Whites 6000 serie 2 with 9.50 coil???

[Malospelos]

Good morning, thank you for your time.

I tried the original 8" coil and the 12" DD coil.

With the 8" the threshold tends to disappear after a while.

With the 12DD coil the threshold tends to increase in the sound.

I also saw that there are two CA3140s, I'm going to change the one on the threshold side, obviously the other one is used for a depth gauge from what I've been able to see.


Thank you all for your help, much appreciated.

[dbanner]

It is normal for the threshold to slowly drift over time. That's the whole point of having a retune button. Also for when switching between modes.

What is abnormal is when the detector cannot maintain a threshold, and constantly drifts out in quick time. This would be indicative of faulty component(s).

[Malospelos]

Thank you for everything, I think it is already within the normal limits, I did not know that the threshold would be lost little by little and I would have to press again, change the capacitors and the integrated one has helped to improve considerably, the truth is that with its coil it is the best way to maintain the threshold, I have tried with a white 6000 Di pro coil and a 12" DD coil, in the end the one that works best is with an 8" 6DB which is the original.

Thanks for all your help.

[Carl-NC]

You can possibly improve it further if you increase the cap (10u or even 100u), replace the opamp with a CMOS type, and find a better switch. You could also try the differential retune circuit from ITMD3:

[Malospelos]

Hi Carl, thank you very much, I'm not ready to try the differential retuning circuit yet, I'll look into it. i will study in the book, I have to be translating and it is even more difficult.

What capacitor are you referring to C23 (1uF)?

And the op amp, which one do you think might be good? Could one be changed for the other or would I have to modify something else? My knowledge is not very good and I don't want to be left without my Whites 6000 .

thanks for your help,

[Carl-NC]

Yes, C23. I like to use the TLV271 CMOS opamp, Ib=1pA which is 10x better than the CA3140. An even better opamp for this function is the LMC6081 which is 0.01pA, but you need a better FET switch first. The opamp can swap directly, but these are SMT so you need to mount them on a thru-hole converter board.

[Malospelos]

Good afternoon, thank you very much for your answers, they have been very helpful.
Do you think I should change both? Or just U8?
When you say FET switch, do you mean transistor? Sorry for the question but all this is new to me, little by little I am trying to understand.
In short, should I change the operational one for a TLV271 or a LMC6081, the C23 capacitor for 10u or 100u and also the transistor? Or would those two things be enough?

With this improvement, what should I achieve? A more stable threshold?

Thank you very much and happy Sunday

[Carl-NC]

The threshold level is held by a sample-and-hold circuit, where Q6 is the sampling switch and C23 holds the threshold voltage. But it can droop with time if there are any leakage currents. Leakage currents can come from Q6, U8, or the capacitor itself. The capacitor voltage droop is V_D = I_L/C₂₃*t where t is time. You can decrease I_L (leakage current) or increase C₂₃ or both. Increasing C23 should help no matter what, but be sure to use a high-quality film capacitor that has low leakage. Decreasing I_L should start with the worst offender. Datasheets suggest Q6 is 1000pA and U8 is 50pA so I would focus first on Q6. This means looking at datasheet leakage currents for other N-channel JFETs and finding something better. But, again, with age either of these components could be way out of spec.

[Malospelos]

Good morning, thanks again for all the help received.

I am learning a lot, I will return the help on the topics I know.

Good morning, I have been looking at compatible transistors and I have asked the "IA", according to her the transistor with similar characteristics and an improvement in gain and transconductance is the BF245 A/B/C or the 2N5457.
He gave me the following:

BF-245
V_DSS:30V
I_D:MAX 50mA
V_Gs(off):-0.5V to -3.0 V

J310 (BETTER TRANSDUCTANCE)
V_DSS: 25V
I_D: MAX 60mA
V_GS(off) -0.5 to -4.0V
g_m: 4500 uS(better than PN4302)

2N5457 (MINIMUM LEAKAGE CURRENT)
V_DSS:25V
I_D :5-15 mA
V_Gs(off):-0.5 a -6.0V

What do you think?