Falcon MD20

[Altra]

It makes me wonder sometimes about people in general. Altra, you knew you had schematic for the MD8 all along and yet you ask me to draw schematic. Why? Possible reasons:
1 you forgot that you had the schematic
2 you failed to recognize that the schematic you posted before was indeed the MD8 schematic
3 #$#%&$%^#@
Now I give you the benefit of the doubt.

1 I did not forget, I wanted verification knowing there were mistakes on the original.

2 The schematic I posted was drawn in 1995 when I posted it 15 years later as favor to the forum. Had no idea it was md2020 or an md8

3 ^%*^%*

4 buy your own, destroy it, and post a perfect schematic

[dbanner]

It makes me wonder sometimes about people in general. Altra, you knew you had schematic for the MD8 all along and yet you ask me to draw schematic. Why? Possible reasons:
1 you forgot that you had the schematic
2 you failed to recognize that the schematic you posted before was indeed the MD8 schematic
3 #$#%&$%^#@
Now I give you the benefit of the doubt.

[Beav]

Agree with all - the pot settings would be SWEET to have! Thanks for all the work of doing this.
Beav

[dbanner]

Great. That is my schematic which I posted a while back. One mistake for sure is the led is reversed.

On the pcb pdf I posted yestarday the TX and shield are reversed, you probably caught that. Tx goes to the collector of Q1

All that remains is to remove the trimmers ONE AT A TIME, and to measure the set resistances. Seeing that there are no setup procedures for this thing, It would be important to know approximately where those trimmers are set, otherwise someone could end up tinkering relentlessly with this little circuit, like a previous comment of the dog chasing its tail.

[Altra]

Altra, were are in luck! This schematic matches up perfectly with your MD8. There are just a couple of errors on the schematic which can be rectified by reference to the PCB of the working device.

Great. That is my schematic which I posted a while back. One mistake for sure is the led is reversed.

On the pcb pdf I posted yestarday the TX and shield are reversed, you probably caught that. Tx goes to the collector of Q1

[dbanner]

Altra, were are in luck! This schematic matches up perfectly with your MD8. There are just a couple of errors on the schematic which can be rectified by reference to the PCB of the working device.

[Goaty]

Notice also that the MD8 above has 3 front panel adjustment pots (Sensitivity, Auto-ground balance and Threshold) whereas the later MD20 has two (Auto-ground Balance and Sensitivity).

[Goaty]

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

Great work Altra - thank you very much!

It would be good if someone could could do the same thing with the MD20 so comparisons could be made. It helps to explain the extra trimpots shown in other circuits. It was obviously a "work-in-progress" with the makers doing improvements over time and helps to explain the variety of circuits presented in this topic.

PS: jtz54 alluded to some differences in post #54

[dbanner]

I'll draw the schematic and post it as soon as time permits. Thanks for the photos.

[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

[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]

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]

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.

[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]

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.