KB 402

[Crackle]

I am currently restoring this radio, https://www.radiomuseum.org/r/kolsterbr_402.html . I have had it for a number of years and the only thing I did with it was to take the chassis out and clean the dial and dial window to take photos. I cant remember now why I did not do any more work, but it may have been because there is no information that I could find on it, up until now.. I also seem to have an idea that I thought the speaker energising coil was open circuit, but it is OK, it measures 3k, the mains transformer and speaker transformer also measure OK.
I have just recently discovered it appears to be the same chassis as the STC 40 and the KB 444, https://www.radiomuseum.org/r/kolsterbr_444.html but in this radio the speaker wiring is hard wired into the chassis rather than via a 4 pin plug and socket on the side of the chassis.
The cabinet is in quite nice original condition, including original speaker cloth, for an 84 year old girl, but there are a few old worm holes on one side. The chassis also looked fairly original, but now on closer inspection there have been some component changes. I thought it was a bit of a mess under the chassis until I saw the photos of Sam's Seymour radio,  
   
One change in particular which is rather odd is the cathode resistor R5 250 ohms, it has had a 140 ohm resistor added in series, even though the grid coupling cap C3 has been changed for a metal Sprague type. The Sprague cap is showing a little leakage so I have replaced it with a nice new one, I also removed the 140 ohm resistor and connected up the 250 ohm one. It actually measures 260 ohms, but that is near enough for me. The resistors that KB used originally during the early 30's are very good at retaining their value.
   
I am continuing to change the caps and check resistors, in this radio Kb had a rather strange habit of using wax paper caps for values as low as 100pF.

Mike

[boater sam]

Ok, you can all poke fun at my untidy Seymour chassis but it all works and is all original .( any new bits were well hidden.)
 I could not wire a chassis as badly if I tried.

That's a lot of waxies to restuff Mike, it will keep you busy for a while yet. What do you restuff a 100pf paper waxy with?

Sam.

[Crackle]

Hi Sam
In that case I used a recycled red 100pf tubular ceramic. In the case of the 300pf cap for C1 which appears to be a tone corrector I used a 220 pf 630v tiny polyester PCB type with extra wire fitted to the short leads.
I dont have a very good selection of small value caps, other than a bunch of large NOS RS flat mica caps which dont fit at all well in a tubular case.
At the moment I am waiting for some green paint on a 200 ohm dogbone (which measures 260 ohm) to dry, so I can use it in place of the 250 ohm dogbone that someone has fitted for R5.  (the one in the chassis measured 400 ohms)
Here is the circuit diagram of the KB 444 which is the same as the KB 346, which in turn is the same as the KB 402, and the STC 40. Or so my documentation tells me. Mentioned towards the end of this document. http://www.kbmuseum.org.uk/documents/KB_...models.pdf
   

Mike

[boater sam]

Grand, I too enjoy maintaining the look of a chassis, even if it is a bird's nest, or in the Seymour case a rat's nest.
Repainting dog bones, I use an oil paint kit from Lidl, it has all the right colours with a little mixing, takes ages to dry though.
To get resistor values higher I file notches in the back or drill through the centre. I have not worked out if the values continue to rise, I suspect that they don't providing the carbon water content remains stable. I dry them thoroughly before reuse and give them time to settle.

[Crackle]

I wonder if the old carbon resistors go high because of moisture absorption causes swelling and then drying leaves the carbon granules less compact.
I have run out of 25uf low voltage electrolytics so now have to wait for some more 50v ones to arrive from Rapid.
Are their any suggestions on how to open up and re-stuff the Sprague capacitors?
   
I have scanned the user instructions for this radio. http://www.kbmuseum.org.uk/user_instruct...2_inst.pdf

Mike

[boater sam]

I tried various methods, cutting the can in the middle ruins the print, filing the swage off one end looks a mess afterwards.
I tried cooking them till they blew but they go with a massive bang and swell up.

