DFX Spectra DD coils

[stefan-s]

Hello,

Im trying building a DD coil for Whites spectra/dfx etc

The winding it self are very close to original but the coil is not working correctly.

I find out (on this forum) that there are resistors and capacitors connected within the coil as well. (i did not have them in the coil that i build.)
I found this picture from the attachment
https://www.geotech1.com/forums/file...1&d=1446770951

But this is a concentric coil, im hoping that someone can help me out with how to add the capacitors and resistors on my DD coil.

Thanks in advanced

[Carl-NC]

I believe those components were needed to dial in the null at 15kHz. You should not need them for DD.

[stefan-s]

I found this one on the russian forum they also seems to be in the 10inch DD coil

[kt315]

Параметры катушки DFX кольцо 25см:

TX - L=538мкГн R=2.63 Ом
RX - L=13.75мГн R=35.6 Ом
------
1.Кольцо D=300 мм
TR: обмотка ?290 мм, R=3.2 ом, L=609 мкГн. Расчетное число витков N=30, провод ?0.75 мм
RC: обмотка ?139 мм, R=35.5 ом, L=14.25 мГн. Расчетное число витков N=212, провод ?0.21 мм

2. DD 12х15"
TR: обмотка ?900 мм (эквивалентный диаметр), R=1.4 ом, L=595 мкГн. Расчетное число витков N=16, провод ?0.81 мм
RC: обмотка ?900 мм (эквивалентный диаметр), R=37.2 ом, L=13.8 мГн. Расчетное число витков N=77, провод ?0.23 мм
----
ТКС отрицательный - при нагреве пальцем сопротивление падает примерно на 0.5 Ом

in russian, but you can translate by google.

[kt315]

[stefan-s]

Thanks for the reply, when i read the schematic above correct this one is for the concentric coil. How should i apply this to a DD coil?

Add a capacitor from 38Pf between RX + and RX -? What about the TX side?

[Carl-NC]

The 38pF cap on the RX coil is probably to suppress fast transients. It should be less necessary on a DD coil but you can try it.
On the TX side there is a thermistor (PTC, probably 3900ppm/°C) to cancel the temperature drift of the coil. I don't know the value. It is mostly needed in manual ground balance mode, autotrack should otherwise deal with coil drift.
You never stated what the problem was with the coil. These extra components are for 2nd order effects and I would not expect them to make a non-working coil start working.

[stefan-s]

The issue with my coil is that i don't get a properly ground balance. When i move the coil up and down after i made ground balance im getting signal effect from the ground.

The reaction speed and depth are fine.

[Carl-NC]

You can try adding the cap. Ground signals come in as square waves so it's possible that overshoot is causing the problem. It's also possibly a lift-off effect; usually DD coils are better about this than concentric but it could depend on the geometry of the coils. Can you post a pic of the windings?

[stefan-s]

Im not able to make pictures because i already put resin in the shell. But i think i can mill out some resin and ad the capacitor when is not work i will make some new windings and place some photo's

[Carl-NC]

Just post a sketch of the windings.

[kt315]

The 38pF cap on the RX coil is probably to suppress fast transients.
---
at first picture 150 pF, 38 pF is sure mistake, very low.

On the TX side there is a thermistor (PTC, probably 3900ppm/°C) to cancel the temperature drift of the coil. I don't know the value.
---
at first picture its value noted as 2.7 Ohm., in room temperature. negative temperature Coefficient electric resistance

[Carl-NC]

I'm sorry, the copper wire is PTC, the thermistor needs to be NTC.

[stefan-s]

Just post a sketch of the windings.

[stefan-s]

You were correct the resistor and capacitor didn't make any difference. I was thinking could a mistake in the CX part of the coil make the problems of the ground effect? Because i remember i had some trouble with soldering the wires. Maybe one of the wires or soldering connections is broken.