KB 214 "Transportable" radio

[Crackle]

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.

Hi Trevor
That could be a useful circuit to try, you can publish the schematic and part numbers.
Thanks
Mike

[Murphyv310]

Hi Mike.
Its years ago but I'll put my memory into gear and draw out a circuit, values may be out a bit but you'll get the drift.

[Crackle]

The speaker energising coil is finished, refitted and tested.
I used the original cloth tape which had no stick left, and worked it through the center and around the coils, and modern cloth tape to over bind around the edge of the coils.
I then coated the finished lump with copious quantities of molten wax (from old box type capacitors)
I plan to make a new speaker surround to replace the chamois leather which had gone very brittle and was crumbling.
I have decided to use ripstop nylon (the type used for kites) as the new surround, and possibly coat it in a rubber type solution, I will let you know how it goes.
   

I have just finished the new back panels. Well they were panels which were cut by the previous owner, but they needed a fair bit of tidying up where the cutting out was a bit wobbly.
I sanded the black paint off the outside of the panels and stained and lacquered them to try and match the oak veneer finish that the original ones had. I used home made stained varnish, a mix of Ronseal outdoor varnish, mixed with dark jacobean stain, and some redwood Sadolin to help add some filler to the paint and tone down the softwood grain.
A bit of a mix up you may say, but these oil based stains and varnishes are getting difficult to find now, and I just dont get on with the water based stuff.
   
All the labels, trims and hardware are now refitted to the new back panels and I think they look the part.
   

I would like to know what that brass cone, which is fitted onto the back, is for. Any ideas, have you seen anything like that before?

Mike

[Crackle]

I took a look at the power supply today, the mains transformer seems OK but it looks like it may have had a hard time, the mains tapping was set to 200 volts. I suppose that may have been correct, but chances are it may have been put there to attempt to boost a failing performance.
There is a large choke which is O/C, that shouldn't be too much problem though.
I found a 10k resistor, marked as 10T I suppose the T may stand for thousands.
   

I also found out what the brass cone was for. It is a port hole to access the jack socket for the "gram" input on the rear of the radio chassis.
Mike

[Refugee]

That looks more like a very old carbon film resistor. If it is it will most likely be 10K as you say.

[Crackle]

The new capacitor block is complete, I used a secondhand tag board to mount the new polyester capacitors on.
The reservoir cap should be 4uF, the smoothing one 4uF and there are 2 more 2uF's and a 0.1uF, making a total of 12.1uF, as stated on the top of the box.
The open circuit on the choke was just 1 out of the 3 sections, so I have bridged the faulty section. Because the effect of the choke will be less I have doubled up on the smoothing cap to help compensate.
I may have to fit a dropper resistor in the circuit to compensate for the lost windings on the speaker energising coil and choke. I still haven't worked out the rest of the circuit yet so I am not sure if the energising coil is across the HT or in series, KB have used both an energising coil and choke in series on other models.

Here is the new capacitor block
   
and here it is fitted to the PSU chassis.
   

I heated the base of the capacitor block can with an electric blow lamp, till the solder melted then pulled the whole base off.
Once off, further heat was used to soften all the pitch inside enough to pull the guts out.
The base of the cap block was cut down to allow the metal case to fit over the new capacitor block. It was then re-soldered to the can so that the existing mounting holes could be reused. All the heat had caused some marking to the original green paint so the whole can was resprayed, even though it will be hidden in the PSU when the cover is on.
   
Here is the power supply fully assembled but with the cover off.
   

The ripstop material arrived today, and I have cut out a piece for the new diaphragm surround.
As this is a bit of an experiment I will update when I know it has all gone well and the speaker is working again.

Mike

p.s. Have you seen a 5 amp plug like this before, I have never heard of Goltone
   

[Crackle]

I am very happy with the way the speaker has turned out with the new diaphragm surround.
As I mentioned before the original surround was made of an extremely thin chamois leather, which had gone very dry and was crumbling away every time it was touched. I really had a bad feeling about how it would stand up to the vibrations of the diaphragm in use, considering the radio was supposed to have an more than 1 watt.

Anyway the plan was to make a new surround out of ripstop nylon.
An actual size drawing was made on the computer and then printed onto 4 sheets of A4 paper, they were glued together to make a life size template.
I glued the paper template onto the nylon, with Pritt glue on the water proofed side, and then cut around the inner most circle which represented the inner edge of the overlap to glue to the rim of the diaphragm. I then made 1/4 inch cuts into this edge of the circle to make it possible to turn the edge in, to glue to the rim of the diaphragm.
       
Once the PVA glue I used to fix the nylon to the diaphragm had dried I fitted the diaphragm into the basket and magnet and wedged it in at its lower limit with 3 plastic shims in the speech coil gap, this helps to centre the coil and holds the diaphragm steady whilst the outer rim of the nylon surround was glued to the rim of the speaker basket.

There was a concern that the diaphragm must not move too far in to the magnet as the spider mounting bolts would cause damage to the diaphragm.
   
Here is the speaker
   

I still have to fix the outer rim onto the basket, I will probably use rivets here.

Here is a short video of the speaker in operation. It starts off with the left channel of the ITT music centre on faintly, with the balance control turned right over to the KB 214 speaker on the right channel.
https://youtu.be/CI55YDSsWkQ

Mike

[peter scott]

That's a really good result on the speaker.

Well done!

Peter

[Crackle]

Well that is strange, the intervalve transformer between V2 detector, and v3 1st AF, is not a transformer at all.
I have not seen this in a radio before, using a standard Brandes audio transformer Bakelite housing to house a capacitor resistor coupling network.
   
It looks original, or at least the resistors do in their little insulation sleeves.
So I believe that means I wont need to get any transformers rewound now, as I have the spare intervalve transformer which Dave so kindly donated, that's certainly a welcome saving.
I have made a drawing of how the radio chassis is wired, I now just need to redraw it on a computer so it is more logical to follow as a schematic.
   
Mike

[Crackle]

Here is a rather scruffy working drawing of the PSU wiring.
   

Mike