Falcon MD20

[chemelec]

I have both the First oscillator and second transistor with the two 1N34a diodes on the breadboard.
Also the T2, So T1, T2 and T3 all wired up.
I first tried it with 2N2222's and than with 2N3904's

My coil measures 145uh and also made that copper ring for it.

GOT IT GOING NOW, Bad Connection.

[homefire]

Gotta hit it with a Defibrillator. 20KV 10uS Pulse. 10 Pulse Train Pulses separated by 40uS Delay. LOL Yea, dat should get the ticker ticking. Are you using PN2222 ? Some times they just don't act the same. T2 going to the coil , is T1 in circuit ? Maybe it needs the feed back to sing.

[chemelec]

I was just playing with this circuit on a Breadboard.
I Assume the first 2N2222 with the coil on it, Should show an oscillation of about 300 KHz, measuring after the 150pF cap?
I Can't get it to Oscillate.

[homefire]

Good STuff from You Sir. Thank You.

[dom44]

yes it is the the one marked MD20-final circuit board should be right now i just have to check last resistor values i used around VCO and this is the final values i have soldered in I would suggest some one check against the circuit diagrams as i had to reverse engineer some of it because of the changes i had to make.

[homefire]

Which one is the Last? Fixed for the Attached md20.rar ???

[dom44]

[homefire]

Can you post just the Copper Layer ?

[dom44]

sorry guys this is the final ignore previous 2 lay files had to add the 78L05 to it



md20-final.rar

[dom44]

I was asked to share the circuit for 4093 VCO version here you go have some more work to do on it but i will post as is the MD20-fixed.RAR is the one to use had some values missing and small amendments to make.




md20-fixed.rar

[dom44]

I am familiarizing my self with this circuit right now and experimenting I have built one circuit that is more sensitive picks up a grain of solder the size of a pin head at 10mm easy but needs a lot of work, the threshold gives problems with stability.
I still think a VCO is the way to go on the MD20, the short fall in the design is the basic on off sounder or 555 timer sounder and the fact they use transistors for amplification when op amps do a better job.

what most people don't realize is the sensitive adjustment of the sens pot I was watching a youtube video where one guy was only just picking up good size nuggets touching the tip, where on the bench i get from 5mm-20mm for the same size it shows bad adjustment
and makes you wonder how many where missed.

[Tim]

Great stuff dom !! Anyone going for a pan or a crack-scratching needs one of these beauties. My [bought] one, I set up on a pvc cane-probe pressed into the base, cord up the middle and box mounted at the top for probing around mullock heaps and creekbeds.

Keep up the good work- good to see the renewed interest in this !!!

[dom44]

I was messing arround on the bench today made another improvement or two if you add a 1M resistor at the base of T3 to ground helps a lot with stability and add a 100k pot in series with R14 at T4 collector gives you the fine adjustment you need to get best sensitivity.

the way to use it is turn the ground adjust R10 till LED comes on then back off till goes out, adjust R14 till LED comes on then back off, adjust fine tune 100K pot then just back off again to a position that is stable you have more movement to play with the 100k this gives excellent sensitivity, but the R6 trimmer is important set for most stable with sensitivity as this adjust the amplitude of the oscillator signal.

[dom44]

9V is ok i like to use 12V AA battery pack will last a long time specially if you are running from one battery and not two the audio section and LED use most of the power. I added a 5 volt reg to the oscillator as the unit becomes unstable at high gain settings. It goes into a a kind of feedback and the sens pot is hair touch sensitive just a fraction ether way makes a big difference to the distance it picks up small metal pieces .
helps but doesn't totally fix it. I am working on it as well as a radically different design but be a while have to test the various elements of the circuit.

[homefire]

9V is what the Original uses. Nothing in the circuit demands much in the line of current.