Need help! Building 1981 VLF Project

[Op04]

Hello everyone,
I need help for finishing project from old 1981 elektor magazine, here's the link for pdf I need more info before I put the ICs and voltage, is this Project have PCB error or anything else I should know? If anyone had successful built it please info...thanks,,

Taufik

[Skippy]

I suggest you download the December '81, Jan/Feb '82 issues, and look for any additional articles, errata, etc on the project.

Your big problem will be the search-coil, are you using a commercial coil to increase your chance of success ? As it runs at about 8 kHz, there are numerous choices.

[Op04]

Thank you Skippy I appreciate you for this!
I will look asap. I've build it yesterday for the coil
I'll update soon here

[Skippy]

Coil building is hard, and especially so if you have no working detector to test it on. It depends on why you are doing this project, but if you can afford to buy or borrow a commercial coil, I recommend it. Then when you've got the detector working fine, make your own compatible coil that you're happy works OK. You can then sell on the original coil if need be, to reclaim your expense.
Coils for the Fisher F5 / Teknetics Eurotek/Omega / Gamma /Delta ( and others ) work at 7.8 kHz, and are quite broadband, so will be fine with this project. They often appear secondhand on eBay etc, from several manufacturers. Here's one example from the U.K.
https://www.ebay.co.uk/itm/MARS-SNIP...EAAOSw3ZFfoAG0

[Op04]

I interested to build this because I have no experience build VLF, it has detailed build guide. Years ago all my PI project works fine. This time my first attemp is build tgsl but stopped coz can't find JFETs, I was attracted by vintage MD so I decided to build this, sadly I just believe what it said on that magazine, turn out after I search this forum that project have errror, even ivconic said there were 7 mistakes, anyway it already soldered so I must finish this. after look into several edition as you said I still can't find which error

[Op04]

My coil was based from this before molding at least it reach the right inductance & ohm on both coil

[Op04]

I do lot google, found this
With Google translation found the 7 mistake.

Here the original France language, if anyone can help please do..
1. La bobine de detection doit etre alimentee en +5 et -5 volts, (et non pas en +9 et -9 volts). Pour ce faire, effectuer les modifications suivantes sur le circuit imprim?: couper le piste reliant les r?sistance R1 - R3 a R28 aux environs de IC3, et faire la liaison entre le morceau de piste R1 - R3 et la broche 16 de IC2. Voil? pour le +5 V. Connecter I'ilot recevant R2 et R6 a la piste qui va a la broche 7 de IC4, voila pour le -5 V. Ilest ?vident qu'il n'est plus question de mettre ces points au +9 et -9v.

2. Le sens de rotation du discriminateur P5 est inverse en figure 4. Le "0" se trouve pres du -5V. Il est impossible de r?gler le montage, si cela n'est pas modifie.

3. Si toutes les gammes fonctionnent correctement, a l'exception de la gamme R1, il faut inverser l'una des bobines

4. Remplacer IC8 par un 4093, et R64 par un pont de fil de c?blage. Une petite pression sur S3 fera basculer les flip-flop N3 N4.

5. Faire passer la valeur des r?sistance R59 et R61 a 2M2, permet de ralentir quelque peu la vitesse d'int?gration.

6. Modifier la valeur de R12 et prendre une 27k. L'amplitude du signal sur la bobine d'emission est de l'ordre de 8V cr?te a cr?te environ

7. Remplacer ?ventuellement IC6 par un CA3140

After translated:

1. The detection coil must be supplied with +5 and -5 volts, (and not with +9 and -9 volts). To do this, carry out the following modifications on the printed circuit: cut the track connecting resistor R1 - R3 to R28 around IC3, and make the connection between the piece of track R1 - R3 and pin 16 of IC2. So much for the +5 V. Connect the island receiving R2 and R6 to the track that goes to pin 7 of IC4, that's for the -5 V. It is obvious that there is no longer any question of putting these points at + 9 and -9v.

2. The direction of rotation of discriminator P5 is opposite in figure 4. The "0" is located near -5V. It is impossible to adjust the assembly, if it is not modified.

