Sniffer XR-71 help

[kt315]

neutral part of the pre amp output signal is zero, that i do not see in your pics.

[kt315]

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.

[Jafar]

meant you have a trouble in this circuit. check T2 T3 and pinouts of the transistors. you have be confident one is PNP second is NPN and they sure soldered in their places right.
FIRST STROBE is Tr4 output.

I’ve checked the transistors T2 and T3 — there’s no mistake in their pinouts or types.
But as long as R13 is connected, I don’t see any signal from D2 and D3.
Could you please guide me?

[kt315]

follow this pic

[kt315]

meant you have a trouble in this circuit. check T2 T3 and pinouts of the transistors. you have be confident one is PNP second is NPN and they sure soldered in their places right.
FIRST STROBE is Tr4 output.

[Jafar]

R13, T2, T3 is a part doing overloading protection. you can see its work just with a coil, with a metalic near it you will move it to the coil and back, look how FIRST STROBE (Tr4) will move at right side from some moment, on your scope, D1B-out as a reference.

without coil and without metal piece you will see nothing.

Thanks for your guidance.
When I disconnect R13, D2 and D3 work, but when it's connected, they don't.
To check what you mentioned, should I connect R13 or leave it disconnected?
And where exactly should I place the second probe to observe the FIRST STROBE in the circuit?

[kt315]

R13, T2, T3 is a part doing overloading protection. you can see its work just with a coil, with a metalic near it you will move it to the coil and back, look how FIRST STROBE (Tr4) will move at right side from some moment, on your scope, D1B-out as a reference.

without coil and without metal piece you will see nothing.

[Jafar]

The signal from T2 is what triggers the CD4538 and it may not be working correctly. Next thing I would do is look at the output signal of D4 and see if it is behaving as expected.

Hi Carl,
I checked D4 (LF357N) and took photos in two conditions — one with R13 disconnected and the other with it connected. I captured the signal from pin 6 of D4 in both cases.


R13 not connect


R13 connect

R13 not connect & coil connect

R13 connect & coil connect

[Carl-NC]

The signal from T2 is what triggers the CD4538 and it may not be working correctly. Next thing I would do is look at the output signal of D4 and see if it is behaving as expected.

[Jafar]

First, KT is right, when you measure the pulses on D2 and D3 you need to use D1B-out as a reference so you can see how the delays vary. You have a 2-channel oscope, I hope it came with 2 probes.

But it looks like the pulses on D2 and D3 do not have enough voltage. Measure the voltage from +E to -E, it should be 10.5-10.8V. I assume it is because the D1 pulse voltage looks OK. You could replace VD8 and VD9 with, say, 1N4001 diodes but if the 4148's are working just leave them. It's possible the reset-hold from transistor T2 is not working right. Try disconnecting T2-pin1 (or, if easier, pull up one leg of R13) to disable T2, then see if the pulses start working.

You cannot use the 74HCT4538 with the 10.5V supply. If you reduce +E to 5V (just for D2 and D3) then the pulses will not have enough amplitude to drive D9 and D10. Then you would have to start redesigning the whole circuit to make it work.

I assume there is another schematic for a micro-driven display. You do not need it for the main circuit to work.

Hi Carl,
I disconnected resistor R13, and now D2 and D3 are working — thank you so much!

What should I do next?

[Jafar]

First, KT is right, when you measure the pulses on D2 and D3 you need to use D1B-out as a reference so you can see how the delays vary. You have a 2-channel oscope, I hope it came with 2 probes.

But it looks like the pulses on D2 and D3 do not have enough voltage. Measure the voltage from +E to -E, it should be 10.5-10.8V. I assume it is because the D1 pulse voltage looks OK. You could replace VD8 and VD9 with, say, 1N4001 diodes but if the 4148's are working just leave them. It's possible the reset-hold from transistor T2 is not working right. Try disconnecting T2-pin1 (or, if easier, pull up one leg of R13) to disable T2, then see if the pulses start working.

You cannot use the 74HCT4538 with the 10.5V supply. If you reduce +E to 5V (just for D2 and D3) then the pulses will not have enough amplitude to drive D9 and D10. Then you would have to start redesigning the whole circuit to make it work.

I assume there is another schematic for a micro-driven display. You do not need it for the main circuit to work.

Hi,
I replaced the VD8 and VD9 diodes with 1N4007 because the original ones were burnt.
I measured the voltage between E+ and E−, and it was 10.12V. My battery is currently charged to 11.54V, so I’ll test again when it reaches 12V.
I haven’t yet tried the suggestion regarding T2, but I’ll let you know once I do.

[Carl-NC]

First, KT is right, when you measure the pulses on D2 and D3 you need to use D1B-out as a reference so you can see how the delays vary. You have a 2-channel oscope, I hope it came with 2 probes.

But it looks like the pulses on D2 and D3 do not have enough voltage. Measure the voltage from +E to -E, it should be 10.5-10.8V. I assume it is because the D1 pulse voltage looks OK. You could replace VD8 and VD9 with, say, 1N4001 diodes but if the 4148's are working just leave them. It's possible the reset-hold from transistor T2 is not working right. Try disconnecting T2-pin1 (or, if easier, pull up one leg of R13) to disable T2, then see if the pulses start working.

You cannot use the 74HCT4538 with the 10.5V supply. If you reduce +E to 5V (just for D2 and D3) then the pulses will not have enough amplitude to drive D9 and D10. Then you would have to start redesigning the whole circuit to make it work.

I assume there is another schematic for a micro-driven display. You do not need it for the main circuit to work.

[kt315]

I DID NOT SUGGEST YOU TO REPLACE THE DIODES. i do that JUST IN MY BOARDS.

[Jafar]

in my design i changed VD8 VD9 on 1N400x type. 1N4148's are burnt in customers bought boards. chinese made 1N4148's are poor on current.
do not desolder the diodes from board, check them by a tester on DIODE CHECK scale right in board.

If I replace those two diodes you mentioned with 1N400x types, do you think D2 and D3 will start working, or is it unrelated?
My D2 and D3 are CD4538BE ICs — do you have any thoughts on what might be preventing them from operating?

[kt315]

in my design i changed VD8 VD9 on 1N400x type. 1N4148's are burnt in customers bought boards. chinese made 1N4148's are poor on current.
do not desolder the diodes from board, check them by a tester on DIODE CHECK scale right in board.