KB BR40T

[Crackle]

Spurred on by finding and fixing the cause of my severe interference, which was making it almost impossible to receive anything on MW, I have struggled upstairs to my workshop with this radio. It has a magic eye tuning indicator, an RF stage before the frequency changer and sports a good selection of SW bands and so it should be fun to have a play around on the bands at night.
   
It is a KB BR40T and proudly proclaims to be Prototype no. 2.
   
The chassis is larger than the cabinet and it slides into the back, upwards so the dial is almost at the top.
There is a large choke under the chassis and it looks like the mains transformer has been replaced, you can see a shadow in the rust where the original one was under the chassis. But was this maybe done by KB at the time it was made?
   
I only have a schematic for the BR40T, no other information, so I will need to compare notes with the information I have on the BR40.
For some reason I don’t have any photos of the underside of my BR40, so I think I will get that down off the shelf to check how that looks underneath.

At the moment the large electrolytic capacitor is reforming.

Mike

[Murphyv310]

That's a nice rare set to have Mike. How many KB sets do you have?

[Crackle]

Hi Trevor
My spreadsheet tells me I have 139 KB, 37 ITT KB, 77 ITT, and 71 Other radios.

[Crackle]

I have taken a photo of the BR40 chassis and it is very similar except for the mains transformer.
Unless KB used a faulty BR40 to adapt, it makes me think the transformer in this BR40T has been a service replacement at a later time.
BR40T on the left, BR40 on the right
   
Often on a tropicalised KB radio (with a T suffix) the mains transformer would have taps from 100v to 250v but the domestic BR40 already had the full range of voltage taps.
The BR40T has a different RF amplifier valve a 6SG7, whereas the BR40 had a 6K7G

The other main differences in the 2 radios are that LW on the BR40T has been replaced by a SW band covering SW 40m to 94m (7.5 to 3.2MHz)
The other bands are;
Medium Wave 195 to 560m (1540 to 535 kHz)
31m band (9.85 to 9.35MHz)
25m band (12.1 to 11.5MHz)
19m band (15.69 to 14.95MHz)
16m band (18.2 to 17.4MHz)
13m band (22.1 to 22.0MHz)

The Domestic BR40 wave bands are as follows;
MW band (1540 to 535 kHz)
LW band (400 to 135 kHz)
49m band (6.28 to 5.92MHz)
31m band (9.85 to 9.35MHz)
25m band (12.1 to 11.5MHz)
19m band (15.69 to 14.95MHz)
16m band (18.2 to 17.4MHz)

[pwdrive]

The 6SG7 has two cathode leadouts to reduce the overall cathode leadout inductance, usually better stability on higher bands if wired properly.

Lawrence.

[Crackle]

if I need a temporary replacement 6SG7, I can look in the AR88, there seems to be about 5 in there.

[Crackle]

I have done a complete recap, changing all the wax paper capacitors, and electrolytics, the radio is now working, got a few other things to check.
The mains transformer gets a little warm after an hour, not too hot to touch, but I think it is slightly underside for the number of valves..
The Magic eye was very dim but I happened to find a NOS 6U5 on the shelf which is nice and bright.
   
The tuning drive cord is a little slack and I need to dismantle it to clean some rust of the large capacitor drum and the capstan, so have dismantled the dial screen to get at it.
This is the 6SG7 socket connections, so it appears KB didnt use the 2nd cathode connection.
   

There are a few differences in the radio to the schematic I have for the BR40T.  The schematic shows an additional winding on the output transformer. The schematic is now in the KB museum website. http://kbmuseum.org.uk/kb_service_manual...0T_cir.pdf
But I cant see this winding in the radio, just looks like primary and secondary wires.
I have taken a photo of some alterations that have been made to one of the LW coils. I assume this was this the testing and modifications done by KB.
   

Mike

[Terry]

LW coil?

I thought this version didn't have LW, Mike ...

[Crackle]

That's correct, the LW coil and capacitor was swapped for a SW coil, it looks a little Heath Robinson.
I didn't explain it very well.

Mike

[Crackle]

I have finished the radio and put it back together, and given it a polish.
I have not been over impressed with the radio, it has been difficult to peak up MW. The MW aerial coil was incorrectly wired, think it is OK now, but I cant get the MW oscillator to go up as far as 1548 kHz to pull in Gold. The concentric type of trimmer capacitors are pretty rubbish.
This chassis that KB used to make these adaptations must have been one of the early ones. There are alterations explained in the BR40 service manual (revised) on my website that explain a lot of the differences that I found.
Here are some photos.
http://kbmuseum.org.uk/kb_images/br40T/kb_br40T.htm

Mike