White 3900D

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But then there is this quote by member "Geo" from the same thread:

"Before years i checked the coil of MD5002 and it was as Carl says. Rx coil was tuned at 3x the Tx frequency."
END QUOTE

So there is some posibility that the 6B coil is so tuned to work this way....maybe.
Interestingly, for the Whites detectors which came with the 6B coil, no other Whites coils would work correctly on them!!! Clue??

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Now I am more than curious to see what the oscilloscope has to say about this circuit, the 3900. and similar circuits which used the 6B coil.


At this point, I strongly suspect that the 6B coil has a feedback resistor/capacitor arrangement between the Tx and Rx within the coil housing in order to set the correct phase at null, even a voltage divider network and feedback for fine nulling. Otherwise I suspect a self made coil with correct Tx and Rx inductance alone, and just the 1nF cap will not work in order to get the circuit to discriminate.
That's probably why sayan1345 and other can't get the circuit to discriminate!! Because their self-made coils, although nulled, are not correctly PHASED.

[kt315]

i think smit has to say something. 3900 was released in 2008 ye. 11 years passed by you...
so thats ansient issue http://www.md4u.ru/viewtopic.php?f=61&t=2436&hilit=3900
i have not access in Zakrzhevski' forum, so can not give you pics from it.

[dbanner]

i think smit has to say something. 3900 was released in 2008 ye. 11 years passed by you...
so thats ansient issue http://www.md4u.ru/viewtopic.php?f=61&t=2436&hilit=3900
i have not access in Zakrzhevski' forum, so can not give you pics from it.

Thanks, this was a big help.
Seems someone had good success on Russian forum with the 3900D.
He said he changed C4 from 68nF to 6.8nF. (maybe there is mistake in schematic, and C4 should be 6.8nF)
Looks as well R18 and R19 might be 100K resistors instead of 200K. A look at the original pcb photos, it is difficult to be sure.

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Unfortunately, there is no resolution on the Russian forum about the Rx/Tx setup.
This isn't going to be hard to figure out though. I can simulate the front end and see what's required at the switching gate.
Then i'll know the phase required at the null. At this point I suspect near to 180 degress.

[dbanner]

Although this circuit is ancient, for me it is more about figuring out the internal scheme construction of the 6B coil.

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From more research, it seems the Bluemax 256 coil is interchangable on machines using the 6B coil. That would mean you could use a bluemax 256 coil on a 3900D machine. Getting closer to figuring it all out.
Whites have such a wide range of coils! It sometimes get a little confusing.

The bluemax 256 is a "widescan" coil, which is another name for DD(double D). Now all I have to do is get the details of the Bluemax 256 coil.

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Now I am thinking phase shift is nearer to 90 degrees, that's more consitent and seems likely.
Soon I will know.

[Artyom]

Now I am thinking phase shift is nearer to 90 degrees, that's more consitent and seems likely.
Soon I will know.

Everything is messed up with this detector. Good luck to you!

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Everything is messed up with this detector. Good luck to you!

Yes, all is messed up at the moment, but soon it will be good. Then I will show Youtube video of working unit, and publish full details of CORRECT schematic and double-D coil construction.

[sayan1345]

My friend opened the coil , The coil of receiver has a 220n capacitor
Now you can calculate the frequency of the receiver

[Artyom]

My friend opened the coil , The coil of receiver has a 220n capacitor
Now you can calculate the frequency of the receiver

With an inductance of 16.2 mH and a capacitance of 220nf the frequency is 2.66 kHz

[dbanner]

These lowball numbers are strange, I was expecting Rx resonance to be a lot closer to 4.6-4.9kHz.

So I must ask sayan, what coil did your friend open up? Did he take photos of internals? Why people are so inept when it comes to these things, I don't know.

First rule of reverse engineering is to take HI RESOLUTION PICS of the subject matter.
Second rule, take proper measurements and draw schematic of the coil arrangement, showing how coils are connected, including the shield wires.

Only then can you be sure to convince anybody of correct information.

Ask your friend for photos, after all, how could someone open up a factory coil and be stupid not to take good photos before closing???

[MartyJ1963]

Hiya DB,,,,,I may be wrong but should there be a jumper across these two pads? Regards,,,Marty

[dbanner]

Hiya DB,,,,,I may be wrong but should there be a jumper across these two pads? Regards,,,Marty

Yes, also pin 3 of the nearby chip goes to wiper of discrim pot. I updated the layout. Thanks for pointing that out.