KB 214 "Transportable" radio

[boater sam]

If you can find the layer that is open cct, can you not bridge the gap?
If it open then it is not a shorted turn or turns.

[Crackle]

Thats a good idea Sam, but I have started to unwind the outer layers and so far I have not come across more than a couple of hundred turns before another break. The breaks are caused by multiple "green spots" of corrosion presumably where there were pinholes in the original enamel.
You can see the "green spots" in this photo
   
This is the point at which continuity ends.
   
I am wondering now if the 11k ohms measured is almost double my estimate because of resistance through the green spots. My efforts to reuse what good turns remain may be in vain, and another spot fail as soon as a little current goes through it.


Mike

[boater sam]

I was afraid that you would find the dreaded green spot corrosion had wrecked the winding, its all too common.
You could try passing a current through it for a while till it warms slightly but I think you will end up rewinding it completely.
11k is very high, usually I find these to be 4k to 5k only. From wire table you could find the DC resistance from your estimate of length taking the mean diameter of the winding.

[Murphyv310]

Hi.
The one I have here is 4.8k DC resistance.

[boater sam]

That's what I would expect too.
Its a beast of a coil to wind, you will need to make a bobbin I assume.
Just a thought, would circular ferrite magnets out of magnetrons fit and turn it into a PM speaker? Save a lot of work, then you need a smoothing choke for the HT, or is it across the HT?

[Crackle]

This wire is 0.15mm, about 35AWG, according to the wire tables I found on the internet https://en.wikipedia.org/wiki/American_wire_gauge , the resistance is 1.08 ohm per meter, and a current carrying capacity of about 200mA.
I suspect the 11k, which is almost twice my original estimated resistance of 6k, is because of further green spots which although there is continuity, there may be only a few copper atoms thickness where there are greenspots. The test will be, as Sam says, if I try and pass current through the remainder of the coil, say start off at 10mA and increase voltage to get the current up to 30 or 40mA, and then see if it holds, or fuses a dodgy green spot.
I have variable o to 300 volts supply which should do it, I believe it is capable of 100mA.
Back to unwinding the layers, I gave up using my little coil winder as there were so many breaks and nothing worth salvaging, and resorted to just pulling it off hand over hand. I have what looks looks a mass of coarse auburn hair in my bin.
Mike

[Doodlebug]

I had a Ferranti intervalve open cct.
Just bought a good, smaller one in size, took out its innards and put into Ferranti
case.
Job done.
Just like restuffing old C's.

[Crackle]

The good news is I have a continuous coil, I found the connection only 16 layers in, so have about an additional 5 layers than expected, it is measuring 11.8k ohms DC.
I tested it as mentioned, the power supply I used for testing it had a constant current function and I took the supply up slowly to 40mA (the max it would supply) at this current it was putting 460 volts into the coil, which I sustained for around 8 minutes.
Fingers crossed it should still work in the speaker, though for how long nobody knows.
   
This is how to make a join in the wire, whilst winding, this one was done in 1930. I wonder if it was an accidental break or a change of reel.
   
Mike

[Murphyv310]

Hi.
Not exactly pukka, I have opened up an intervalve transformer and added an anode load resistor, FET and another few components to drive the output with total success, only one other external connection is needed namely ground that can be an existing ferrule used for a mounting screw. Like I say not pukka but only you would know...... Oh and anyone you tell.

[Murphyv310]

The good news is I have a continuous coil, I found the connection only 16 layers in, so have about an additional 5 layers than expected, it is measuring 11.8k ohms DC.
I tested it as mentioned, the power supply I used for testing it had a constant current function and I took the supply up slowly to 40mA (the max it would supply) at this current it was putting 460 volts into the coil, which I sustained for around 8 minutes.
Fingers crossed it should still work in the speaker, though for how long nobody knows.

This is how to make a join in the wire, whilst winding, this one was done in 1930. I wonder if it was an accidental break or a change of reel.

Mike

Hi Mike.
At 11k it must go between HT and deck, I've come across this type before, there are some advantages, if it does the set will have a choke if not its unusual to be so high, there will be a fair old voltage drop.