GS4 questions

[Paul Harris]

Hi,

I'm have been building the GS4 as per Alex Schematic, the front end is complete, that is up to NE5534.


I'm trying to figure out the working of the circuit after this stage, at about 20uS after the clock signal goes low, I take it that is the point, 20uS after the clock goes low, at which the 4538's wake up and activate the second stage of the amplification process? And also what is the configuration of the U10.B op-amp is this an integrator circuit, can anyone give me a rundown on its working in relation to the jfet and the 4538's?

And can I get way with using just two 4538's since I'm only interested in the gold circuit for now?

Many thanks.

[Jose]

U10: B and U10: C are part of the integrator, their operation was discussed in a forum.
https://www.geotech1.com/forums/show...032#post270032

[Paul Harris]

Thanks Jose, took a look at the link you provided. What I don't get I think are these 4538's. And what kind of switch is SW1-C?

[dbanner]

The 4538's are CD14538. They provide the the pulses to switch the jfets.

[dbanner]

Hi,

I'm have been building the GS4 as per Alex Schematic, the front end is complete, that is up to NE5534.


I'm trying to figure out the working of the circuit after this stage, at about 20uS after the clock signal goes low, I take it that is the point, 20uS after the clock goes low, at which the 4538's wake up and activate the second stage of the amplification process? And also what is the configuration of the U10.B op-amp is this an integrator circuit, can anyone give me a rundown on its working in relation to the jfet and the 4538's?

And can I get way with using just two 4538's since I'm only interested in the gold circuit for now?

Many thanks.

[ATTACH=CONFIG]53710[/ATTACH]

The gold circuit won't work without the other channel. You've got to build both, they work in tandem.
Don't get me wrong, you'll detect metal, just not fine gold in mineralized soil. The second channel provides ground balance.

[Paul Harris]

What sequence are the jfets switched in relation to the clock pulse?

How does the detector perform on the beach and are both circuits required for the beach?

[Jose]

What sequence are the jfets switched in relation to the clock pulse?

How does the detector perform on the beach and are both circuits required for the beach?

Attached are the time sequences, taken from my built GS4.
The separation between channels of the GS4 is 45 uSeg.
The SW1-C switch widens the separation between channels, and modifies the GB settings and the responses to different objectives, what I can remember is that it had the mission of eliminating the soil, but continuing to detect certain types of coins that exist in it, UK, I can't really give more details, because I don't use it.
In my opinion, you can build the GS4 without the iron channel, in this case, it would not have GB, but a sensitive detector, which can operate with a minimum delay of 15 uSeg.

[Paul Harris]

Thanks for the info. I'm trying to build the timing circuit, but with no success at the moment, need to recheck the wiring. My dad has a Deepstar which was built for him by Eric many moons ago, nice depth, great machine anyhow he let me take a look inside and it has only two 4538's. He also has a Surfscan another of Erics an that one has no mosfet and again has good depth, not quite as deep as the Deepstar but not far off.

You say you built the GS4, what schematic did you build it from?? I'm looking at a circuit reversed by Alex, and I've noticed the pre-amp/op-amp is not the same as the original board and is missing a cap. I'm in the process of redrawing the circuit in Altium, I need a working machine to test since I have some ideas of my own.

[Ferric Toes]

Thanks for the info. I'm trying to build the timing circuit, but with no success at the moment, need to recheck the wiring. My dad has a Deepstar which was built for him by Eric many moons ago, nice depth, great machine anyhow he let me take a look inside and it has only two 4538's. He also has a Surfscan another of Erics an that one has no mosfet and again has good depth, not quite as deep as the Deepstar but not far off. You say you built the GS4, what schematic did you build it from?? I'm looking at a circuit reversed by Alex, and I've noticed the pre-amp/op-amp is not the same as the original board and is missing a cap. I'm in the process of redrawing the circuit in Altium, I need a working machine to test since I have some ideas of my own.

I guess your Dad is Willie the Wizard. The original Goldquest had the same circuit board as the Goldscan/GS4 but minus the ground balance channel; hence only two 14538's. Same with the Deepstar. The Surfscan did have a Mosfet, but a very small one (IRFD113) as it pulsed a much lower current but at a high repetition rate. Does he still have the 'BeachBurner'? That was the big daddy of high power 'keep your hands warm on the coil' beach detectors. Only one was made so it is unique. Eric.

