Falcon MD20

[homefire]

Why can't people just build something without screwing around with it ?

[Goaty]

@Goaty,,,,,It's not correct,,,I forgot to reverse it with it beng SMD,,,,I will correct it and and re-post,,,I was drawing it as if it was all Through-hole,,silly mistake I know,,I'm glad I didn't build it yet,,,lol,,,Sorry for posting the above Layout. I think the Layout just needs to 'Flipped' horizontally but I'll check before re-posting it again,,,Regards,,,Marty

OK Marty
Thanks for that.
It will be a while before I can build it as I am still playing with several versions of the Falcon and have a new Mirage to build as well!

[MartyJ1963]

Hiya Goaty,,,,,Me too!,,I'm just waiting for my 2 x new Mirage pcb's to arrivefrom Elliot also,,,Looking forward to building them,,the Mirage is my favourite detector out of all the detectors I have,,love it to bits! Regards,,,Marty

[Goaty]

Over the last week I've had the opportunity to play with several MD20s. From one that is about 3 years old to a current production one. The one I've been playing with and a current one are the same. I replaced all my ceramic caps with poly ones but I've noticed the two in the front end are already poly. I've concluded that these are probably the critical ones and are in there for stability. Between mine and a current one there is little difference in thermal stability whereas the older one drifts quite a bit. Also the older probe and the newest ones are not compatible which tells me they changed the inductance on the probe. With my cheap LC meter I measured about 300uh on the current ones. The detection distance on a 1 grain piece of gold between the newest and older model is about 50%. I think they have improved the basic design and is probably as good as it get without a redesign. For what I use the Falcon for (detecting gold inside rocks not visible on the outside) I think this is an invaluable tool.

I've been trying to make sense of the plethora of MD20 circuits presented in this thread and would like to see how they differ from an "original" board as shown in jtz54's photos.

I would greatly appreciate if anyone could post some oblique shots of this board which clearly show the component values used.

[dbanner]

This layout seems to have been copied from original PCB, just rotate the image 180 degrees, there it is. I can't say much about the component values listed though, they seem to be directly copied from the original.

[Altra]

I've been trying to make sense of the plethora of MD20 circuits presented in this thread and would like to see how they differ from an "original" board as shown in jtz54's photos.

I would greatly appreciate if anyone could post some oblique shots of this board which clearly show the component values used.

Hi Goaty,

I have an original MD20. One thing I have noticed on the forum about the reworked circuit diagrams and pcb's is someone decided that the second 9v battery could be eliminated. The original designer put a second battery there for a reason. The MD20 oscillator needs a super stable voltage. This is because the circuit detects the peak voltage. When metal comes close to the inductor the oscillator voltage increases or decreases. With one battery, a metal is detected the led and speaker go on. This in turn loads the oscillator voltage down and is detected. With this feedback the sensitivity now has to be lowered to keep it stable. By powering the audio/LED with a separate battery it isolates the affects on the oscillator allowing greater sensitivity. Maybe the second battery could be eliminated with careful regulator design?

[homefire]

It would seem it's IMPOSSIBLE for these people to replicate a working circuit without altering some damn thing. I would love to see photo's , schematic and BOM of the original MD20 circuit. I have what is believed to be the original schematic.

Do you have any information on this original ?

[Goaty]

This layout seems to have been copied from original PCB, just rotate the image 180 degrees, there it is.

I have built a test board using Walkman's layout which seems to be a re-vamped version of the one above.
It works but I feel that it could be better and was considering the two battery approach as suggested by Altra - just have not had the time to get to it yet.
Dom44 used a 5v reg in his designs to stabilise the oscillator power and isolate the audio section.
Trying to tune up the board is no easy task, it's a bit like a dog chasing it's tail with so many variables which all interact.
It seems like the original designers have made a few mods along the way too as it appears that a couple of the trim pots which appear on some of the circuit diagrams have been replace by fixed resistors.
To date, I have built versions of Dom44's and Walkman's designs and another just using the front-end of Dom's driving a small piezo buzzer - all work but need tweaking.