Now I cut the lead off flush at one end and drill down to almost the other end with a drill just smaller than the end of the can swaging. 
This removes most of the old capacitor guts  and leaves enough space for a new poly capacitor.
Cut the other old lead off and a  1mm drill through  the remaining end cap for the new lead, insert new capacitor and fill open end with matching colour silicone sealant smoothed to look like an end cap.
I've not found it necessary to ground the can though some makers did.

Most of the ones I have tested pass well enough for a valve set provided that they are not in a critical position.

[Crackle]

Thanks Sam, I will give that a try next time, this time I used an old, more of the period, cardboard tube BMI capacitor and re-stuffed that. .
I was in 2 minds as to what to do with C9, a 200pf waxed paper cap. It was showing a very small leakage, about 3uA at 300v DC, it is rated at 1500v. But from the circuit diagram it did not seem to have any DC on it which could leak. In the end I changed it as when I measured it on my DMM it was measuring 267pf.
When I came to C6 today I got really confused, it was not present, I thought maybe I had cut it out yesterday but it could not be found anywhere.
I finally found out that the large brown waxed cap, in the lower left centre of the chassis photo, was in a roundabout way connected to the same points, so this large .4uf cap must have been substituted for the original .1uf one at some time in the distant past.
It is interesting to see this superhet radio has a "reaction control" in the IF valve circuit. It is a trimmer type cap with a small black knob fitted and located at the read of the chassis, so it is probably something which is adjusted to suit your local reception area.
In the top left of the circuit diagram there are 2 unnumbered capacitors, these are made up of twisted enamelled wire, I wondered what they were at first.

Here is a photo of the Underside of the chassis after the restoration. All capacitors have been changed now, except for the two 25uf cathode bypass caps for V2 and V3, still waiting on delivery of these. These will go in the 2 cardboard tubes with metal end caps bolted to a bracket in the centre of the chassis.
   
Mike

[boater sam]

Appears never to have been touched, Excellent! As it should be. 
Glad that there is another me too.

[Crackle]

I have been having problems with the tuning capacitor drive. The moving vanes seem very heavy and tend to drop back under their own weight.  The rear bearing on the tuning cap is not adjustable, it is a spring plate fitted over the ball bearings and secured with 2 screws.
I removed and tried to increase the tension in the brass strips which rub on the shaft to earth the moving vanes, this helped a little but the vanes still tend to drop sometimes.
   
It is not helped by the fact that somehow the housing and the bearing surface in the concentric slow motion drive has been damaged. There were cracks in the pressed steel cup which forms the outer face for the 2 ball bearings to engage on.
One of the problems I believe is the slow motion drive only used 2 ball bearings, most modern ones have 3 or 4 bearings, giving greater grip between shaft and outer bearing.

I have had several attempts to fashion some sort of repair on the small pressed steel cup. first was to reshape it and press the crack together, that did not work it just opened up again. Then I tried covering the crack with a large blob of solder to stop it opening,
   
   
that still left a slightly pitted surface inside the cup, so then I tried smoothing the inside bearing surface with solder to help to even out the cracks. But the pressure caused by the ball bearings rolling round the surface is too much for soft solder to take.
   
I have now tried it with a steel wire shaped into a circular ring and soldered all round the housing rim to strengthen and help stop the crack from opening again.
   
Using a small grindstone in a Dremel I have ground down the bearing surface to help smooth the edges of the cracks and even out the pitting. This seems to be working more reliably but the vanes still occasionally slip down a little.

I wonder if anyone has a spare tuning cap of the same type and could let me have a replacement bearing cup.

Mike

[Terry]

Mike, have you considered the possibility of adding a counterweight to balance the gang?

Is the tuning scale fixing by screw or just friction? If the latter, could you add a suitable clamp to improve matters then glue a semi-circular weight - possibly lead sheet - to it? Possibly paint the weight and the back of the scale to make it completely invisible if anyone should want to investigate the interior of the set in the future (I don't know how much of the scale is visible from behind)?