GA-72Cd Magnetic locator

[ionut_mtb]

Hello. Does anyone has the audio output schematic for this device? I got very low audio output on both headphones and piezo (3pin).

[ivconic]

It is unlikely that you will find someone who can directly help with such exotic devices.
But there is no need.
It is most likely a 555 audio generator and if the output (3rd pin) does not go directly to the piezo;
then you should also check those few components that look like transistors.
As well as powering that part on the pcb.
It is clear that someone has already desoldered those two black diodes and the component near them that looks like a transistor.
There's probably a problem somewhere there.
And one more important thing to mention; tantalum capacitors!
They are very reliable, but once they start to fail; then they know to cause a lot of serious problems.
I marked with red cicrles all the components I would check first: (you can zoom In to see clearly)

[ivconic]

Let me pretend to be clairvoyant; this component is probably not a transistor but a voltage regulator/stabilizer?

[ivconic]

These two can be audio "driver output".
Signal comming from 555 is "buffered" through them.
Just an assumption, of course.

[ivconic]

I would be very interested to see detailed pictures of the sensor, provided it is not totally covered in epoxy.

[ivconic]

What particularly catches my attention (it has nothing to do with the audio glitch) is this component, it looks like an incremental encoder?
Or is it just some special switch with multiple positions?
If it can go round and round endlessly; then it is an encoder (perhaps even an older "BCD" type) and if it cannot be rotated infinitely
in a circle; then it's just a potentiometer or simple switch?

[ivconic]

My reason for writing and commenting so much is because I am very interested in this type of device.
According to a bunch of high quality opamps presence; I assume that it is a refined "preamplifier" of the signal coming from the passive sensor?
Probably a coil with very thin wire and a large number of turns?

[ivconic]

Oh it has 2 sensors actually!
Becoming more and more interesting!
I would like to do reverse engineering on it and produce complete documentation (Sch&Pcb).
Also the same with sensors!
But that directly means "destruction" of the device!

[ivconic]

But unfortunately there is one major deterrent to such ideas; and those are opamps that are very hard to get in local stores and I guess very expensive.
Another "obstacle" is the sensors themselves.
If they are well encased in epoxy: chances are high that they will be completely destroyed during disassembly without knowing all the specifications of those sensors in the end!
A very expensive game!
Often this kind of motivation and enthusiasm COST me a LOT in life!

[ivconic]

Very expensive game indeed!

OP482 x 2 Quad Low Power, High Speed JFET Operational Amplifier $12
OP282GP x 3 Dual Low Power, High Speed JFET Operational Amplifier SMD $5
OP213EP x 1 Single- or dual-supply operation, SMD $8
DG303ACJ x 1 TTL-Compatible, CMOS Analog Switches $22
CD4047 x 1 CMOS LOW-POWER MONOSTABLE/ASTABLE MULTIVIBRATOR
SI7661CJ x 1 (possible x2 on the pcb, one is covered, can't see the label) Si7661 is a monolithic charge pump voltage inverter $5
L555CN x 1

As a joke, I listed only the chips that are clearly visible, which is why I started looking for them on the net.
At local suppliers, there is NO one to be found except 4047 and 555.
With foreign suppliers, I had a lot of problems to find everything in one place.
And the additional problem is that through-hole versions are harder to find and mostly SMT versions are offered.
And already now (based on the incomplete list) the "net" prices without additional costs already reach $100!
And that's just for a small handful of parts.
Conclusion; FORGET!
But...!!!
Two ideas arise from this:
Find such a totally broken device on ads for a small price... find a correct/semi-correct device like that for a small price.
Either fix such one.
Either harvest those opamps from it and use them for other works.
Conclusion: such and similar devices are a good source of high-quality opamps, provided that such a broken device can be found cheaply on sale.

[ivconic]

Sometimes these random searches are done more as a joke; they know how to pay off!
I stumbled upon a treasure cave!

https://www.prc68.com/I/MagneticLocators.shtml#GA-22

[Altra]

I have a GA-52 and two GA-72Cd's. On the GA 72 the green switch 5pos . Is on/off, L, M, H and XH gain. I think it switches the 4 resistors that are near

Wanted to reverse the GA-52 pcb, but never got around to it. It uses a RC4152 for the vco, The tx/demod oscillator is HEF4047, DC-DC ICL7660,,OP421, OP421. CD4053. The blue chip is a 10K resistor array.

Here are some photos of the pcb

[ionut_mtb]

Hello Ivconic, thank you for helping me.
I will try to reply on all your post but is hard from the mobile phone.

#2 - that side of the pcb is exactly how you described but 555 pin 3 does not go to the piezo/headphone, it goes into the yellow resistor network (ladder).
#5 - i tried to remove the sensor for you but I cannot pull him out from the case and i’m afraid damaging it…
#6 - it is a switch - on/off and gain settings (L, M, H, XH) si it is a 4 position switch.
#7 - i don’t know much and i’m not so interested in it because my time for metal detecting is very limited for now (lost of interest unfortunately- maybe because of hot weather) but i can inspect more if you want. This device is for industrial use - a friend has it. Two CuEm wires (from the pcb coil) going inside the shaft and 3 small insulated also.

The pcb had a lot of corrosion on some pins and i tried to resolder and clean. The result is: device not power on anymore

[ionut_mtb]

My upper post is incomplete.. before i post i copied the text because i was afraid that something will go wrong and i need to rewrite everything from phone

[ionut_mtb]

Something is very strange. I pasted some text but the forum did not added to the post.. hmm i will try without the smiles emoji. This forum version still have bugs…


In Romania we have a saying: “ before was broken but was working, now is fixed but not working anymore” and “if it’s working don’t touch it”

Where the 555 is it is connected the power supply (2x9v battery which is going separately to the pcb.


The piezo wires are going to the other corner of the pcb (opposite to ic 555) to op282 connected at pin 12 and pin 7 (which is output).