Late 1960's BFO

[chemelec]

Variable chokes like that are fairly common.

Now its time to Correct the Schematic.

[Skippy]

I have to admit I was wondering if they had used the same basic circuit for both oscillators, when I saw the polystyrene capacitor/ 0.04 ceramic cap combination used in both circuits. Might we guess the 10mm coil and the search-coil are basically the same inductance? If the 10mm coil is only 5 - 25 microhenry, there won't be many turns on it. They typically have 100 - 150 turns of wire in 455KHz IF coils, with several hundred microhenry inductance. There's possibly less than 20 turns on this one. The internals of the coil are classic 10E / 10EZ type. I've just looked at my Toko data, it's probably around 15 turns on that core.

[HECTOR]

Well, this just simplified things X100!
A single 1.5 ohm coil.

Look at the pics and weep! (and laugh)
Who would have known!
A single coil with a movable slug. What's that called??? Adjustable choke? Is it still a transformer? I think not. Very tricky indeed
Time to move on with ease

[HECTOR]

WAIT TIL YOU SEE WHAT'S COMING!!!!

Those tricky Japs!

[Skippy]

The search-coil construction intrigued me, too, especially when Hector described it as 'double-wound'. Is the red cable twin-core? In which case the number of turns could be 10, increasing the L to around 25 microhenries. I wonder how they fed the red cable through the braided screen? That stuff can be expanded quite a bit in diameter, but it doesn't look mass-production friendly.

[satdaveuk]

The screening on the search coil is interesting, looks overboard, and got about inch gap but oviously did the job.

[jap]

Hi Skippy
Yes, it refitted it in Maintenance - you - arrived to answer - all of the above sell.
-I thank him - for drawing Except (T2) .
thanks Skippy
best time.
j.

[Skippy]

Post no.9, Quote:'Just ran it downstairs and put it on the freq counter. 75KHz'. Hence my comments about the 10mm tx in post no.15.
It's very easy to pull the pins on these transformers partially out of the plastic body, breaking the attached very fine wires in the process. The obvious question is - does the detector/oscillator still function correctly now the transformer has been refitted?

[jap]

be sure

Hi HECTOR
-(T2)10mm transformer(A critical piece) .(If you are must be sure it did not damage , the new mode is like this).
-Can you put a clear picture of the bottom of(T2) unknown transformer like this.
best time.
j.

[chemelec]

When I had it removed, I only got continuity between 1 & 3. The resistance was low but I don't remember the #.
I could take it out again. I have very nice soldering/desoldering gear.

Well it seems that either your Schematic is Incorrect or your Resistance readings are incorrect.
And does that T1 have a PART Number on it?

[HECTOR]

Based on the search-coil details, I estimate it has an inductance of 6600nH / 6.6uH. It resonates with caps C9,C10. If C9 were 1nF (can you find out what it is, Hector?) that would give a resonant freq of 1.9MHz, which seems way too high. If C9 were 10nF, the resonant freq. would be 680KHz, which is close to the measured reference oscillator freq of 662KHz, which would actually make sense. However, Hector measured the operating frequency of the detector as 75KHz, so I'm unsure of what frequency anything runs at.
I'm also puzzled by the 10mm transformer connections, surely pins 4 and 2 (and 6) must be joined internally to something?

I don't remember saying 75KHZ.
The operating freq measured at the SEARCH coil leads, with the threshold at a low hum, the freq was 662KHZ. 659-665KHZ Varies with thresh control.
When I tested the stand alone search coil with a sweeping sine wave and monitored it on the scope, the voltage doubled when it hit 950KHZ. If I swept it just a little either way, the voltage would quickly drop.
I'd be fine with taking the Tokyo coil out to test it again, if needed.
My test gear is mostly for audio. Is there a test we can do with an oscillator and a scope?
Here is a list of the equipment that I have.
Leader LBO-506A Oscope.
Phillips PM5131 Freq gen.
Goldstar AO 3001C freq counter/gen.
Fluke 8020B Multimeter.

Thanks again you guys!
Where did Homefire go?

[HECTOR]

Hi HECTOR
-Thanks,of a clear picture of the COIL.
-What are the measuring of(T2) unknown transformer in ohms between the pins1,2,3 - 4,6 ,,,,,,,5not used......(T2) in free from pcb.
best time.
j.

When I had it removed, I only got continuity between 1 & 3. The resistance was low but I don't remember the #.
I could take it out again. I have very nice soldering/desoldering gear.

[Skippy]

Based on the search-coil details, I estimate it has an inductance of 6600nH / 6.6uH. It resonates with caps C9,C10. If C9 were 1nF (can you find out what it is, Hector?) that would give a resonant freq of 1.9MHz, which seems way too high. If C9 were 10nF, the resonant freq. would be 680KHz, which is close to the measured reference oscillator freq of 662KHz, which would actually make sense. However, Hector measured the operating frequency of the detector as 75KHz, so I'm unsure of what frequency anything runs at.
I'm also puzzled by the 10mm transformer connections, surely pins 4 and 2 (and 6) must be joined internally to something?

[jap]

Hi HECTOR
-Thanks,of a clear picture of the COIL.
-What are the measuring of(T2) unknown transformer in ohms between the pins1,2,3 - 4,6 ,,,,,,,5not used......(T2) in free from pcb.
best time.
j.

[HECTOR]

I don't have an inductance meter to list the search coil specs. I checked the resonant freq with an oscillator hooked up to the open coil, and a scope to monitor voltage gain.
Voltage gain doubled at 950KHZ.
It's a 5 turn, 5"OD paired loop, shielded mono coil of something like #24 stranded. Just under 1 ohm resistance. (about .8 ohm)

The unknown transformer on the PCB is a 5 pin, not 6. I have already removed the part and tested for continuity. With the pins viewed as in my schematic, (starting from top left) there was only a measurable connection between pins 1 & 3. No continuity between any other combination of terminals. (meter set to 20M)
I have listed the operating freq at 662KHZ (which varies greatly from the resonant freq of the open search coil at 950KHZ).
I think that's all the information that I can give at this time.
Note that in my updated schematic, the +9V power trace has been routed correctly.