Falcon MD20

[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

[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.

[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 ?

[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?

[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.

[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.

[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]

@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!

[homefire]

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

[Goaty]

Nice work Dom - your efforts are appreciated!

[dom44]

have been running the circuit on and off quite a while, mine is locked at 277.7khz with 60 turns on a 50mm fer-rite rod, and is very amplitude sensitive to metal objects.

[dom44]

I thought i would play around with the circuit a bit more been to busy with other things lately, just built this test circuit to improve the oscillator using current mirrors to give thermal stability to the circuit, so far looks good built a test circuit works as expected. any suggestion would be appreciated.

[Goaty]

Yes, I would appreciate a copy as well. Which version is this one ?

I am hoping it is this one -



I have marked the fixes required (Yellow - links, red - cut tracks) but will need re-checking.

There don't seem to be any component values around the 555 audio section but these could be gleened from other circuits in this topic.
This one has a LED indicator which is quite useful when tuning the front end.

It would appear that the original MD20 board had extra trimpots which were later replaced with fixed resistors - all adding to the confusion around which version to build!

[homefire]

Yes, I would appreciate a copy as well. Which version is this one ?

[Goaty]

Please do not laugh at me, just wiring test
In addition my laser printer is not too good, therefore routes are thicker diameter.
For bernte_one most likely only weekend I will have some time working[ATTACH]33718[/ATTACH]

Hi Noro Could you please post just the copper pattern as a .jpg or pdf as I can not open Sprint files in Linux
Thanks