Late 1960's BFO

[Skippy]

Now it's starting to make sense. My original estimate of 25 microhenry for the search-coil was pretty close. With a parallel cap of 2000pF / 2nF, and a measured 660KHz oscillation, implies the search-coil inductance is close to 30 microhenry. (2nF and 30 uH resonate at 650KHz). The reference oscillator has a 1000pF capacitor, so I deduce that the 10mm inductor will be 60 microhenry in value. (it is likely to have around 50 turns on its bobbin core)

[HECTOR]

Major error: a non-existant connection between the audio transformer primary and +9V supply has just recently appeared, shorting out the transformer.
Resistor queries:- R4: check the value, it made sense when you showed it as 68K, now it's become 6K8 I'm not sure. Similarly R12. 60K is a NON-standard value, re-check the resistor, I expect it to be the same value as R4, probably 68K. R14: 78K is a NON-standard value, check it again, please.
Capacitors: You have to establish the values of C4 and C9. If C4 is marked as you show on the circuit, then it's value is 10nF, which seems plausible. C9 I expect to be the same value, can you confirm what it says on it? It is absolutely not 1 microfarad, it would be 2 inches long if it were.
Now we have established the search-coil construction, could the circuit be amended to show this? Remove the reference to 950KHz and 5 turns, just label it as 25 microhenry, 120mm diameter. This is a reasonable working value, it could be 20 or 30 uH, it's not that important.
Minor details: If you can add a representation of the 10mm inductor's screening can, connected to ground it would be nice. It sounds trivial, but it plays an important part in the way the circuit functions, stopping capacitive and magnetic coupling. What is the odd extra loop of 0 Volt wire around the 5K pot symbolising?
Maybe jap could add his suggestions, too?

Thank you many times for the clarification Skippy!
I believe the necessary corrections have been made.
The threshold pot (R20) is PCB mounted on metal standoffs with a connected metal piece covering the resistor area. The main body of the pot is plastic. It's supposed to represent shielding (screening).

[jap]

Hi Skippy
-Good,,,All u correction true.
best time.
j.

[Skippy]

Major error: a non-existant connection between the audio transformer primary and +9V supply has just recently appeared, shorting out the transformer.
Resistor queries:- R4: check the value, it made sense when you showed it as 68K, now it's become 6K8 I'm not sure. Similarly R12. 60K is a NON-standard value, re-check the resistor, I expect it to be the same value as R4, probably 68K. R14: 78K is a NON-standard value, check it again, please.
Capacitors: You have to establish the values of C4 and C9. If C4 is marked as you show on the circuit, then it's value is 10nF, which seems plausible. C9 I expect to be the same value, can you confirm what it says on it? It is absolutely not 1 microfarad, it would be 2 inches long if it were.
Now we have established the search-coil construction, could the circuit be amended to show this? Remove the reference to 950KHz and 5 turns, just label it as 25 microhenry, 120mm diameter. This is a reasonable working value, it could be 20 or 30 uH, it's not that important.
Minor details: If you can add a representation of the 10mm inductor's screening can, connected to ground it would be nice. It sounds trivial, but it plays an important part in the way the circuit functions, stopping capacitive and magnetic coupling. What is the odd extra loop of 0 Volt wire around the 5K pot symbolising?
Maybe jap could add his suggestions, too?

[jap]

Hi Skippy
Step by step,not in a hurry.
best time.
j.

[HECTOR]

One mistake? There's still several circuit diagram mistakes/oddities/queries.

OK.....
I thought that I was doing a fair job of criticizing myself
So, besides the mislabeling the 10 turn search coil, what do you find amiss?
I posted this little project here so you genius designer/engineers could give my inexperienced butt a boost (and maybe you would find it interesting enough to breadboard it for experimentation) Now that we've solved the coil(s) mystery, it should be close to the point where someone could try a working model. I could try, but I figured you guys could do in an hour, what would take me a week
Once again, my electronics experience is very basic, mostly audio diagnostic & repair.
I can solder with the best of them
As you probably already know, audio circuits are MUCH easier to understand than this stuff.
I'm happy to correct any mistakes that are pointed out to me.

[Skippy]

One mistake? There's still several circuit diagram mistakes/oddities/queries.

[jap]

It's a 5 turn, 5"OD paired loop, shielded mono coil of something like #24 stranded.



I think that's all the information that I can give at this time.
Note that in my updated schematic.

Hi HECTOR
It's just counting the number of turnes,There are now 10 tu and previously 5 tu,Where trust in what establish?
-there is one mistake in modified pictur!!!!!!!

j.

[HECTOR]

It was really bugging me so I exposed the coil a bit more.
I haven't made so many mistakes in so long, I forgot what it was like.
Why didn't I do this before?
Disregard previous description of coil. Logic was telling me that it would just cancel itself out.
Pretty sure the wire is #28 7 strand.
Looks like 10 winds.

[HECTOR]

Just realized I forgot to delete a connection on R7.

[HECTOR]

I've made some corrections.
Also,
To clarify my description of the search coil. It's wire folded in half, or a flattened out U wound into a coil, instead of a continuous coil. That would make it fairly easy to insert into the shielding.
How would you calculate that??



Should I just get one of those cheap LC meters on ebay, from china?

[jap]

Hi HECTOR
thanks!
Correct the Schematic
best time.
j.

[jap]

Hi Skippy
- u correction true ,Q4 is not the mixer!!!(u here in MD area-not radio)
best time.
j.

[Skippy]

There's an error in the R7 connections, it doesn't connect to C10. R12 is likely to be 68K, by comparing to the other oscillator. C9 value, is it 10nF?
The two oscillators are almost identical, the freq shift of the reference one is done by bleeding in a variable amount of 50pF cap in parallel with the 10nF oscillator cap, giving around 2KHz frequency shift. Q4 is the mixer, it appears to be linearly biased, but it is evidently driven hard, so it saturates and the non-linearity does the mixing. Cap C15 filters out the high frequency component, leaving the mixed audio, which goes to the power amp Q3.

[HECTOR]

Variable chokes like that are fairly common.

Now its time to Correct the Schematic.

Yes, I do believe I've achieved success!
It runs now with no errors.
I'll double check the values. It's been a long time since I tried to read color bars on resistors. I usually stick a meter on them to avoid errors. I'll do that . Caps get me sometimes too.