Fisher 1266 PART 2

[mayky]

There is good quality image from the top side? I can not wire the top side. Too dificult for me.

[Davor]

Ouch! Now you must be extra cautious because it did not blow on its own.

As for the wet fields, especially those regularly exposed to fertilisers, none of the classic VLFs have a proper GB to work on. Even if you extend the GB phase shift, you'll ruin the discrimination range in a process.

[Koala]

I have you in mind, unless someone else sends you photos first. My gear is some 30km away. I have a faulty 1266 electronics that I bought cheap. Perhaps one day...

Thanks.


Get yours fixed.


Not quite sure which fields it excels on. But on some it seems to out preforms virtually every other machine out there. Last rally I was on it was superb found part of the rim of a Charles 1st penny, William 3rd sixpence turn in jewellery and a Victoria four penny, part of a lead alloy hawking whistle and a lead tobacco jar lid. Not bad for a field in the middle of nowhere.

Doesn't work well on waterlogged fields though.

Mines gone worse. Stopped working altogether. Quick look around and the FET VP0104N3 has smoked.

[Davor]

I have you in mind, unless someone else sends you photos first. My gear is some 30km away. I have a faulty 1266 electronics that I bought cheap. Perhaps one day...

[Koala]

If someone could share a photograph the original wiring to the toggle switch. Mine has always played up. Changed it and still the same. Beeped a few of there wires through and it appears to be different from the schematics.

Found a few photos but the switch is always just out of the shot.

[kt315]

http://www.md4u.ru/viewtopic.php?f=63&t=1291&start=1200

praktik did share lay file of the pcb in DipTrace, smd. if you are still not in course.

[Sean_Goddard]

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

Was on a hard drive wich didn't learn 2 fly properly and fell down the stairs. No long dead sorry. Shame as it took a long time to put that together

There's time I'll never get back.

[Sean_Goddard]

Hello Sean your right , Thanks for the information you posted earlier . NO FOCUS .....I do not need to use the T-1 at all !! I always like to keep my projects as close to the original design as possible and also use "almost identical" parts . I have seen other vlf detectors using this method for 180 degree phase shift , RF not audio !!!! .............I am good now , I have all the information I need and Thanks again for the schematics too !!!................Eugene

Sorry didnt see your post

To others: Use a transistor splitter fergodsake..Please take a look at what the tfr ACTUALLY DOES otherwise ANY bogostandarto 600R audio tfr will do. I doubt the turns ratio will be critical given it is only used to provide two OUT OF PHASE outputs (correct me if I'm wrong please Dave J).

[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.

[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?

[Dave J.]

I think everyone looks at SNR. Intensely.

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

--Dave J.

[Dave J.]

Davor, you bit the bait!

--Dave J.

[still just havin' some fun]

[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.]

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]

[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!