chance pi build

[baum7154]

I posted photos in threads #161 and #162 of this thread yesterday and in the photo 5.JPG of the new amp board installed in CHANCE you can also see the location of the 2ea blue 470pf caps in series from R21 to ground on the motherboard that forms the last 677 kHz low pass filter.

Dan

[baum7154]

[QUOTE=Tepco;190006]Few comments on this:Effects of 2 diodes on first amp feedback and 14 on second one (omit?) I don’t have 2x OP37 at hand to reconstruct this, just watching schematics.


Regarding the 14 -1N4148 diodes in series in the feedback loop of the second stage(2 Stage Amp.jpg)forming a 10 volt limiter to keep the amp out of saturation the question of 'Why didn't I just use a Zener diode' arose. I did try a Zener first and found that the wave form of the output suffered. Looking at the capacitance of the zener I found it was very high and that isn't desirable in this application. So then I put a couple of 1N4148s in series with it to lower the capacitance but the waveform at the output still suffered. So it ended up with the 1N4148 diodes in series. I like the speed and low capacitance of the 1N4148s made even lower when they are in series. The low capacitance is easily included in the Low Pass filter design.

Dan

[baum7154]

Increased the first stage gain today to 22 so with the second stage at 28 the total gain is now 616. The original single stage amp was a gain of 470 so this is an increase of 31%. One of the standard gold targets I use is a 4.5 grain 14kt gold head for mounting a 4mm stone. The detector now sees this at 4 inches in air. The other target I use is a 1/4" X 1/4" square of aluminum pop can and the detector now sees this at over 4" in air. Previously the detector saw these two targets at 2 to 2 1/4". These are both fast decaying targets and the speed of the new amp appears to help make them detectable.

I have noticed that metal discrimination seems to be better with objects in the soil as opposed to air tests. There is more stability in the VDI display with the coil near the ground especially with the higher gain amp in place and the audio seems to lock in better as well.

I will do some in soil tests the next time I go to my prospect area.

Dan

[6666]

Hi Dan

I general terms how deep in real ground will the chance go and still give reasonable VDI figures to show iron from other targets ?

[baum7154]

Hi Dan

I general terms how deep in real ground will the chance go and still give reasonable VDI figures to show iron from other targets ?

Since I modded the detector with the 2 stage amp I have not been out in the field with it but I can tell you that with the old 1 stage amp I have had good indications of a piece of 10.8 Grain gold at 4.5 " with a solid indication of gold/aluminum and 5 increments of signal strength. This is with the only size coil I have and that is 8" dia. Just poking around the yard I have recovered a 3/8 " ball bearing at a depth of 4" and a strong indication of Iron. There is also something undefined in my front yard at a depth of 6" and this is strongly indicated as iron. I used a probe to poke through the sod and it seems to be a piece of pipe, but not the water pipe for the house. Really don't want to tear up the front yard to dig it up though. On the gold fields I have detected what I thought was gold only to find a sliver of aluminum foil at more than 5" depth. A common soda can is seen at 20 inches or more with the new amp and the 8" coil. I have dug numerous coins from the front yard at depths from 2 to 5" and they were nickels and pennies all properly indicated. Dug a rusty old thin razor blade that showed as gold instead of Iron at a depth of 4" or so. Dug a .3" X .018" thick flake of iron rust that looked like gold. Also have dug flakes of lead bullets approximately .125 "at a depth of 3" or so. I intend to run some more controlled tests in the coming weeks to determine detection depths on small gold and to define the discrimination better with the new amp. One other thing I have been experimenting with is the best A/D input point using the OP37 offset control and it does have a significant effect on disc and sensitivity. Will post the results as soon as they are completed.

Regards,

Dan

[6666]

With renewed interest in the chance project, I have been having a closer look at the schematic.
Its looks like Q3 is the coil switching fet, but what does Q9 do DAMPING ? it looks to be under the control of T1
sorry for the dumb question.
6666

[Silver Dollar]

It shorts out the flyback. Though looking at the code it's not really switched properly for that...

It might hold the current in the coil for build up during the 16 short pulse mode...

[baum7154]

It shorts out the flyback. Though looking at the code it's not really switched properly for that...

It might hold the current in the coil for build up during the 16 short pulse mode...

-----------------------------------------

Good Morning, Your second statement is correct. There has been much misinterpretation on the function of Q9 performing active damping. According to TEPCO and reviewing the scope traces posted on Andy F CHANCE page, Q9 under the control of T1 does short the coil between the short pulse intervals but the short prevents flyback and allows much of the coil energy to be held. I have to go back and look closely at the scope traces and T1 timing to see the short duration between pulses.

Dan

[baum7154]

6666 & Silver Dollar do you have your CHANCE built yet? I reread a comment in this thread that Silver Dollar made that you need to get yours going so that you can see how the menus are presented…. I can help with that if needed.

Dan

[baum7154]

With renewed interest in the chance project, I have been having a closer look at the schematic.
Its looks like Q3 is the coil switching fet, but what does Q9 do DAMPING ? it looks to be under the control of T1
sorry for the dumb question.
6666

____________________

On the MD4U Russian forum some outlined the exact part of CHANCE that you did and asked the same question and got the short quote from Andy F below:

Q3 - the power stage, the usual key.
Q9 - controlled shunt.

Short & sweet he calls Q9 function a "SHUNT".

[Silver Dollar]

Mines all built but I haven't programmed it yet. As noted previously in this thread I haven't figured out the
fuse settings for the program I'm using. I've been playing with TGSL and Bandito lately and got a DeltaPulse
board recently too. So Chance has been waiting patiently for me to return to it...

[baum7154]

In the GRAPHITE POWDER COIL SHIELDING' thread some questions have arisen about the 2 Stage Amp modification I did on CHANCE PI. In an effort to avoid 'highjacking' that thread I'll continue the discussion here:

First the description of the problem and what was initially done is in POST 125 of this thread.

The photo of the schematic of the amplifier is in POST 154 of this thread.

Photos of the board and its placement in the detector are in POST 161 of this thread.

'Before mod' and 'After mod' scope trace photos of the input to the A/D are in POST 162 of this thread.

I hope this helps,

Dan

[baum7154]

[QUOTE=baum7154;192658]Hi Dave, Yes a 2 Stage could be added in most cases where faster sampling is needed. An appropriate design for the specific detector would need to be implemented. In my case it was added by interfacing through the single stage IC socket so no significant mother board changes were necessary.

Regards
Dan
Hi Dan
When or if you get some spare time have a ponder over the Barracuda and let me know in your opinion if anything along those lines could be done there for added improvement .

Regards
Dave

--------------------------------------------------------------
Did a quick look at the single stage preamp of the Barracuda. Everyone probably knows it is a non-inverting single stage amp with a gain of 1000. I think this could be replaced with an inverting 1st stage with a gain of about 25 -28, followed by an inverting 2nd stage with a gain of 40 - 36. Does any one have a post of a scope picture of a fast coil decay and minimum possible sample delay? It would be interesting to know how long that amp is in saturation compared to the coil decay. I would go with the OP37 amps in this mod.

Just my thoughts,

Dan

[baum7154]

Or another way would be to keep the Barracuda first stage configuration but make it a gain of 28, add diode limiting and filter in the feedback loop, couple it to a second non-inverting stage with a gain of 36 with diode limiting and filter in the feedback loop. This would keep basic circuit configuration but move it to a small circuit board containing the new 2 stage amp and interfacing through the IC socket or preferably, direct solder it in place of the socket/chip.

Dan

[baum7154]

I need the schematic of the original non-motion Barracuda that uses the TIP127. Can someone please post the link?

Dan