Late 1960's BFO

[Skippy]

I wonder if now is the time to think about 'modernising' and improving the circuit. I think adding some high-frequency power rail decoupling would be worth doing, such as 47nF from 9V to 0V. The non-standard (today) 0.04nF ceramic caps could safely be replaced by 47nF ones in all locations, I think. C7 could be replaced by 47pF, assuming the '50' means the cap is 50pF. Quick calculations suggest that adding 50pF in parallel with 2000pF will cause an audio output shift of 17KHz, which seems a bit excessive. I wonder if C7 may be 5.0pF? This would then give 5KHz shift, which is more sensible. Maybe this detail is one to sort out later. Anyone think that R20 (pot) and C7 should swap positions? Then the pot wiper and one fixed end could be grounded, the other fixed end going to C7.

[jap]

I wonder if now is the time to think about 'modernising' and improving the circuit. I think adding some high-frequency power rail decoupling would be worth doing, such as 47nF from 9V to 0V. The non-standard (today) 0.04nF ceramic caps could safely be replaced by 47nF ones in all locations, I think. C7 could be replaced by 47pF, assuming the '50' means the cap is 50pF. Quick calculations suggest that adding 50pF in parallel with 2000pF will cause an audio output shift of 17KHz, which seems a bit excessive. I wonder if C7 may be 5.0pF? This would then give 5KHz shift, which is more sensible. Maybe this detail is one to sort out later. Anyone think that R20 (pot) and C7 should swap positions? Then the pot wiper and one fixed end could be grounded, the other fixed end going to C7.

Hi Skippy
-(today) 0.04nF ceramic caps..... rong its 0.04uf=40nf safely be replaced 47nf ....ok
-I think. C7 could be replaced by 47pF, assuming the '50' means the cap is 50pF....ok its C7=50pf =~ 47pf.
-- There is an error in connected R20
best time
j.

[jap]

To Skippy ,HECTOR
best time.
j.

[HECTOR]

The screw must be seated with thread lock. It won't budge.
It is a 5K switched pot on metal, grounded standoffs. (they serve as the switch)
There are no hidden circuit traces underneath.
I double, double checked the R-20 connections. Can't find any errors.
I didn't show the R-20/switch combined because CircuitMaker 6 Student doesn't have that option. This is also the first time that I've used this free online circuit design program.
I have piles of junk PCB's and surplus components. Could probably do a breadboard of this if I had the time. Was hoping you guys could help put things in fast forward.
I've looked at several basic BFO designs online, but none look like this one.
Do you guys think that this one is somewhat unique?
A long range pinpointer was my first thought. Maybe it has other potential?
I most certainly appreciate the interest and assistance

[DavidB]

It looks like the detector was copied from a Jetco Mustang. I traced one about 40yrs ago when I was in high school. I still have that drawing somewhere.

[HECTOR]

It looks like the detector was copied from a Jetco Mustang. I traced one about 40yrs ago when I was in high school. I still have that drawing somewhere.

Very interesting. You're probably right.
I looked it up online and the description sounds dead on. There's a pic of the back of the PCB too. I think that with a modified coil and some love from the Geotech crew, it would make a nifty pinpointer. Maybe put an LED ladder on it for those who don't want to hear it whine?

[Skippy]

Speaking of LEDs etc, I was wondering if the two missing components next to the audio transformer might be for a meter or LED connection? Looks like one part is a diode (it's labelled on the PCB), one maybe a resistor, with some other device wired in to the PCB.

[jap]

To Skippy
And the one missing components next to (T2)10mm transformer maybe a resistor or
ceramic capacitor.
best time.
j.

[HECTOR]

Any suggestions for a small pinpoint type coil?
I've been playing with an old AM antenna (ferrite rod), using different wire gauges and # of turns.
Can't get it close without a tuning ring, even then, it doesn't have much distance.
?

[HECTOR]

Hi Hector

It might be worth redrawing the circuit in this sort of layout, only neater and without any errors I may have added:

OK, I did the hard part.
Who wants to connect the dots and make it reel purdy?
You can download the program for free.

http://babbage.cs.qc.edu/courses/cs343/Circuit_Maker/

Now all that I need to do is figure out how to export the file so Geotech will accept it as a valid file.

PS....I was successful in making a small coil but I'd like to get more than an inch.

[Skippy]

Why is R12 still shown as 60K? Obviously, we don't have any views of the components that enable us to see this part, but it has to be something different, like 68K? Maybe the colours have faded or something, I suppose you could ultimately remove one leg and measure it with a meter.....
I think ferrite cores are unsuitable for BFO coils, they are very temperature sensitive. A suitable size for an air-cored coil,to start with is 35 to 40mm diameter. I suggest you take a look at the Scarborough EPE magazine mini-pinpointer design for ideas. I've made a 35mm coil for that, with self-adhesive foil screen. It would find a 25mm copper coin at 30mm distance.
http://david.alfa-romeo.eu/elektro/EPE/2003/EPE%202003-07.pdf

[HECTOR]

Why is R12 still shown as 60K? Obviously, we don't have any views of the components that enable us to see this part, but it has to be something different, like 68K? Maybe the colours have faded or something, I suppose you could ultimately remove one leg and measure it with a meter.....
I think ferrite cores are unsuitable for BFO coils, they are very temperature sensitive. A suitable size for an air-cored coil,to start with is 35 to 40mm diameter. I suggest you take a look at the Scarborough EPE magazine mini-pinpointer design for ideas. I've made a 35mm coil for that, with self-adhesive foil screen. It would find a 25mm copper coin at 30mm distance.
http://david.alfa-romeo.eu/elektro/EPE/2003/EPE%202003-07.pdf

R12 is a 68K (65.5 to be exact)
Thanks for the link. That's a simple, compact unit. A wristband mount and a finger mounted coil would be slick. I'd like to hear what it sounds like.
I found that if I make about a 20MM 40 turn coil and split it, then I can move the two pieces to tune. It reacts to a quarter at about 25MM. Will it make a big difference if I shield it carefully? I like the way the tone varies with proximity.
My CenTech pinpointer is either off or on. Has anybody around here broken into one of those yet?

[Skippy]

I felt that the foil screen was a bit excessive, and probably reduced sensitivity. I think a graphited-paper or similar screen would help a bit, and it would be worth trying on your circuit. One problem I found with the EPE circuit is the sensitivity to hand capacitance on the RC oscillator/pot area, it needs to be shielded well. It's also not stable thermally, amongst many other shortcomings.

[homefire]

Some of the Old BFO used Copper Tubing for Shielding on the coil.

[Skippy]

I've just had another study of my Toko datasheets, to pin down construction details of the 10mm can inductor. Though Hectors coil is not a Toko coil, I think it will be similar to Toko's 10E and 10EZ range, commonly used in 455KHz IF stages in radios. The inductance of these cores is determined by the number of turns and the specific inductance,AL (big A, little subscript L).

Inductance = AL x (number of turns)squared

Analysing the data for a number of these coils, it seems that AL is close to 25 nH for the 10E core. From this, to make a 60 microhenry coil for this project, we need 49 turns of wire. Commonly, 455KHz coils/transformers will have around 170 turns, so it should be possible to salvage enough wire to rewind them for this project, but Q would be improved if slightly thicker wire were used, or you may be able to rewind the original wire bifilar. These cores have a wide adjustment range, possibly +/-20%, so number of turns is not too critical.
I also have data for 7mm and 5mm coils if anyone fancies miniaturising it, eg. for pinpointer use.