[Goaty]

It would seem it's IMPOSSIBLE for these people to replicate a working circuit without altering some damn thing. I would love to see photo's , schematic and BOM of the original MD20 circuit. I have what is believed to be the original schematic.

Do you have any information on this original ?

I agree and understand your frustration but it is a fairly old design and could probably be improved upon - I am looking forward to trying Dom's stabilised oscillator design

[dbanner]

I agree that the power supply loading can definitely have adverse effects on the circuit. But this seems not to be the main problem when cloning this ridiculously simple circuit. I think the main problem is the construction of the search coil itself. It must be precisely replicated as akin to the original parameters. Not to mention a long probe between the main circuit and the coil. This can be problematic with drift.
If the original works a gem, then it stands to reason that a perfectly replicated clone should perform just as well as the original. So the problem ain't the circuit, its the builders who don.t pay attention to the finest of details.

[Goaty]

I think the main problem is the construction of the search coil itself. It must be precisely replicated as akin to the original parameters. Not to mention a long probe between the main circuit and the coil.

A good point. I noticed that the original uses RG58C/U coax for the cable whereas I have just used 2 core shielded mic cable of unknown impedance. My coil is around 50 turns of enamelled Cu wire wound between 2 PCB Cu clad disks on a 50mm ferrite rod & wrapped with a gapped turn of Cu tape soldered at the edges to the Cu on the PCB disks. Oh! and I stabilised the turns with nail varnish. Before sealing the coil up I removed turns until it oscillated at 300kHz.

Incidentally, I did notice that some of the circuits described used a 1n8 cap in the tank circuit and others used 2n2. This would affect feedback and the resonance frequency - not sure what it would do to the sensitivity of the device.

As you say, a simple circuit, but lots of interacting variables.

[dbanner]

Yes, seems like you have done quite a lot of work on this one. Some time back, maybe four or five years ago, I spent a lot of time experimenting with some pinpointer circuits I found on the forum. They all suffered from drift, and always required tinkering with sensitivity pot, always having to retune on a regular basis. It just wouldn't hold a threshold for long, none of these simple designs are self adjusting threshold. Unless you are into a specific type of prospecting, I don't really see the value in messing around too much with obsolete circuits, when you can get an off the shelf pinpointer for cheap price nowadays.
The best way to clone a device is to have the original in your possession, that way you can really see what the deal is with the circuitry. Some devices are rather simple but there are some hidden secrets which make the performance outstanding, just a single little detail gone unnoticed can make all the difference, such as the choice of type of capacitors, cabling between coil and circuit, the layout with stray capacitances on main board, sheilding etc. The ferrite core used on the original also could be very useful to know precisely which ferrite.

[Altra]

It would seem it's IMPOSSIBLE for these people to replicate a working circuit without altering some damn thing. I would love to see photo's , schematic and BOM of the original MD20 circuit. I have what is believed to be the original schematic.

Do you have any information on this original ?

I'll tear it apart and take some photos this week.

PS: I was wrong. I actually have an MD 8 still works the same except no 555 timer

[dbanner]

I'll tear it apart and take some photos this week.

PS: I was wrong. I actually have an MD 8 still works the same except no 555 timer

If you put photos here Altra, I'll definitely build the thing.

[Altra]

If you put photos here Altra, I'll definitely build the thing.

I opened the probe it's has a small diamond wraped coil on a 2.5"x 3/8" powered iron rod. Typical AM loopstick antenna type. The coil measures 150uh and about 2.2 ohms with cable . Cable is Belden RG 58 A/U white. The coil was shielded with thin aluminum foil connected to the braided shield on the cable.

Dbanner if you draw a schematic I'll remove the trimpots and measure their values plus where they are set

Attached some photos of the coil and pcb