Need help optimizing dual field

[bklein]

I need some help tuning up a Dual Field. I wasn’t sure I’d I need to have a damping resistor across the coil at the pcb. If too low value (390 ohms) it kills the outer coil. I tried a resistor jig and as you adjust it the signal saturates. You don’t see the usual random pulses in the waveform. It looks good but it advances out in time as you tweak it. So I don’t know where to best set it. The attached photo shows where it is now, but a surf pi pro is quieter and has better target depth. Value is 600 ohms.

[ivconic]

I see a young naked girl on attached photo!?

...
Put the damping resistor at coil itself, closest possible to coil leads.

[bklein]

I see a young naked girl on attached photo!?

...
Put the damping resistor at coil itself, closest possible to coil leads.

Fig tree that refuses to grow more….

I can’t put a resistor at the coil as it would likely cause trouble from water intrusion. This is a production machine so is the resistor in the coil already?

[Carl-NC]

Your attachments didn't come through.

There is a damping resistor inside the coil but only for the inner coil. There is also an overall damping resistor on the main board. The traditional value (non-DF) is 360Ω but (as I recall) the SMPI coil inductance is somewhat low at around 250uH. Even then, 360Ω sounds too low. I don't know what the DF coil specs are or what damping resistor should be used.

[ivconic]

Your attachments didn't come through.

There is a damping resistor inside the coil but only for the inner coil. There is also an overall damping resistor on the main board. The traditional value (non-DF) is 360Ω but (as I recall) the SMPI coil inductance is somewhat low at around 250uH. Even then, 360Ω sounds too low. I don't know what the DF coil specs are or what damping resistor should be used.

Oh so!
Problem, problem!
...
Fig tree that refuses to grow more….

No use from growing up!



What you gonna do now?
You have indeed delicate problem there!

[bklein]

I took a screen capture as it was of a short video and only way to get a screen shot without aliasing causing a dark diagonal band across the image. So hopefully no attachments.

With the 600 ohms it is as seen in the photo - about 15uS saturation. If I decrease the value this saturation lengthens. No other artifacts unless maybe preamp offset is then needing adjustment. So I don’t know if the 15uS is optimum. It could go to 10us for example but outer coil starts to be dead.

[Carl-NC]

it is as seen in the photo

This is what I see in the first post:


Photos aren't showing.

In any case, with the DF coil the overall damping R is related to the internal damping R (for the inner coil) so I assume there is a limited range of values for which the coil will work reasonably well. Question: are you trying to adapt a DF coil to the SMPI, or is this an SMPI-DF you are trying to optimize?

[bklein]

Stock DF detector. I have to check if there is a resistor on the board already.
There was a 390 ohm across the coil pins solder side I put 5 years ago. Now I’m revisiting this as the outer coil seemed dead.

[ivconic]

...This is what I see in the first post:...

You don't see a young naked girl on attached photo!?


Forum engine is playing games with us!

[bklein]

No, but thanks for increasing the views.

[bklein]

Carl, your memory is still great - 360 ohms across the coil via resistor on the board. I’ll take it out and see if then I get something with the resistance jig that is reasonable and brings this unit back to previous glory.

[ivconic]

No, but thanks for increasing the views.

At your service Sir!

[ivconic]

Carl see this: https://www.geotech1.com/forums/foru...ash#post414624

The same problem happened to me on that topic, I posted 6 photos and George can see only last 2 of those, while I can still see all 6 of them???
bklein I am sorry for making jokes, obviously you can still see your posted picture, while Carl and I can't.
Forum engine is gone wild.
Carl please see what is going on.

[Carl-NC]

Barry, I just saw the pic on Steve's forum. I assume for that photo you had the pulse delay cranked up, it appears to be 25us. Saturation ends at 15us which is the best you can do for pulse delay, but the coil may be overdamped. So there is hope that if you increase the damping R you may be able to reduce the sample delay. Two other options for a lower delay are to replace the preamp with a faster recovery opamp, and to increase the clamp resistor R41.

Question: Has this unit ever performed as well as the SMPI-Pro?

[bklein]

I bought the DF used, the pi pro seems better depth and less chatter.

I removed the damping resistor that was stock 360 ohms on the board. Adjusting damping resistor jig on DF coil input. Left at 278 ohms. Is this the correct setting? Now adjust sample pulse to start just after this (once I solder in a fixed resistance of about this resistance)?

https://youtu.be/m1OxyTNfppM?si=docYbCCWpOKYO2jA