KB 429, A 1935 Battery TRF radio

[Crackle]

There was a problem with the volume intermittently going up and down, it eventually stayed loud, I thought that was OK and working correctly, but later I found I could not reduce it all the way to off on some of the louder stations.
My thoughts were the variable resistor, I was getting strange readings from it whilst connected up, but when disconnected it all seemed OK.
I measured the voltage on C2 and it was near zero however I adjusted VR1. After following the circuit around and checking R6 R7 and C2 again I could find no faults.
But then I noticed that R7 leg was protruding below the cardboard tag board and was probably touching the chassis due to the cardboard warping over the ages. Effectively shorting the wiper of VR1 to ground through the rust.
I have managed to bend the leg up slightly and I can now adjust the bias voltage of V1 from -2.8v to -8.8v and the volume goes to zero on all stations.
I feel lucky that VR1 was not damaged by the current from the bias battery.
I think I will try and fix some insulating material there to stop the cardboard warping and shorting out.
       

Mike

[Crackle]

The loudspeaker was enclosed in a black muslin bag, which has disintegrated around the magnet due to the rust.
The bag is to protect the speaker speech coil, which is otherwise open at the front and at the back of the diaphragm, from dirt getting into the magnetic gap.
I will need to make a new bag, so I am off to look for some black muslin.

Mike

[Murphyv310]

Very nice restore Mike.
Would this set be one of the last TRF sets from KB? 1935 is late for a TRF, Philips of course were late too but their Superinductance TRF sets were class leading. I do like TRF sets with their minimal component count and how well a good designed set will actually perform.

[radioman]

Yes, that's a nice restoration Mike and an interesting write-up.
I've got a mains-powered KB 510 TRF that came out the following year, so that might be the last TRF model that KB released Trevor.
I'm sure Mike could confirm.

Andy

[Crackle]

The last KB TRF radio which I am aware of is the KB 810 from about 1940, this was probably the last TRF design that KB made.  https://www.radiomuseum.org/r/kolsterbr_810.html
Kolster Brandes prided themselves as making quality affordable radios, and I suppose the TRF principle is one of the simplest ways to make a radio. The later TRF radios that they made were often battery ones, again I suppose the low valve count helped with the battery life. http://www.kbmuseum.org.uk/kb_1934_catalogue.htm

Mike

[Crackle]

According to the Radio Museum there were only a handful or so British made TRF sets later than the KB 810, including an Ever Ready, a PAM and the Sinclair Micro 6 and a few other less well known makes.

Mike

[Crackle]

I mentioned in my first post that the tuning dial had no frequency or wavelength markings, only station names.
I was mistaken, the wavelength markings are marked on the plastic/cellulose dial cover.

I have also realised I have a second chassis for this model (no cabinet). Oops, my memory is getting worse.
I will look it out and see if it is worth swapping anything. I am not too concerned with the minor spotting of rust, or the slightly pitted aluminium coil cans. But if the dial is more presentable it would be nice to swap them as they are so visible.
The spare chassis can be seen in the radio museum, I uploaded the photos of it before I got hold of the radio with the cabinet.
The photo of the complete radio in the Radio Museum is the same as the one in this thread, The photo of it was taken when it was still wet from having a rub down with a cleaner. After drying it returned to its original dull dry and slightly crazed lacquered finish.

I have now given the cabinet a further clean and have applied a furniture rejuvenator which will seep into the cracks and pores, masks the scratches and brings out the grain again making it brighter.
When this has had 24 hours to dry I use a wax polish and go over it vigorously, this removes the thin surface coating of rejuvenator but leaves the grain still looking nice and the finish original.
Some photos will follow when I have the radio back together.

Mike

[Crackle]

I made a new bag for the speaker to go in. The material was not quite as sound transparent as the original which was little more than gauze.
My hand sewing was pretty terrible, but when the bag was turned inside out it looks fairly original.
   
Mike

[Crackle]

It is all back together.
Here are some photos of the finished radio..
       

and here is a photo showing the battery iliminator, I seem to remember that this is the radio which the EKCO eliminator came with when I bought the radio at RWB last year.
   

I have made another video of the radio working in its cabinet, this time filmed during the daytime so there are less stations received.
https://youtu.be/D6h-ZzMTsoM

Mike