KB 830, 1939 superhet radio

[Crackle]

The second choice to the KB 333 in my recent poll was the KB 830. This is a fairly standard design superhet radio from 1939. Just imagine all the news of the war it must have brought to its owners.
   

A long time ago now I came across a packing case for one of these radios, now 4 years later I have the radio to go with it.
   
   
Tucked in amongst the items on the top of the chassis was a very long mains lead and a "Clix" 15 amp plug. I have seen hundreds of 15 amp plugs in my time working as Chief electrician in the Towngate Theatre, but I have not come across this make before.
   
   
   
So it looks like this radio has not been used for some time either, although it would appear that it has had various services in the past.
   
First checks show the speaker energising coil to read 1.2k and the speaker transformer primary 460 ohms, so they dont seem too bad.
The mains transformer primary is 31 ohms, seems a bit low!! and the secondary is about 200 ohms.

At the moment one of the 2 caps in the main electrolytic can is attempting to reform, or at least it is soaking up 5mA from my electrolytic reformer. This doesn’t seem to good either but shouldn’t be too much of a problem if they need replacing.

I am a bit worried about the mains transformer primary so I am going to do some more tests on that.

Mike

[pwdrive]

As a comparison the Bush AC81 mains tx primary is given as 30 ohms on the 230 volt tapping.

Lawrence.

[pwdrive]

Trader sheet for the 830 gives:

Main heater winding 0.4 ohms.
Rect. heater winding 0.2 ohms.
HT winding (total) 192 ohms.
Primary winding (total) 29 ohms.

Lawrence.

[Crackle]

Thanks Lawrence I should have looked up those values first before worrying, they confirm my findings, I guess you have Paul's DVD now.
Yes the mains transformer checks out OK giving 296 v AC on the 250v transformer setting, (should be 316v AC but the mains here is about 235).
The lamp limiter I used to start the Mains transformer up with was giving a very very faint glow, with no load, but the transformer is showing no signs of getting warm.

Interesting, I found a 12AT6 lurking inside one of the valve screening cans in place of V3 (11D5). The old B7 valve base has been removed and a B7G valve base has been fitted to an adaptor plate and bolted on to the chassis.

The 16uF section of the aluminium can has dropped to 3mA now, so it looks like there may be some hope for the electrolytic if left over night.

Mike

[Terry]

I must admit I've never seen a 15A 3P Clix plug before but Clix 5A 2P plugs used to be very common and had exactly the same style of split pin which clamped the conductor when the top was screwed on. I had one on my Henley Solon iron for many years as it fitted the mains lead of many TVs and a razor adaptor was useful in most cases where that wasn't possible. I think I also carried a short lead with a 5A 2P female and a 5A 3P male but it was a long time ago.




Mostly, where you found 15A 3P sockets, they had adaptors fitted with 5A 3P outlets as well as one 15A 3P outlet, so you didn't normally need to worry about those.

[Crackle]

That bottom photo is of a "clang" the other type of adaptor we used was the "grelco", the grelco was far better, it would survive being dropped 30 feet from the lighting grid. The clangs would disintegrate if dropped from a great height..
They were both very sought after as they were no longer made, so many theatre companies had to resort to making up splitters. (15 amp plug with 2 sockets.)
Mike

[Crackle]

I dont think I mentioned, the dial cord is broke and I cant find a diagram of anything similar to this apart from the 810, and it is not quite the same, so it looks like trial and error.

Mike

[Crackle]

Restringing the dial all became suddenly a lot easier when I decided that it was impossible to drive the pointer and capacitor with the one cord and therfore it must have 2 separate cords, one to drive the capacitor and another to drive the pointer.
The tuning drive cord is all working again, and to top it all off the pointer goes to the HF end of the scale when the cap is at minimum, AND the knob seems to rotate in the correct direction to move the pointer. Woopee.

The radio has an indicator which shows which wave band is selected, this is a piece of metal, or plastic which is attached to a cord which goes to a cam on the wave change switch spindle. The only problem is that the only bit of this that still exists is the cam/arm on the wave change spindle. So I will have to fabricate something for this.
Meanwhile the 8uF cap C25, is proving a little stubborn and is going to require a lot more time if it is going to reform at all. The trader service sheets show this as 16uf.

The rectifier in this radio is an IW4-350, (supposed to be a R2) I was checking to see what the max capacitor can be for the smoothing capacitor when I came across this, http://www.r-type.org/pdfs/iw3.pdf. where it states that the IW4-350 should not be used with indirectly heated output valves which take longer than 20 seconds to heat up.
I have not come across this before, I wonder why.

Mike

[Crackle]

The main electrolytic is never going to reform satisfactorily, in the morning the current leakage in the 8uF reservoir cap had gone from 3mA back to 5mA. So I have now cut the base off and scraped the insides out ready for restuffing.
Most circuits I have seen using a IW4-350 seem to have an 8uF capacitor as the reservoir, it only seems to be KB that used 16uF occasionally.
Can any one help and confirm what the max value capacitor should be as a reservoir on a IW4-350 rectifier.

Thanks
Mike

[pwdrive]

Can't find any reservoir info for the IW4-350 but the IW4-500 data states 16uF max.

I'm not sure what that 20 seconds warm up time is all about either.

EDIT: Maybe post the 20 secs question on UKVRR?

Lawrence.