Falcon MD20

[dbanner]

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

[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

[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).

[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.

[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]

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.

[Beav]

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

[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.

[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]

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

Splendid, Now having gotten that out of the way, I wish to point out that on the schematic, one leg of the trimmer should be connected to its wiper, as shown in the attachment, highlighted in the red box. When all corrections have been made, then a final schematic can be drawn.

[dbanner]

Moving right along, I found another error, this time a 10n bypass capacitor omitted from the schematic but is on the actual pcb. I have now included it on the schematic. Right near the red box, on the emitter of the transistor, there is where it should be.

[dbanner]

Yet another correction, this time a 10n ceramic capacitor just before the MPSA13, omitted from the schematic. I've now included it. Highlighted again in a red box. I'm going through the circuit step by step, to illustrate just how easy it can be to just have a tiny error or a missing component, and the circuit is kaputz.

[dbanner]

And finally, a 10uF electrolytic capacitor from collector of MPSA13 to ground. Missing on the schematic, I have added it in.
Well that looks to be it. Now I can redraw the schematic to include all the changes.

[dbanner]

Altra, please check the orientation of your MPS5172(shown as 2N3906 on schematic). Please verify that the emitter shown on your drawing is indeed the emitter, as the schematic shows the emitter and collector swapped. This is the only thing remaining to be checked before I can finalize the schematic.

[Altra]

I agree on all of your corrections. The big mistake was the 3906, and yes reverse the emitter collector for the mps5172. Also the led needs reversed. Get a rough schematic and I can fill in the pot values and cw/ccw connections. Most of the trim pots are set near mid-point.