Who knows differences?

[Gervm]

Looking through old bulgarian magazine I found article with circuit of heathkit GD-48. Enough interesting project. I also heared that it was modified to GD-348. Who knows differences betwin GD-48 and GD-348 and thier parameters? Article say that GD-48 can find very large object at 1,8m. Is it so? May be any information abot search head. I would be very much obliged if somebody send me circuit or describtion of GD-348.
[email protected]

[Geo]

Take it!!!!

[Gervm]

Thank you very much.
I saw they changed the first model more than I supposed.

[Esteban]

Coils for both, GD-48 and GD-348. Also you can build O-O type, works good. More details soon.

[Esteban]

Gervm, see your email box.

[Gervm]

Thank you. For me it is very interesting. I like old detectors when you hear your own transmiting signsal ... They are not so sensitive but they are realy nice and almost every is original. I`m waiting

[Gervm]

I`ve found - interesting

[Esteban]

The schematic I post you on your email is not very "clean", but is understandable.

Oscillator and preamp RX coil is inside housing coil. 30 pF capacitor must be good quality, "high voltage", 200-400 V, can be mica. Also the trimmer capacitor, good if porcelain, no plastic, except if you found very good quality. This is automodulated in audio, frequency is 98-104 Khz. You must adjust the coils for no ferrous (zero adjust, helping with trimmer connected between TX and RX cols). You can use fix capacitor for to adjust receiver coil, no need here the tuning trimmer. Better if you use capacitance box and put in critic point. In general coils are not shielded, but you can shield only receiver coil. Resistor of emitter in oscillator transistor can be between 56-68 ohms, better 1% here.

This reject nails and wire of fences, but iron relics as weapon are very good detectable. You must put small audio and hear variations. When you detect nail or wire of fences, audio vanishes, but non ferrous targets increases in audio. Here is good a headphone 500-600 ohms, also seems soil effect is not very remarcable with this kind of headphones.

[Gervm]

I`ve understood details. Seems they made rather good device.

[Esteban]

Simplicity + good discrimination + sensibility, nice for a 1968 detector.

[Esteban]

You can glued aluminium paper and connect to negative of the PCB (oscillator and preamp). This is in housing coil. In general, the grass produce falses in part of the capacitor trimmer. With this implementation you solve it. This is a modification. All metalic parts must be fixed before zero adjusting, include final lenght of cable. More critic point is obtained if you move one of the wire of receiver coil (one of the extremes), this acts as a final compensator. Invert extremes of receiver coil and put your hand in both sides of rec. coil, time by time. You note in one extreme capacitive effect (a kind of null) when you put your hand. If the capacitive effect is produced in down part (this is, a kind of null), invert the connections of the extremes of the rec. coil. Is better the increasing of audio in the down part when you put your hand, this is, the part of rec. coil near the soil.

[Gervm]

Tank you for detais of construction.

[Gervm]

Esteban, look your E-mail. I posted one circuit that did not see here. What model is it?

[Esteban]

Thanks very much. Here the Heathkit and some finds. One photo is of a Heathkit with big coil (note here a common variable AM cap. instead porcelain trimmer). Big coil is not sensitive to coins, but good for big mass. Other coils are o-o type. Note that receiver wire is more high number of tx coil. With this, capacitance effect is more less, maybe this difference in wire is applicable to all kind of detectors. Remember: you must to leave a hole for to access the capacitor trimmer (or the AM tandem) for future adjustment. In my case, never I need to re-adjust this. Maybe in cold weather oscillator don't start... C1 0.033 (in series with 150 ohm resistor) you can select for desirable tone for you, minimun is 10 nF, I try also 330 nF, but here sound is very gravis, but can be interesting. Transistor of oscillator you must select the best, today I use BC547A.

[Esteban]

An extra component is a 0.1 uF capacitor in paralell with the electrolitic. Is better with this. See here: