KB 279 TRF radio from 1931

[Nowhere-Man]

With these battery sets, I found the filament circuit the most puzzling. Mine had a 100 or so ohms between HT and LT negative. Also baffling was the pin to chassis readings, with two filament positive pins testing zero to chassis. And that was in the end with LT wire desoldered and valves unplugged. I felt it was wrong as the schematic showed LT+ in isolation so maybe the holders had issues.
In the end the valves proved to be faulty so HT was never applied. Each valve was quoted at 20 quid each so I decided to wait a while. That makes the oldest radio I repaired to be 1947 and not 1938. However now as I creep towards the Sixties I feel just out of my depth. Transistors are new to me. Mike's radio reminds of a time where the radio was switched on for brief periods and homes had no AC mains.

[Crackle]

Hi NWM
This KB 279 is mains driven not battery, it has a full isolating mains transformer with three 4 volt outputs (heaters for rectifier, op pentode, & RF and detector valves) and a centre tapped HT winding giving a massive 775 volts AC. I dont know yet what the final rectified HT voltage will be but I suspect it will probably be around 500v DC on the rectifier cathode and 250 to 275v by the time it has been dropped by the 2.3k ohm speaker field coil.
When I come to power it up I will be treating it with some respect.
This is one of the reasons I am planning to use two 4uF 630v polyester capacitors rather than an 8uf electrolytic as the main smoothing capacitor.

Mike

[Murphyv310]

Hi Mike.
You might find the HT on the output valve's Anode higher than you would imagine so take extra care.
In the past I have used 450 or 500v electrolytic caps in series with balancing resistors to give a decent overhead for surges.

[Crackle]

The KB 279 lives again. I just have to fit it back into the cabinet.
   
   

Here is the WW schematic which I have been updating with component values, notes and some voltages.
   

There is a bit more detail on my thread on UKVRR
Mike

[pwdrive]

Proper job Mike, good documentation.

Lawrence.

[Murphyv310]

Brilliant job Mike.
How well does the set perform?

[Crackle]

Copied from UKVRRR
I have given the cabinet a clean with Liberon wax & polish remover, lightly touched up a few chips in the lacquer and finally a good wax polish and rub down.
It is working very well, I can hear and resolve 3 really strong stations on Long Waves and about 3 more quiet ones. Lots of strong stations on Medium Waves but it is quite easy to resolve them with careful use of the fine tuning control and aerial capacitor. On MW the Reaction control needs no adjustment it is just left fully anticlockwise. On LW the reaction control has a more pronounced effect and some adjustment is an advantage as you scan across the wave band.
Volume is very loud now that the speaker is in the cabinet and facing me. You need to back off the aerial capacitor and detune slightly to get a reasonable comfortable listening level. All in all a very good radio and quite easy to operate with the ganged tuning control and fine tuning capacitor.

The finished service information now resided on the KB Museum website. http://kbmuseum.org.uk/kb_service_manuals/KB_279_WW.pdf
Here are some photos of the finished radio.

Mike

[Terry]

I find that circuit interesting, Mike.

Many of the resistor values are odd by modern standards - 230, 400, 50k and 70k, for example but we can see the beginnings of the transition to the logarithmic series we are all familiar with today, with values such as 22k and 120k.

And have you noticed the the valve base diagram for the 1807 looks like a very sad face?

Anyway, an excellent restoration - as always.  

[Crackle]

Hi Terry
Yes the old values were more "rounded", the 22k in the tone corrector circuit was something I added as it and the capacitor were not present at all, but it sounded very harsh with out them, I copied the idea from the KB 286 schematic.

Mike

[Murphyv310]

Very much a credit to your work.
Lovely looking set and another one saved and restored.
Great stuff.