Fisher Gold Bug

[dbanner]

I'm pleased that you're working on this circuit. Could you please throw away the battery check going forward, no need for that rubbish.

The original discreet rx front end contained errors, but I corrected those errors and got it to work nicely in ltspice sometime ago.
But I am in agreement that the front end should be re-designed with opamp. This is by far a more practical solution.

As for the power supply, bipolar 5V gives adequate headroom for tweaking with the gain of those stages also more flexibilty with using different opamps, but will have no effect on performance of the circuit as is. The amplifiers/ filters will simply take what it needs from the rails whether it is a 6 V spread or a 10 V spread.

I like the CD4060 approach, since you can also use one of its outputs to drive a voltage inverter for the negative rail.

Have a look at the minelab 15000 schematic, more specifically the tx circuit. I don't recall anybody lifting that tx circuit for implementation on any diy device, maybe it can be used here as an experiment, it is after all a gold detector.

I don't really care about the power supply, you can do whatever solution you think is best, just make it as frugal by way of components so I can build the thing with the parts most diy'ers might have on hand or are easy to obtain. 10Mhz crystal is common enough. I see those quite often.

[ivconic]

I am not simulator freak.
However... Proteus simulator is giving me very interesting results!
When i put 2.5V zener diode there; it will not work, giving me "nan" at voltmeter display.
But if i change 2.5V to 2.7V value; it works.
Giving 3.18V output.
I guess this is due improper choice of Q2, R1 and R2.
Anyway; point is: if there is 2.5V zener there; output will most probably be close to 3.0V.
Actually the right half of the supply.
So it is symmetrical after all. Rails are +/-3V in this case.
Yet i don't like simulators, this must be checked in real world.
In meantime i'll try to trim Q2, R1 and R2 to make it work with 2.5V zener.
Here is what's in simulator:

[ivconic]

When i made TX the other day; i used alternative TX output (from square on schematic).
This is forced TX, output gives interesting behavior on scope.
When switched On; it shows immediate clean sinus for half a second, than it settles down to bit distorted sinus with a known "notch" on the rising edge.
I guess this is not good and it will need trimming the amplitude. Which is btw ~12vpp.
So the original TX output looks more promising than alternative one.

[ivconic]

Nice rework on the Front end
Original is really sophisticated, the guy who made it had a really good day that day!
I did not really understand the Fisher type power supply, why 6 volts and 2.5volts can you explain it please?
regards

I can't explain it to myself either.
Similar to 12xx series.
We elaborated 1265 model here on forum, quite while ago.
I forgot all the important conclusions we had than. should use search option to remind myself.
But from a first glance is obvious that SiVP0104 fet is having complex role there.
Kind a "soft start", protection and regulator.
I added on schematic possible substitution without that fet, since is impossible to obtain it in local shops.
Also i recall that i successfully replaced it on one broken 1265, with ordinary fet. Can't recall which one.
OP37 i put more like "principally" solution, in reality i don't plan it but another more modern opamp with better specs.
From what i already checked on the bench is TX part, 4060 based oscillator and 4013 based phase shifter.
I can confirm that stage is working alright. Much better solution than damn transformer.
I took 9.5Mhz crystal value as just probing example. I wonder how would it perform with higher frequency.
Later on when complete device finished; will be easy to check this, by simple crystal replacement
10Mhz, 10.450Mhz, 10.7Mhz. All close. Giving slightly higher working frequencies.
So, by now; TX part with phase shifter is tested on bench and proven.
Problem is; i work parallel on 2 other projects, quite different than this one.
So it will add some delay in my work on this.

[Nandor]

Nice rework on the Front end
Original is really sophisticated, the guy who made it had a really good day that day!
I did not really understand the Fisher type power supply, why 6 volts and 2.5volts can you explain it please?

regards

[Nandor]

I like this topic, it gives me that good old TGSL topic vibes with great talks and interesting ideas. I hope it will be interesting to others to as it's for you and me!

[ivconic]

I agree with you dbanner.
Few "awkward" (for diyer) moments in all those schematics from that series.
Obsolete components, transformer etc.
But all solvable today.
I am great Fisher fan since '90s.
I had dozen models... except Gold Bug.
Had one, broken from a start. At the time wasn't ready to fix it, so i resold it.
I simply adore 12xx series. I think is pity that series is not in production anymore.
Now... on this schematic; i am not sure if it is accurate. I use existing one as starting point.
And intend to change lot on it. Would it be success? I can't tell.
I find great pleasure in journey, more than reaching final destination, that's all.

[dbanner]

I assume the coil shield is connected to battery negative (ground) on original schematic, with the PS rails at ground and +6V, the virtual common being at 2.5V.......there is something strange about this, the entire circuit is always above ground throughout all its transients, perhaps someone has the pin out and schematic for the coil.
I wonder if perhaps a more conventional bipolar power supply with +5V and -5V and battery minus serving as ground as well for shield ground.
I noticed fisher labs using this unconventional and strange power supply topology since the start of their Mscope series beginning with the 1210x. You might say the entire circuit has been level shifted, but why?
I don't like one bit. Unless there is some real benefit in terms of performance, I'd dispense with it in favour of something a little more mundane.
Also, using more mainstream junkbox opamps to replace those not-so-regular ones, shouldn't make much difference to extreme performance.

[ivconic]

I am "contemplating" more wider options now.

[kt315]

still one sch from Russia. i think this is Alex Serbin. hybrid.

[dom44]

good job ivconic I have been playing with the circuit for a while now, except i moded it i have a lobo front end amp circuit works at 32khz and 4069 crystal driver for the transmit circuit driving at 32khz to experiment I will use your circuit to check against mine to see if i have any mistakes on my circuit. I have also just finished the lobo diablo running 25khz with a 4060 and 3.2768mhz crystal.



this part of the circuit works 100% i have built it.

[ivconic]

So that would be first Gold Bug, working at 19.2kHz.
Gold Bug 2 uses MAX412 at the RX front end, instead "poor man's opamp" solution as in this case.
I am looking up now for the Gold Bug 2 schematic or at least parts of schematic with RX&TX frontends.
I think i saw it somewhere ...

[ivconic]

Thats a great contribution!
Congratulations for your work.

best regards.

Sometimes I kill boredom like that.
And is good to have easier insight in the design.
Cheer!

[Nandor]

Thats a great contribution!
Congratulations for your work.

best regards.

[ivconic]

It wont be any sooner. It is long way from unproven schematic to appropriate pcb and workable device.
Besides, if i dare to try to make it; it will be most probably based on Atmega mcu with lot of mods and changes.
Long way! Long shot!
I just wanted to "decipher" scary gibberish schematic that we had so far.