[Paul Harris]

Hello Eric, Willie the Wizard is he. He still has the BeachBurner and from what he tells me he is very fond of that machine, he's not let me near it, but he said what you just said about warming your hands on the coil. I believe the BeachBurner has excellent depth. Funny, I feel I know you Eric, the dad has told me many stories about yourself and your excellent detectors and how, by sheer luck ,crossed paths with a Jerry Brown and that is how he came to know you. He has told me many funny stories about his adventures in detecting.

[Jose]

I built the GS4 with this attached schematic, as far as I know, it has no errors, the front end was modified by better integrated circuits, there are several possibilities, the 5534 would be a good option.
In red it indicates, the mods, that worked well, as far as I can understand.
You are lucky to see these machines made by Eric, I know him only from photographs.

[Paul Harris]

Hi Jose, ah yes I found that diagram, in fact I have several versions of it. I also have photos from here of the front and rear sides of the board. Now, assuming those photos are of the original board, then both diodes connecting to pin 2 of the 709 are incorrect as shown on the circuit diagram you linked to. From the photos both anodes actually connect to pin two, and this is shown in later versions of the circuit diagram. These diodes have been bothering me because I read an article by Mark Stuart which is floating around the net and he describes the purpose of diodes in a similar configuration to these in GS4 and he says:

Diodes D1 and D2 clip part of the
coil voltage passed via R2 and so limit the voltage
swing which is passed to the amplifier IC3 to one
volt. The diodes are of different types because D2 has to handle
only 50mA during the time that TR1 is turned on, whilst D1
must handle 3A peaks from the coil as TR1 is turned off. This
clipping only affects voltages from the coil above one volt, it
leaves the low level area of interest completely intact.

He says it doesn't effect the signal. I'm no authority on this so don't know the significance of these diodes being incorrectly placed. Maybe Eric would cast some light on this??

[Vadim18]

Hi Paul!
For absolute safety i use double stage clamp with traditional 1n4148 + second stage with Shottky Bat15 diodes. Bat15 has a low forward voltage 0.25V at 10 mA. This simple solution also helps to reduce op-amp recovery time, a little bit.



*In second stage 100 Ohm SMD resistor may act as fuse if one 1n4148 will open.

[Jose]

Hi Jose, ah yes I found that diagram, in fact I have several versions of it. I also have photos from here of the front and rear sides of the board. Now, assuming those photos are of the original board, then both diodes connecting to pin 2 of the 709 are incorrect as shown on the circuit diagram you linked to. From the photos both anodes actually connect to pin two, and this is shown in later versions of the circuit diagram. These diodes have been bothering me because I read an article by Mark Stuart which is floating around the net and he describes the purpose of diodes in a similar configuration to these in GS4 and he says:

Diodes D1 and D2 clip part of the
coil voltage passed via R2 and so limit the voltage
swing which is passed to the amplifier IC3 to one
volt. The diodes are of different types because D2 has to handle
only 50mA during the time that TR1 is turned on, whilst D1
must handle 3A peaks from the coil as TR1 is turned off. This
clipping only affects voltages from the coil above one volt, it
leaves the low level area of interest completely intact.

He says it doesn't effect the signal. I'm no authority on this so don't know the significance of these diodes being incorrectly placed. Maybe Eric would cast some light on this??

Hello Paul.
Somewhere in the forum this topic of diodes is discussed, but I don't remember where.
What I remember is that in the original GS4, the diodes are installed in the same direction and that happened by mistake according to Eric, but fortunately the arrangement did not affect the proper functioning of the equipment, because it suppressed the return pulse.
In my homemade build of the GS4, the diodes were installed as shown in the diagram uploaded before, and I did not have any problems.

[Paul Harris]

Thanks for the info Vadim. Much appreciated.

Jose, ah I see, yes that explains it. I have them diodes installed as per the diagram, but will be reversing the one diode today. Well actually I may leave it for now and change it when I've tested the circuit to see if there are any performance issues.

I'm very happy today because I have finally got the timing circuit to work after finding a fault. So I can now make progress.