MPP Beach REV-E

[Qiaozhi]

[ATTACH]46279[/ATTACH]I'm just getting back to the MPP-Beach after final coil potting, and going over the preset adjustments.

I read about the benefits of a 'fast coil'....in Step 6, Adjusting the sample delay on TP6, instructions state 10uS, I can adjust for a minimum of 5uS before it cuts out.... is this a measure of how fast the coil is? and better for detecting than 10uS?

The fact that you can adjust the sample delay down to 5us doesn't tell you anything about the speed of your coil. You need to connect channel 1 of your scope to the TX oscillator, and channel 2 to the preamp output. Trigger the scope on channel 1. The preamp output will first go into saturation (output hits the supply rail) and eventually come out of saturation after a few microseconds. You need to start sampling on the rising edge of the preamp signal.

Also, note that the MPP does not have a ground balance facility, so there's not much point going below a sample delay of 10 to 15us. Although some builders have been somewhat successful using the MPP below 10us delay, they have to sweep slowly and accept the fact that it will react to the ground conditions if the coil is not kept parallel to the ground.

[samsearcher]

Can the MPP be preset adjusted to not react to a piece of ferrite like the TGSL?
Thanks Q, whilst having a probe around the damping resistor flyback pulse, the detector stopped working.....
U1 the 555 timer had failed!
What do I need to do to view the effect of the damping resistor?

[Qiaozhi]

Can the MPP be preset adjusted to not react to a piece of ferrite like the TGSL?

That's a different issue. The TGSL is a VLF design, and the ground balance adjustment moves the GEB sample point slightly away from the zero-crossing to more closely align with real ground conditions. For the TGSL, the ferrite creates a phase shift in the RX signal due to the balanced coil system. The MPP, however, relies on detecting eddy currents flowing in the target. Since the eddy currents in ferrite die away quickly, when compared to more conductive targets, you only have to increase the sample delay in order to ignore the ferrite.

What do I need to do to view the effect of the damping resistor?

Use a damping resistor tool to find the correct value -> https://www.geotech1.com/forums/show...785#post227785
Look at the output of the preamp with a scope.

[samsearcher]

Do you mean TP3 Rx preamp output? I can view that by triggering from TP2, adjusting with the 'damping tool pot' but no real sign of ringing .
If I examine the very top of the flyback pulse voltage spike (across the resistor) I can see a little ripple, but nothing very significant at any setting.
However the detector is working well.

[samsearcher]

I've just looked at my previous post (#99) when building the MPP, and can see what the waveform used to look like . The tool pot adjustment was right at the end of travel, giving a damping resistor value of only 220 Ohms.

[Qiaozhi]

Do you mean TP3 Rx preamp output? I can view that by triggering from TP2, adjusting with the 'damping tool pot' but no real sign of ringing .
If I examine the very top of the flyback pulse voltage spike (across the resistor) I can see a little ripple, but nothing very significant at any setting.
However the detector is working well.

Trigger on TP2, while measuring on TP3.
If you remove the damping resistor completely, you will definitely see the ringing. The damping resistor should adjusted to critically damp the preamp output. If it's under-damped, the signal will overshoot, and if it's over-damped you will lose sensitivity.
http://hyperphysics.phy-astr.gsu.edu/hbase/oscda2.html

[samsearcher]

These are the waveforms on TP3, overdamped, underdamped and OK?

[Qiaozhi]

These are the waveforms on TP3, overdamped, underdamped and OK?
[ATTACH]46350[/ATTACH]
[ATTACH]46351[/ATTACH]
[ATTACH]46352[/ATTACH]

The OK image is still slightly underdamped. When critically damped there should be no overshoot.
However, the detector will be a bit more sensitive with the OK setting, but have less stability in the threshold.

[samsearcher]

Thanks Q, yes the pot is a bit 'jittery' at the extreme end, I will put some fixed resistors in to fine tune.

[tomatos666]

Hello, if i want use 2 coils with ~400?H for TX and RX
what exactly need to modify of the settings and how value must have the resistor on RX (RD)?

[waltr]

Hello, if i want use 2 coils with ~400?H for TX and RX
what exactly need to modify of the settings and how value must have the resistor on RX (RD)?

Do you mean a DD or Concentric coil?

RD? -damping resistor value for RX coil is selected by observing TX off waveform at output of pre-amp on O'scope.
Adjust TX damping R value for best (critical) damping first. Then do same for RX damping R.

What settings are you asking about??
Only thing needed to do is separate the TX & RX coil connections.

[tomatos666]

I will made a DD coil.
Is there some example how to set the damping resistor?
At the moment are only on the TX the 470ohm 2W resistor because i have only a mono coil.

[waltr]

I will made a DD coil.
Is there some example how to set the damping resistor?
At the moment are only on the TX the 470ohm 2W resistor because i have only a mono coil.

Not that I know of specific to the MPP. You just need to solder the RXd resistor from the RX coil connector to 0V.
Better yet, put the RXd resistor inside the coil connector. This way you can simply switch coils since the damping resistors specific to each coil is in that coils's connector.

I built and use a modified Hammer Head with DD & concentric coils which is similar to the MPP. Read through my thread on what I did to use a double coil.
https://www.geotech1.com/forums/show...ake-on-the-HH2

[Suvabard]

So I just got hold of a REV-D board. I would ideally prefer to build the REV-E version. Is it worth adapting the REV-D board? Is it possible? How much work is involved? I will dive into the docs later this weekend.

Also is there a gerber file for the REV-E board? I am tempted to try to make the board myself.

Thank you!

[Suvabard]

So I just got hold of a REV-D board. I would ideally prefer to build the REV-E version. Is it worth adapting the REV-D board? Is it possible? How much work is involved? I will dive into the docs later this weekend.

Also is there a gerber file for the REV-E board? I am tempted to try to make the board myself.

Thank you!

Note: the REV-D PCB I have is currently unpopulated. I see from the following post it may be more complicated than first seems:

https://www.geotech1.com/forums/show...-to-RevD-build