KB Kobra 305 (AC 3)

[crackle]

I am currently restoring this radio.

An AC mains version of the TRF radio Kobra 274 which I restored a little while ago.

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

[crackle]

A kind gentleman on UKVRR has sent me the circuit so that has saved me a headache trying to trace the wiring.

ike

[Michael Watterson]

I look forward to the final report. Looks interesting,

[crackle]

I spent most of yesterday cleaning up the image of the circuit that I received.
I have attached a reduced copy of my cleaned up circuit here, and on UKVRR is a PDF with a larger image.

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.
As I had originally thought that the energizing coil was in series with the HT I did think that addition of an additional 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 are 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. Other differences in my version is that C1 and C2 are NOT ganged, as mentioned in the notes on the circuit diagram.

At the moment I have the radio running with a spare speaker with a 22k resistor in series with the energising coil. the HT has also now risen from 106 volts (full HT across energising coil) to 240 volts.
If winding a new coil proves too expensive then I will rewire the spare speaker in series with the HT.

Mike

[Michael Watterson]

Strange that the field coil isn't used as a choke!

[Cathovisor]

Strange that the field coil isn't used as a choke!

Murphy did this with the D28 console: there may well have been insufficient magnetising current if used in the 'traditional' manner in the KB. Ekco famously used a resistor that did nothing but get hot in the AD65 to provide said extra current consumption for the field coil.

[crackle]

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 at 190v.
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.

Experimenting with the controls, very fiddly to set all 4 to the right point, I managed to get the radio to resolve 6 stations as I went up and down MW. On LW, not so good, just R4 with a hint of something else. Reaction on LW does not seem to do anything, it probably needs a 100m long wire aerial to get much more on LW.
One annoying thing is the lack of a proper volume control.

Mike

[BriancuffRest In Peace]

One annoying thing is the lack of a proper volume control.Mike

I have seen reaction controls on early sets labelled "volume" so I suppose, with not too many stations around in the early days, it was OK to lose gain/selectivity thus volume without resorting to a costly pot.

[crackle]

Hi Brian
Yes it probably would have been easier in those days, but I find it almost impossible to resolve (separate) BBC Essex and Smooth at the high frequency end of MW with out having the set at almost full volume.

My tests showed that with a 12k load in place of the energising coil the HT was about 200 volts.
It works out to just over 5.5km of .1mm copper wire needed, as it has to be over 10k ohms in order to be able to work on the 200 volt HT. I am advised that the safe working current on .1mm copper in a winding like this works out to about 23mA.
I am planning to ask Ed Dinning to wind a new coil for the speaker.

Can any one suggest a good reason why KB made the modification of adding the 500k resistor across the energising coil.
My set did not have the modification, I wonder, should I make it, I cant see any point in the resistor?

Mike

[crackle]

I have put the radio back together whilst I wait for the speaker to be repaired.
In the meantime here are some photos,

On UKVRR I have uploaded a new PDF containing annotated schematic with additional notes, and annotated component layout photos.

Mike

[crackle]

Here is the final instalment of this restoration.
The energizing coil arrived back from Ed Dinning today. He has put 32,000 turns of 0.1mm wire on giving it a resistance of 10K ohms.
I have reassembled the speaker and made a couple of "improvements" to the appearance of the coil, to make it look more in keeping with the age of the radio.
It is working very well and the radio is surprisingly loud and very good quality audio for a 3 valve TRF, easily as loud as any 50's valve radio.
Now I have worked out how to adjust it, it is also quite sensitive and selective, not quite as good as a superhet on selectivity but very impressive for a TRF.
Some more photos of the speaker and finished radio.

Mike

[BriancuffRest In Peace]

What an excellent job, Mike and what an unusual construction & layout - one that really needs saving for posterity.
I notice that there is a 2pin 5A plug inserted into the paxolin panel at the rear. Is this the power input or a safety interlock? I'm asking as it was typical for electric player pianos and other instruments to have a socket mounted on them for power input meaning a bare plug on the end of the cable from the household socket . This even extended to a German orchestrion I owned until recently, which needed 220VDC input. The electrics were incredibly crude with two huge brass terminals standing proudly on top of the open framed motor, all at 220VDC

[crackle]

Hi Brian
"No veneer in ere" well there maybe in the speaker baffle.
Yes, the round 5 amp plug was inserted into the safety cut out by me to enable the radio to work with the back removed. The back has a flush fitted 2 pronged plug mounted on it. I useful safety feature which seemed to be forgotten as time went on. I am lead to believe this was a standard method for service engineers to use and is why it was designed this way.
I am impressed with the originality of this radio and also the great sound reproduction and operation. One of the reasons that I was happy to pay to have the speaker energising coil rewound. I didn't consider it a job that I could tackle as the wire was so thin.
The audio quality seems far superior to the quality of the KB 274 Battery Kobra which has the balanced armature speaker, described as sounding like a wasp in a bottle.

Mike

[EDDINNING]

Hi Mike, speaker really looks the part now, glad to hear the whole set sounds good too.

Ed