Re: Question for Dave Johnson

[Reg]

Hi Dave,

I recently did some testing with the GB 2 and found I could readily detect a nugget sitting right next to an rather large nail in the discriminate mode. This is something I have not been able to do with any other detector.

My qustion is what is so different about the GB 2 that allows the detection of the nugget?

I was hoping to be able to purchase the schematic for this detector, but it seems that the new owners of the company have changed their policy about selling them. At least the schematic would give me some insight.

Thank you for any information you can supply,

Reg Sniff

[C.Pruysers]

Re: Question for Dave Johnson

Reg. The question is that even in the discriminating mode , WHAT did you detect . The nail or the nugget ? Kees

[Reg]

Re: Question for Dave Johnson

Hi,

With the GB 2, in the discriminate mode, you can pass the coil over a large nail and get little or no response indicating the nail is being rejected or ignored.

However, when I placed a 1/4 oz nugget close to the nail, the GB 2 responded with a distinct positive response, quite similar to the response from the nugget by itself.

I have tried this same test with quite a few other detectors in the discriminate mode and when the nugget is placed near the nail, there is no audio response from the nugget, just like it isn't there.

Reg

[C.Pruysers]

Re: Question for Dave Johnson

Reg . Don't get me wrong but ..... you wonder about the schematic while you say you made the test with your detector . If you have the detector , don't you already have the schematic ? If you have the detector why not coppy the circuit and let us know about it ? Something is fishy here ! Kees

[Reg]

Re: Question for Dave Johnson

Hi Kees,

Fisher doesn't release the schematic with their detectors. As far as copying the the circuit, that is a pain in the butt. I have done it in the past years ago and didn't like it then.

Producing a schematic by tracing the pc board can take days and right now, I am in the middle of developing a modified design of another detector, a PI which is taking up most of my time.

Although I haven't taken the GB 2 detector apart yet, I am sure it is a double sided board which makes things more difficult.

Yes, I do have the detector. In fact I have about 14 different detectors at the present time.

Most US manufacturers do not release their schematics. However, at one time, Fisher did, and a person could buy a copy of them, but they do not sell them now. Had I known they were going to change their policy, I would have purchased one some time ago.

I am just hoping somebody did purchase the schematic when it was possible and would be willing to share it.

I have no desire to copy the detector, but simply know why this detector can detect a nugget easier in the disc mode when the nugget is close to an iron object. All other detectors I have tried, and there have been quite a few, have failed this test.

The reason could be as simple as the target separation that occurs at higher frequency, or it could be a combination of things including filter response speed. Whatever the reason is, I would like to know.

If you want to see a picture of the detector, visit my website below. The website only references a few of the detectors I have owned. I really need to update the site and include several more detectors.

Reg



my website

[Dave Johnson]

GB2 question

I don't have a schematic myself. However, there is nothing extraordinarily special about the discriminator in the GB2. I suspect that the best explanation is the high 71 kHz operating frequency which provides more separation between the nugget and iron, than a typical "coinshooting" frequency would.

--Dave J.

[Reg]

Re: Thanks Dave

Hi Dave,

Thanks for the info, that is basically what I needed to know.

I have to say you efficiently used the board space on that design. My technical term for that board is it is crammed packed. I took one look at it and decided to not try to reverse engineer it at this time.

I do have a couple of other questions though. At the higher frequencies such as the 71Khz, does the signal from iron reduce if compared to the signal at a frequency of, lets say, 20Khz?

The second question is, when designing the GB 2, how did the frequency of 71Khz come about? Finally, did you try higher frequencies such as 100Khz?

Thanks for your help in trying to figure all this out.

Reg

[Dave Johnson]

Re: Thanks Dave

Iron stays about the same with variations in frequency; it's the nonferrous stuff that looks more conductive as you go up in frequency.

100 kHz would have been a poor choice because the LORAN-C radionavigation signal occupies that frequency.

There's no special magic about 71 kHz. It had to be some frequency, and that's the one we happened to choose.

--Dave J.

[Roy]

Re: Thanks Dave

mmmmm... I don't want to be disrespectful, but I think Big D is keeping things under his hat!
Rgds Roy

[Dave Johnson]

yep, Roy, since I work in this field, certain things have to be kept under the hat. n/t

[Reg]

Re: I understand

Hi Dave,

I understand the reason you have to limit your response.

However, your answers to my questions were specific enough for my purposes.

Thanks again for your responses.

Reg