Originally posted by Mullihaka
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http://www.geotech1.com/forums/showp...postcount=3896
So many things to check. Plus and Minus voltage rails are first, especially minus, which should be around -4 to -6 volts. Plus rail about 8 volts.
Then oscillator freq and amplitude. Should be about 14.5 Khz, and 10 to 16 volts peak-to-peak.
Make sure coil shielding has a gap (not continuous around coil). Make sure the Receive (RX) coil and Transmit (TX) coils shields do not electrically contact and create a continous loop.
Check RX coil leads are correct phase -- you can try reversing them to see how it changes the results (you may have to null again).
Then I would work my way from "front" (coils) to "back" (speaker) checking the various signals using oscilloscope, comparing to reference pictures.
There are two "channels", the discrim channel, and the "ground balance" channel. If one is not working right, can cause problem.
It is easy to put JFets in wrong way, double-check that.
Then check the signals at the gates of the JFets of each channel's detector with switch in discrimination mode (not all-metal). The signals should look like almost square waves that have a max at about zero volts and a min at about V-. They should have different phases, which varies with the discrimination knob.
Turn the discrimination knob and the phase of the discrim gate signal should shift relative to the TX signal. It can shift maybe 90 deg.
You can check the Ground Balance gate signal similarly. Turn the GB knob, and the gate signal phase should shift relative to the TX signal. It shifts typically 45 to 50 deg, then may disappear at one end.
If the peak of either gate signal drops below zero suddenly as you turn the knob, then you may have your coil null point set on the wrong side. This causes the filtered voltage on the Source side of the JFet to be negative instead of positive, which causes the JFet to be forward biased at times, dragging down the gate voltage -- and probably causing the detector to function poorly. Voltage at JFet Source should be positive, maybe up to .5 volts. To fix, shift the coils slightly to other side of null point. But this assumes your circuit is working perfectly.
Then what I do is set my oscilloscope to a very slow sweep, maybe .2 cm/second. I put a probe on each output of the LM308s and set the vertical sensitivity to maybe .1 volts per cm or whatever is necessary. Then sweep the coin over your coil. You should see pulses in both channels. If the both pulses go postive, you should hear a tone in the speaker. If the pulses go in opposite directions, you are discriminating out the coin. If both pulses go negative, some phase is backwards.
Try turning the discrim knob to make the pulses go in the same direction. If they can't, then it sounds like your phases are wrong in the front end. You could have wrong capacitors in the discrim or GB circuits, or coil leads reversed, etc. Check carefully the signal phases.
If the pulses are just tiny when you sweep the coin in both channels, then maybe you have problem with ICs or wrong resistors, etc.
It takes time to understand the circuit to find problems, but that is part of the fun (maybe).
It is a lot easier when you have one project finished and working well, then you can compare. But first one is always hard.
Let us know what you find.
Regards,
-SB
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