circuit must be turn off.

[Jafar]

abnormal signal. huge. it is not limited by the diodes VD3 VD4. 'delta pulse' must be approx 2V P-P. not more.

The device is connected to the coil.
If you think the diodes might be faulty, I’ll replace them.

[Jafar]

something wrong in your pcb but i can not help. you have to check all resistors and voltage polarity of supply. this part is working perfectly, no need to delete R13.
give an o-scope pic D4 pin 6 again.

I didn’t remove R13 — I just disconnected one of its legs, as suggested by Karl. When it was disconnected, D2 and D3 started working.
But when the part of the circuit with T2 and T3 is connected to pin 2 of D2, D2 and D3 completely stop working.

If you need photos of any part of the circuit, I can check and send them to you.

[kt315]

abnormal signal. huge. it is not limited by the diodes VD3 VD4. 'delta pulse' must be approx 2V P-P. not more.

[Jafar]

I used a 22.5-ohm resistor in series with the coil for damping.

[kt315]

reference point is GND, output of D6A. THE level you have to do the measuring vs.

[Jafar]

something wrong in your pcb but i can not help. you have to check all resistors and voltage polarity of supply. this part is working perfectly, no need to delete R13.
give an o-scope pic D4 pin 6 again.