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Old 5th Apr 2016, 10:29 am   #1
crackle
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Default KB Kobra 305 TRF radio

I am currently restoring this radio.

An AC mains version of the TRF radio Kobra 274.

It looks all to be very original, and the most amazing thing is the 1931 capacitors in it all test as working very well. There is a large metal can capacitor block, it is labelled as 10.1uF. It is actually made up of 4uF 4uF 2uF and a 0.1uF. None of them draw any more than about 0.05mA at 350v and they all have lower ESR readings than any of my new components. They are also holding a charge well, so they will be left in circuit.


There is also 2x 1uF capacitor cans marked Kolster Brandes, these also have a very good result on the ESR meter and leakage current. The only capacitor that I will be replacing, when I get one, is a 200pF wax paper that shows a little too much leakage and as it is used to connect the aerial to the mains it will need a new one fitted, at the moment it is disconnected.

The mains transformer tested out as working well, as with the OP transformer.
There is no intervalve transformers so no problems there.
There is an on/off switch missing off the side, and a small paxolin strip which is a push fit into a lever which turns a coil in another coil. No problem there as trimmed Magnum lolly stick fits perfectly.

There is only one real problem and that is the energising coil for the speaker is open circuit. But using a replacement speaker I have found the radio works reasonably well as can be expected for a simple circuit like this.

I am looking for a circuit for the radio, cant seem to find one anywhere, I have PM'ed a couple of members on UKVRR who were talking about having the circuit back in 2010, but dont hold out much hope as they have not posted in the last 4 years. Has anyone on here got one?

Thanks
Mike
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Old 5th Apr 2016, 1:07 pm   #2
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Default Re: KB Kobra 305 TRF radio

Brilliant, I have just been sent a circuit, thanks John.

Mike
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Old 6th Apr 2016, 10:53 am   #3
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Default Re: KB Kobra 305 TRF radio

I spent most of yesterday cleaning up the image of the circuit that I received.
I have attached my cleaned up version here, and and also attached it in PDF format.

Having now seen the circuit I am surprised to find that the speaker energising coil, instead of being in series with the HT, is in fact wired across the full HT.
I did think that the smoothing choke was a bit over the top, but maybe necessary because the capacitors were only 4uF.

Anyway I am lucky the speaker I was testing the radio with was not damaged, it must have been taking in the region of 100mA through the coil.
I suspect the energising coil on the original speaker is a rather special one with a very high resistance.
I have dismantled the speaker and removed the coil and exposed the winding.
There is signs of corrosion in at least 2 places with quite a few wires broken. So as far as I am concerned it is beyond me to repair this coil.
I will have a word with Ed Dinning and maybe Mike Barker to see if either think a new coil could be made.
Attached is a drawing of the coil with dimensions.
Would anybody here be able to work out the length of wire in the windings and hence the approximate DC resistance.

The circuit diagram shows a modification of adding a 500k resistor wired across the energizing coil, I have no idea why this has been added but my radio does not have the modification.

Mike
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Last edited by crackle; 6th Apr 2016 at 11:14 am.
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Old 6th Apr 2016, 4:53 pm   #4
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Default Re: KB Kobra 305 TRF radio

I am not too sure what I am doing, but I have been trying to work out some details for the energising coil. i.e.
1.) what would the resistance of the coil be; seems like it is about 9.8k
2.) current taken by winding of coil, approx 24mA

And this is where I fall down as information seems conflicting and confusing.
3.) what is the current capacity of a coil of .11mm enamelled wire?

These are my calculations, see attached image, can anyone please confirm if I am on the right track, or put me right?

Thanks
Mike
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Old 6th Apr 2016, 7:58 pm   #5
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Default Re: KB Kobra 305 TRF radio

Length of wire in mm; start at (32.5 * (37/0.11=(336.37 coil former length)) * Pi = 1057
Then 32.5 + 0.22(layer2 32.72, layer3,ect)(coil thickness each side 0.11=0.22) * (0.11*336.37) * Pi (1057) until 58 is reached.

Not sure if this is the right way to work it out, each successive winding increases by 0.22 diameter, that is 116 layers (58-32.5)(/0.22). This is only if the windings are on directly top of each other, in practice the windings will go in the 'grove' of the previous winding reducing the actual winding diameter by the grove depth. The easy way is just to get a large coil of wire and start winding. Be good to get the original speaker working.
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Old 6th Apr 2016, 8:24 pm   #6
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Default Re: KB Kobra 305 TRF radio

I have tried working out the length of each layer in the way hetrodyne suggested in a spreadsheet, and the answer comes to 5556 metres. Thats pretty close to the 5543 metres that I estimated using the average diameter, so it looks like I am on the right track.
Mike
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Old 6th Apr 2016, 10:54 pm   #7
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Default Re: KB Kobra 305 TRF radio

According to the Preece equation 0.11mm copper wire will fuse at 2.9A. I have no idea how much to derate this value for your application.

Al
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Old 7th Apr 2016, 10:27 am   #8
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Default Re: KB Kobra 305 TRF radio

The insulation thickness should also be added to the overall diameter. A table shows these figures for soft annealed copper.

Metric 1.12 (American Wire Gauge)
0.00441 (thousandth inch, 41 SWG)
0.112 (millimetre)
533.8 (Ohms per 1000 foot)
1750 (Ohms per Kilometre)
0.163 ( maximum Ampere chassis wiring)
0.0277 (maximum Ampere power transmission)
1400 kHz (maximum skin frequency)
breaking force approx. 8 ounces
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Old 7th Apr 2016, 10:39 am   #9
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Default Re: KB Kobra 305 TRF radio

Hi Hetyrodyne
I think that is probably one of the sites I looked at. But what is the safe current in a tight close wound winding such as an energising coil.

