Fisher 1266 PART 2

[Funfinder]

Thanks for your answer, I'm happy that you like it.

This Forum really makes new appetite built your own
detectors - which I've done already 15 years ago.

Found an IC LM324M op-amp, perhaps i can invent something.

[Mikes]

Ivconic!







Best regards!

[Gyur666]

I’m going to build a 10,5” DD coil for 1265. Can somebody help me about it with parameters of coil?

[Gyur666]

Nobody made a good DD coil for 1265/66? I use fisher spider coil, but it has destroyed. Our country the 10,5 spider 190 euro

[Funfinder]

try to get a chinese MD3030 and use the coil of this MD for your Fisher.

Fisher 1266 PART 2 - Page 3 - Geotech Forums
7 Jan 2009 ... Page 3- Fisher 1266 PART 2 Schematics. ... For coil i am planning to use CC coil from MD3030. ... Much better than original Fisher coil. ...

[Sean_Goddard]

SOMEWHERE I have a schematic of a 1266 with oscillograms of ALL important points. It may help those of you attempting to build your own. I'll have a dig around and if I find it I will Zip it up and post it.

[Davor]

I looked at the Fisher 1266 schematics, and it seem to me there is a part that can be omitted which would make this design somewhat more desirable to replicate.
It is the 8:800 transformer. The LM339-s have PNP input transistors, and the common mode voltage includes negative rail. That means that the comparators can be hooked directly to the 1 ohm resistor, and work happily ever after.

[zoomtronic]

Ivoncic,
Did You managed to finalize detector?
kind regards

[technos]

SOMEWHERE I have a schematic of a 1266 with oscillograms of ALL important points. It may help those of you attempting to build your own. I'll have a dig around and if I find it I will Zip it up and post it.

Ok, this would be good. Dated thread but it would help my understanding of this circuit here!

[Dave J.]

I looked at the Fisher 1266 schematics, and it seem to me there is a part that can be omitted which would make this design somewhat more desirable to replicate.
It is the 8:800 transformer. The LM339-s have PNP input transistors, and the common mode voltage includes negative rail. That means that the comparators can be hooked directly to the 1 ohm resistor, and work happily ever after.

Since that's so obvious, I wonder why the original engineer bothered with the transformer?

--Dave J.

[just havin' some fun here]

[Davor]

From what I've seen in 1225 which uses a transformer and a transistor to amplify a signal off of a 1ohm sensor, and makes a lot of sense, it was only logical step to keep the good solutions in 1266. My thought about omitting a transformer is IMHO also logical, as such transformers are not so common any more. Using 1ohm current sensor is a beautiful solution for producing accurate quadrature phase reference. With ~100mA coil current you have ~100mV at 1ohm, which is plenty for a comparator input. More voltage swing and shifted in the midst of a common mode is a more comfortable way of doing it, but omitting a transformer is also a worthy goal. If risk of going beyond a common mode range is a limiting factor, a simple biassed (LED) diode may shift the sense resistor signal up.

I'll try this some time and share my findings about the transformerless approach.

[Dave J.]

Davor, you bit the bait!

--Dave J.

[still just havin' some fun]

[Dave J.]

I think everyone looks at SNR. Intensely.

Often more intensely after the prototype has been built, than before.

--Dave J.

[dbanner]

Why did this 1265 never turned out to be a viable project? So simple schematic, no exotic parts, transformer easy to salvage, PCB layout available, so why? Perhaps the crystal hard to obtain? This is good project to revive and make complete on single PCB. What say yee?

[Koala]

Why did this 1265 never turned out to be a viable project? So simple schematic, no exotic parts, transformer easy to salvage, PCB layout available, so why? Perhaps the crystal hard to obtain? This is good project to revive and make complete on single PCB. What say yee?

Take a a look over on MD4U.RU both DIP and surface mount. Single and twin boards. Follows on from the work here. Several versions but can't tell you which is the best.