Garrett BFO schematic

[Carl-NC]

I consider the Garrett BFOs collectively to be a "classic" in metal detector history. Garrett was a leader during the brief reign of BFO and the pinnacle of their efforts was the Master Hunter BFO discriminator, the last model sold in 1981 as BFO was gasping its last dying breath. I own a couple of these units, along with the vast array of coils that were available, from a 3" puck to a 24" square, and a weighted coil with a 50-foot cable. The coil selection was one reason people liked those old BFOs so much. Otherwise, they weren't much to talk about.

I've long thought the Master Hunter BFO wasn't really a BFO, but more likely an off-resonance design that gave it the ability to discriminate. I was recently moved by the curiosity spirits to pull open one of my units and trace out the circuit. Of all the schematics floating around the Internet, I've never seen this one, so here it is.

As it turns out, it really is a BFO with a crystal used in the reference oscillator. I suspect the White's "zero-drift" BFOs did the same. But Garrett also takes the amplitude modulation off the search oscillator and combines it with the mixer output to drive the meter. I suppose that in a way similar to off-resonance there is enough variation between the two to get some crude discrimination.

GarrettBFO.pdf

[waltr]

Interesting and thanks for tracing the circuit and drawing the schematic.

One minor error: The inputs to IC1a are backwards. The feed back should be on the inverting (-) input.

What are the op-amps, IC1, IC2 & IC3? My guess is fairly common duals and not critical.
Any idea what the XTAL frequency is?

[Melbeta]

Carl
I used to be a dealer with Garrett back in the good old days. I used the Master Hunter BFO with the entire package of coils. I still have a pristine Garrett green case to hold the entire MH package of coils. I worked the mountains of Colorado with the BFO before I turned to the IB (TR) and the VLF machines. I never had one of them break down so did not have a schematic of them. I have fond memories of the Master Hunter and it had NO drift at all from that crystal. The ones I used had NO discrimination in them at all as back then I wanted gold nuggets and did not want to discriminate anything out.

I also was a dealer for Treasure Tronics through Ray Smith, with his NTHL company. I used the SST Treasure Pro for a year in the Cripple Creek Mining District, and the discrimination did work, but only good/bad kind, and the drifting was bad moving from sun to shade areas. But I loved the light weight of the airplane aluminum.

I never had the weighted coil as no one seemed interested in using it other then for working the mining areas in Colorado.
Melbeta

[Carl-NC]

IC1a is correct, it's used as a comparator with hysteresis. All opamps are LM358.
Haven't measured the crystal yet, my vague recollection is north of 200kHz.

[homefire]

Carl ! Any off hand info on the Coil ?

[Tim]

Much appreciated Carl. Crystal and coil info would be appreciated also. Despite its age, this and the Compass 94B are on my list for the Sprint designs.

[Carl-NC]

Carl ! Any off hand info on the Coil ?

120uH from older measurements.

One thing I like about Garrett BFO coils (and the later Sea Hunter PI coils) is that the cross-section of the loop is perfectly round. I don't recall any other company doing this. A round cross-section is ideal for a loop moved through water.

[waltr]

IC1a is correct, it's used as a comparator with hysteresis. All opamps are LM358.
Haven't measured the crystal yet, my vague recollection is north of 200kHz.

"comparator with hysteresis"- Ok - wired correctly then.
Must study this schematic some more.

Thanks Carl

[dbanner]

120uH from older measurements.

One thing I like about Garrett BFO coils (and the later Sea Hunter PI coils) is that the cross-section of the loop is perfectly round. I don't recall any other company doing this. A round cross-section is ideal for a loop moved through water.

I take it there are two coils in the search head? 120uH each respectively? Smaller for pinpointing? 120uH too small for PI machine? How about two coils in series like dual field setup? Finally, good circuit for pinpointer? Can the varicap be eliminated in favour of variable resistor setup for tuning?

[dbanner]

Is the coil shielded?
https://m.youtube.com/watch?t=15s&v=OO-LTRYUlRs
This one looks brand new! Old school!

[homefire]

120uH from older measurements.

One thing I like about Garrett BFO coils (and the later Sea Hunter PI coils) is that the cross-section of the loop is perfectly round. I don't recall any other company doing this. A round cross-section is ideal for a loop moved through water.

Thank You Sir !
T

[chuubaba]

Carl are there any pics of PCB and Components

[Carl-NC]

No, but before I put it all back together I'll take some & post. I'm in El Paso this week, the Garrett is in Oregon. So it'll be a week or 2.

[homefire]

I'm 100 miles from El Paso.

[dbanner]

Thanks for posting the schematic carl. My first metal detector build was a bfo. I may just build this one to see out of curiosity. I just can't resist seeing a circuit, even a bfo, without making the dam thing. I think it's a disease.