Falcon MD20

[parun14]

Need halp for PCB !

[Pacliff1985]

Here are the values for the pots and trimmers. This is for the MD 8, current model is MD 20. The probe is a 3/8" ferrite core probably 20 to 40 turns of AWG 32? These must be good for something, I believe mine is from the 1980's

good job and thank you for your schematics. I've been researching information on this metal detector for quite some time and try to see if I could duplicate it I was looking at the schematic and I was wondering what type the capacitors are NF, UF and so on. maybe it's obvious I'm just missing something somewhere..

Thanks in advance

[Qiaozhi]

good job and thank you for your schematics. I've been researching information on this metal detector for quite some time and try to see if I could duplicate it I was looking at the schematic and I was wondering what type the capacitors are NF, UF and so on. maybe it's obvious I'm just missing something somewhere..

Thanks in advance

I assume you're asking about the capacitors marked "0.01" and ".018".
If not specifically stated, then these values are in uF. For example, "0.01" means "0.01uF". Or in other words, "10nF".
Likewise, ".018" is "18nF".

[Pacliff1985]

that's what I needed to know. Thank you

[jtz54]

I have the Falcon MD20. It has a 555 chip in it for (I'm assuming) the audio output. I really like the unit but it is a bit unstable during use and has to be constantly adjusted. I don't think the auto ground balance pot is that, think it's a threshold adjustment as is the 500K sensitivity pot. I added a 50K ten turn pot in series with the 500K pot and the unit becomes more stable and I'm able to adjust the threshold to a much finer degree. Any other suggestions to improve on the design?

[Mechanic]

HI Jtz,

For a start I would try replacing the ceramic disk capacitors with either SMD np0 caps or through hole polypropylene or polyester caps. Those ceramic disk caps will change their value dramatically with temperature change. The other trick to narrow down exactly where the main drift is occurring would be to heat up the components individually, that way you can narrow down the temperature sensitive areas.

Cheers Mick

[jtz54]

Thanks Mick! I will start with the caps and report back to this thread.

[jtz54]

Finished replacing ceramic caps with poly ones. When comparing this to another unit I'm thinking there's something else wrong in the circuit. While it does detect metal ok, trying to adjust the threshoild is near impossible. On a working one you adjust external pots until you barely hear a tone then when using the probe it sounds off when going toward metal OR sounds off when pulled away from non-metallic minerals like black sand. Although the range is very short, typically under 2", it is very useful in finding hidden gold inside of rocks. So I need to start trying to figure out why mine cannot adjust the threshold (either off or on but can adjust to the edge of just sounding off). Any ideas on troubleshooting or maybe adjusting the trim pots inside?

[jtz54]

Ok. The above issue with threshold has been fixed. The coil wires on connector were swapped (red wire is shield and gray wire center conductor-weird).
The unit seems a bit more stable with a bit more depth but field testing it over the next few days.

[jtz54]

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.

[frankly]

Is anybody currently working on this project? Was anybody able to increase depth?

[jtz54]

After replacing the ceramic caps with poly ones I played around with the tuning. The two pots can be adjusted for better deth (in my case about 50% more) but at the expense of stability. I found it possible to adjust for maximum depth then back off slightly for stability. There can be a wide difference in depth between units with the same piece of gold.

[Noro]

Hi Jtz54 ! or another member of this forum!
I want to building this metal detector but, I do not have
schematic falcon md with CI 555.
Tnx Noro.

[Qiaozhi]

Hi Jtz54 ! or another member of this forum!
I want to building this metal detector but, I do not have
schematic falcon md with CI 555.
Tnx Noro.

This is all I have by way of a schematic. It's a bit fuzzy, but readable.

[kt315]

Noro did mean new version, with NE555.