3. If all the ranges are working correctly, except the R1 range, one of the coils must be reversed.

4. Replace IC8 with a 4093, and R64 with a wire bridge. A little pressure on S3 will switch flip-flops N3 N4.

5. Changing the value of resistor R59 and R61 to 2M2 allows the integration speed to be slowed down somewhat.

6. Modify the value of R12 and take a 27k. The amplitude of the signal on the transmission coil is around 8V peak to peak approximately

7. If necessary replace IC6 by a CA3140

I'm not very happy with google translation, if any kind French people here can help me pls make better translation above

For no 1 I think I got it, make it to -5/5+ not the +/-9V track

[Skippy]

My interpretation:

1. The search-coil transmit amplifier must be supplied with +5 and -5 Volts, (and not with +9 and -9 Volts). To do this, carry out the following modifications on the printed circuit board: cut the track connecting resistor R1 & R3 to R28 near IC3, and make a new connection between the R1 & R3 track and pin 16 of IC2. That fixes the +5 V supply. Similarly, connect the copper island joining R2 & R6 to the track that goes to pin 7 of IC4, that fixes the -5 V supply. Obviously there is no longer any question about joining these points to + 9 V and -9 V.

2. The rotation direction of Discriminator pot P5 is the opposite of what is shown in figure 4. The "0" is connected to -5V. It is impossible to adjust the detector, if it is not modified this way. (( this means the two fixed ends of the pot need swapping round ))

3. If all the operating ranges are working correctly, except the R1 range, one of the coils must be reversed. (( this means swap the two transmit coil wires over. If you get a double-beep in discrimination mode, swap the transmit wires; if you get a single beep, it is wired correctly ))

4. Replace IC8 with a CD4093 Schmitt NAND gate, and replace resistor R64 with a wire link.

5. Changing the value of resistor R59 and R61 to 2M2 allows the integration speed to be slowed down somewhat.

6. Change the value of resistor R12 to 27k. The amplitude of the signal on the transmission coil is around 8V peak to peak.

7. Ideally, replace op-amp IC6 with a CA3140 (( this is a MOSFET input op-amp, which will work better than the jfet opamp originally specified. 3140's are hard to get now, maybe choose another MOS input device ))

[Op04]

Again,, thank you so much Skippy

[Op04]

Thank God, I finally build this. Will update soon

[Op04]

This is the result after trying with several coils.
1st coil 245mm @5.5mh (tx&rx coil) detection 20cm of half core Tin lead wire, no gold detection
2nd coil 19cm @6mh detect 25cm for tin lead core, no gold detection
3rd coil 85mm @10.5mh detection 15cm for tin lead with gold detection 5cm from the floor. 2.5cm from coil perhaps, had to change the caps, 220nf no gold, change to 100nf+3n3 paralel gold (2.5grams) did make sound
At least it works,, here a video for 3rd coil. Just short & bad video he.he

[Skippy]

Hmm.. it's a start, but clearly needs a lot of skillful adjusting to get it going properly. But that's part of the fun, I suppose.

It's not sounding right for a start, it's supposed to be 270 Hz, not a buzzy 50 Hz.

And I think the search coils are supposed to be in the 1.55 mH region, so you are miles out with the tuning. I would suggest making them 1.4 mH, then use a parallel capacitor on the TX of 242 nF = ( 220nF + 22nF ). You can then fine-tune the 22nF cap value up or down to match the resonance to 8.64 kHz.

[Op04]

Thank you Skippy for your directions. My equipment are not present perhaps another day. When my time meets I will update again...

[dbanner]

Just based on the schematic for this circuit, the performance is supposed to be quite good, on par , I think, with similar circuits.
However, your circuit sounds like crap. You need to get the coil right, assuming there are no errors in assembly of the PCB etc.
There shouldn't be any buzzing threshold, it is supposed to be smooth and pleasant sounding, barely audible when properly adjusted ( threshold).
Do you know the correct parameters for the coil this circuit is meant to utilize?

[dbanner]

I can help you. But I would like to see both sides of the PCB. If you can, take some good hi res photos and upload them. We can have a look too see if everything is ok with the PCB. After that it becomes necessary to make and balance the coils properly, then we can have a chance at getting your project working great.
Don't recall anyone here ever building this project. The circuit schematic is very interesting for a number of reasons.