Sniffer XR-71 help

[kt315]

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[Jafar]

Hello, good time. Sorry if my question is unrelated to our previous topic, but I have a question about the clamp circuit used in the Sniffer XR‑71 device. At first, the diodes used in that circuit were 1N4148, but the circuit did not reduce the output going into the preamp to ±1 V. I replaced the diodes with 1N5819 Schottky types to solve the problem, but it still had no effect on the operation. Could you please guide me? Thank you.

[Carl-NC]

Please post a question only once, and in an appropriate thread.

In the case of the XR71, the diodes should clamp the signal to +E (+12V) so the signal at the input to the opamp should be limited to 12V +/- 1V, or 11V-13V.

[pito]

[kt315]

change the diodes back. no such suggestion to change 1N4148s on Shottky. not exist. you have to see 'classical' view on preamp output.

[Jafar]

[Jafar]

green trace is the output of the LF365 at pin 6, and the yellow trace is the input of the LF357 at pin 2.
I still don’t understand what you mean by the ‘classical’ view.

Also, in my new board I’m using a 7812 to keep the voltage fixed at 12 V.”

[Jafar]

---
“At the input of my op‑amp I don’t have anything like that.
My positive voltage is lower than +12 V, but my negative voltage is much higher than −12 V, and sometimes it reaches close to −90 V or even mo

[Jafar]

---

“These two images were taken separately, each with the same probe fixed on the point you mentioned, so that nothing changes between them.”

[pito]

post #66

The diodes are dead , remove them and check them, the voltage over there should be 0.6Vpp, increase R6 to 1k.

[Jafar]

The diodes are dead , remove them and check them, the voltage over there should be 0.6Vpp, increase R6 to 1k.

“I checked the diodes. Even when the diodes were 1N4148, the same voltage shown in the photos was present. Since this diode was made in China and not suitable for the coil’s instantaneous current, I suspected that this might be the reason the clamp circuit wasn’t working. So I replaced it with a Schottky 1N5819 to see if anything would change, but nothing happened. I tested the diodes and they are all OK.”

[pito]

green trace is the output of the LF365 at pin 6,.

you have power supply +/- 12V so the output of the amplifier cannot be bigger than 24 Vpp, and you have over 100V

[pito]

so you check the diodes with an ohmmeter?
show the picture of the diodes area ( the other side pcb ), if diodes are ok then must be error on pcb.

[Jafar]

so you check the diodes with an ohmmeter?
show the picture of the diodes area ( the other side pcb ), if diodes are ok then must be error on pcb.

I haven’t changed resistor R6 to 1 kΩ yet. I’ll change it and let you know.”

[Jafar]

so you check the diodes with an ohmmeter?
show the picture of the diodes area ( the other side pcb ), if diodes are ok then must be error on pcb.

“I checked the diodes with the diode tester and they are all fine. I also inspected the back side of the PCB according to the schematic, and there is no issue in that area.
In your opinion, is there any need to change the values of R5 and R6?
And do you think R6 might need to be moved after the diodes — meaning between the diodes and the preamp?

This issue is really driving me crazy.”