search-oscillator didn't work on the ETI561 metaldetector

[Radic]

Hi

I've build the ETI561 as described here: http://www.geotech1.com/pages/metdet/projects/eti561/eti561_150.pdf

the red markte part below dosn't oscillate


I simulated the marked part with LTspice but the result was the same as in the real circuit.

The coil was wound on a 160mm PVC-pipe. ~60,75 turns, ~24,4mm length with 0,4mm enameled copper wire and has ~3937,5uH
The coil is connected with a shieled cable and a 3 pin XLR-connector

Here are photos of my PCB: http://info-tbb.de/~radic/mdt/

I hope someone knows what I did wrong.

[der_fisherman]

Hi

I've build the ETI561 as described here: http://www.geotech1.com/pages/metdet/projects/eti561/eti561_150.pdf

the red markte part below dosn't oscillate

(Schematic removed)

I simulated the marked part with LTspice but the result was the same as in the real circuit.

The coil was wound on a 160mm PVC-pipe. ~60,75 turns, ~24,4mm length with 0,4mm enameled copper wire and has ~3937,5uH
The coil is connected with a shieled cable and a 3 pin XLR-connector

Here are photos of my PCB: http://info-tbb.de/~radic/mdt/

I hope someone knows what I did wrong.

Are you using the exact same components? Some replacement transistors have the legs in a different combination, check carefully.
Have you checked your soldering with a good eyeglass? Some of it appears to be very questionable in the fotos you posted, also look for shorts at the same time.
Aso the PCB needs cleaning with an old toothbrush and methylated spirits to get rid of flux residues.......
You appear to have "clipped" many component leads BEFORE soldering, not a good idea, always clip afterwards.....then its easy to touch a soldering iron to each component lead and see the solder melt to be sure a connection is good.
This detector is over 30 years old and although I haven't made one myself, I am sure someone else would have already noticed if it didn't work!!!
As to why LTSpice also doesn't work I have no idea.
Best of luck.
Regards
Andy

[Skippy]

Check the DC conditions are correct. There should be 9V on the transistor collector. The emitter voltage will be about 0.6V less than the base, the base bias should give 4.5V with the 'coarse' pot set mid-position. Try and measure the search-coil resistance accurately, to see if there is a short anywhere.

[Qiaozhi]

Hi

I've build the ETI561 as described here: http://www.geotech1.com/pages/metdet/projects/eti561/eti561_150.pdf

the red markte part below dosn't oscillate


I simulated the marked part with LTspice but the result was the same as in the real circuit.

The coil was wound on a 160mm PVC-pipe. ~60,75 turns, ~24,4mm length with 0,4mm enameled copper wire and has ~3937,5uH
The coil is connected with a shieled cable and a 3 pin XLR-connector

Here are photos of my PCB: http://info-tbb.de/~radic/mdt/

I hope someone knows what I did wrong.

If I understand your markings on the schematic correctly, you have not fitted either R6 or C5. Since this is a simple Colpitts oscillator circuit, then R6 is required to make the connection to the 5V power supply (located at IC2, pin 14). This is probably the problem with your SPICE simulation, although it looks like R6 is actually on the PCB. My suspicion is that Q1 is fitted incorrectly.

[porkluvr]

I don't understand why you would measure almost 4mH on your coil. Pretty sure it should be closer to 2mH. I used 2mH (with 250pF parallel capacitance) and got LTspice to show this circuit oscillating at around 112kHz, right where it should be. But using 4mH would not make the oscillator fail, so something else is wrong with your simulation.

And, believe it or not, some manufacturers use physically different packages for transistors and still use the same part numbers. Two BC548 from different manufacturers might have different pinouts.

[satdaveuk]

I agree with porkluver, although theres alot of room for variants with inductance on this particular oscillator arrangement running with CX should be around 110khz been 2mh over would shut it down.
I suggest check total circuit current consumption guessing should be no more than 150ma, that tells you alot, then transistors for pinout errors plus circuit errors.
If you did the right amount of turns with .4 wire on a 150mm form inductance should be around the 1.5mh maybe a tad lower/higher.
If you done this on a 150mm form and your reading 4mh which 1 would say is almost impossable with 70 turns then your inductance meter is up the spout.

Goes back to the coil tester project I put on here some months ago, you could have proved that stright away by simply connecting the coil to it together with the CX and then bingo, aunts your fanny and grandads got balls.

Just thinking im going to build this project, would make a good pinpointer.
Anyway let us know how you get on either way, dont just go silent on us.

Regards

[Radic]

Are you using the exact same components? Some replacement transistors have the legs in a different combination, check carefully.

they are inserted correctly

Have you checked your soldering with a good eyeglass? Some of it appears to be very questionable in the fotos you posted, also look for shorts at the same time.
Aso the PCB needs cleaning with an old toothbrush and methylated spirits to get rid of flux residues.......

the questionable soldering already checked and now it's cleaned too

Check the DC conditions are correct. There should be 9V on the transistor collector. The emitter voltage will be about 0.6V less than the base, the base bias should give 4.5V with the 'coarse' pot set mid-position.

the emittervoltage is 3.5V and not 3.9V

Try and measure the search-coil resistance accurately, to see if there is a short anywhere.

the search-coil has 5 Ohms

If I understand your markings on the schematic correctly, you have not fitted either R6 or C5.

R6 and C5 exists, but dosn't belong to the search-oscillator.


I did a new simulation of the oscillator like discribet here: http://en.wikipedia.org/wiki/Colpitts_oscillator
and replaced with the parts who are used in the detectorcircuit and I used a coil with 2mH and it works.
I think there's something wrong with my coil. can someone make some photos of the coil from a working detector?

[Skippy]

Your search-coil could have a shorted turn. It would still read as 5 Ohms with DC current, but would behave differently magnetically (with AC passing through it). Make a new search-coil, it is not essential to shield the coil to prove your oscillator functions.
Also ,I've had another look at your component placement, and I think you have the oscillator transistor the wrong way round. TO92 packages are normally EBC or ECB from left to right, when looking at the front. Your PCB looks like the transistor should be CBE, like metal can TO18 transistors. I suggest you check this, or just try fitting the transistor 180 degrees rotated, (so that C and E swapped)

[Radic]

Solved!

It's now working. The problem was the coil.

The left is the non working, right is the working coil

[reza vir]

The problem in the coils
And lack of Sense Field
Please share your test results
Thanks & Regards