Thanks
Mike
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Old 7th Apr 2016, 11:47 am   #10
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Default Re: KB Kobra 305 TRF radio

The speaker coil isn't used as part of a smoothing circuit and is directly across the DC. At a rough guess if the DC is 300 Volts and the coil DC resistance approx. 10K then 30mA is going through the coil, the half meg resistor across the coil is not going to bypass much current. Not sure what happens if 30mA is put through the coil, the wire rating looks just about adequate, though the inductance of the windings will presumably cause some heating of the core.
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Old 7th Apr 2016, 12:46 pm   #11
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Default Re: KB Kobra 305 TRF radio

Experimenting with a 10k resistor in series with another speaker with a 1k R energising coil the HT voltage is 190v, so this gives about 17mA for the energising coil.
The current through the choke to the radio circuits is 11mA.
I expect they did it like this as there was not enough current to energise the speaker magnet if it was acting as the choke.
I still dont know why they added the modification of the 500k resistor across the coil.
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Old 7th Apr 2016, 1:58 pm   #12
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Default Re: KB Kobra 305 TRF radio

The 500K resistor could be to damp any resonance, the speech coil must induce a small varying voltage in the field coil. If the HT is 190V then the field coil current should be within the limits of the wire. Must be quite a job to make such thin wire.
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Old 7th Apr 2016, 10:33 pm   #13
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Default Re: KB Kobra 305 TRF radio

With some help from Ed Dinning, who recommends to use 3A per square mm CSA. the sensible working Current capacity of .1mm copper wire in this type of application works out at about 23 mA. So if we can make a new coil of around 10k ohms or more, then it should not be over loaded on a 200 volt HT.

mike
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Old 8th Apr 2016, 6:23 am   #14
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Default Re: KB Kobra 305 TRF radio

Hi Mike
As a matter of interest, what valves are used in this set?

Peter
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Old 8th Apr 2016, 7:57 am   #15
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Default Re: KB Kobra 305 TRF radio

There are tables of "fill factor" for circular wires, which with the cross-sectional area of your coil will give you another way to estimate the number of turns.

Cross sectional area of former, fill factor, cross sectional area of wire and volume of winding will give you another estimate of the total length.

Weighing the coil, estimating the weight of the former and wire tables will give you another check on the total length.

If your micrometer read 0.11mm for the wire, that is the diameter over the varnish, and is the right diameter for calculating the number of turns. But you need to use the diameter of only the copper for current carrying and resistance calculations. For finer wires the difference in these diameters becomes more important. The resistance will be higher than your existing estimates, so the current will be lower. Of course, the cross section will be less and the fusing current will be lower, but fusing current is for a single strand with space around it.

In this case, using the resistance and the HT voltage, the total power dissipation can be calculated, and all this power has to come out through the surface area of the complete winding. There are other guides to temperature rise versus dissipation which will give an estimate of the outside temperature of the running coil, but the thermal resistance through the coil will create higher temperatures inside the winding.

Modern varnishes have better max temperature ratings than those of the period, at least.

David
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Old 8th Apr 2016, 8:51 am   #16
crackle
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Default Re: KB Kobra 305 TRF radio

Hi Peter
The valves in my radio were MHL HL4 ML4 DW2.
If any one is interested I have attached a PDF with updated information including component layouts.

Mike

EDIT BY MODERATORS: Replacement document sent by Mike and attached, correcting a valve number.
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Old 8th Apr 2016, 9:20 am   #17
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Default Re: KB Kobra 305 TRF radio

Noone seems to have suggested simply fitting a PM speaker. Not original, but very easy and totally reversible if and when.....
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Old 8th Apr 2016, 9:48 am   #18
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Default Re: KB Kobra 305 TRF radio

I know, it is a difficult decision to go ahead and spend around £55 to get the speaker repaired.
I have a small energised speaker which I have been testing the radio on and it does work (with adequate volume) wired in series with the HT.
But fitting a replacement one will mean adapting the shelf the speaker magnet is bolted to and the original speaker will probably get lost in time.

It is not a particularly attractive set, in fact I think it is rather ugly, it is not one that I am likely to want to use regularly. But the radio is probably one of THE MOST original ones I have in the collection. There are no obvious alterations or previous repairs, original speaker cloth with no tears, and a reasonable cabinet and back, and the other amazing thing is all the resistors and capacitors bar 1 are still working within tolerance.
It is nice to think all the radios I have restored are working, and I think this one deserves to be kept as original as possible.
There, I think I have convinced myself, get it repaired.

Mike
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Old 8th Apr 2016, 9:57 am   #19
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Default Re: KB Kobra 305 TRF radio

Thanks, Mike. I think you've made the right decision to spend the money to get the field coil rewound.
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Old 8th Apr 2016, 11:55 am   #20
crackle
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Default Re: KB Kobra 305 TRF radio

I had to make a small escutcheon for the on off switch, I used a piece of paxolin.
So whilst waiting for the speaker I have reassembled it to take some photos.
The 1uF capacitor seen in the photos on the right of the top chassis abd left of the under chassis photos are made by Kolster Brandes, The rectifier valve holder is also marked KB in the Bakelite moulding

Mike
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Last edited by crackle; 8th Apr 2016 at 12:04 pm.
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