KB 830, 1939 superhet radio

[ppppenguin]

If the rectifier warms up faster than the other valves then the HT will be rather high until the other valves warm up. This may harm the electrolytics. I can't see that it would hurt the rectifier. If the electrolytics can take the higher voltage then don't worry.

[Crackle]

That was my thoughts about the 20 seconds thing as well.
Thanks for checking Lawrence, if you couldn't find any information I am sure it doesn't exist.
I have fitted a 10uF 450 volt and a 22uF 450v capacitors into the old can. I was testing the HT to see what it would rise to via a 10k resistor, when bang, puff of smoke and the radio spat the fixed tone corrector capacitor at me.
I thought I had disconnected the HT from the output of the field coil, but what I had disconnected was the return wire to the smoothing cap C26. Anyway no harm done and I would have changed the tiny tiny little hunts mould-seal cap anyway. It must have been way under rated from point of voltage as it was physically smaller than the replacement I put in. The HT rose to just under 430 volts. with no load so things should be OK.
By the way I am using the trader component numbers during this thread.
One advantage of an energised speaker is you can clear the rust and crap out of the speech coil gap easily. Now the diaphragm is moving nice and quietly I have fitted a felt cap over the open speech coil. I made the felt cap by slicing a felt self adhesive pad in half.


Mike

[ppppenguin]

Not just electrolytics I was going to say in my previous post that old 350V paper caps might not have liked it even when they were new. I think you've proven the point.

[Crackle]

I have finished the capacitor change, and applied power to the radio.
Audio is working but nothing at all on the RF side, on any band.
So the next job is to check all the valve voltages. Hopefully this will indicate a faulty resistor. For this I will use the manufacturers service information as it includes the valve pin numbers and saves me looking them up.
Following that if no luck finding the fault it will be a matter of tracing back from the audio, through the IF and then RF with a signal generator.

Mike

[Nowhere-Man]

Don't forget your waveband contacts. I have a 40's radio that didn't work on RF due to this. Also check for isolation of the moving vanes. Hopefully you won't need to replace any valves. Good luck.

[Crackle]

Problem found, it was V1, 20D2 heater is open circuit. Luckily I have a KB 880 which has the same valve line up.
I could also have looked at the 880 to see how the tuning drive went, but thats done now.

The KB 830 is working very well, I have given it a quick RF alignment, not the most sensitive of sets but more than adequate volume and easy to operate smooth 2 speed tuning.
I have changed the tome control as well as the track was burnt out, probably caused by a leaky tone control capacitor. It now has one of those 470k switched pots that Maplin sells.
I have been trying to carefully clean the glass dial, one has to be so careful that the print does not rub off, it is looking cleaner now and still has its print, after a good huff and careful wipe with a clean duster.
I am on the look out for a 20D2 valve with a working heater, I dont think these are so common, so will put something in the wanted sections of here and UKVRR.

Mike

[ppppenguin]

I can't see any references to a 20D2 online. It's a triode hexode. Would the slightly more common 20D4 triode heptode be adapted to the role? Or even a common ECH81?

http://www.mif.pg.gda.pl/homepages/frank...2/20D4.gif

[pwdrive]

20D2 has a 13 volt heater, according to the limited data in the link below the 6K8g has the same characteristics but has a 6.3 volt heater:

http://www.shinjo.info/frank/sheets/155/1/19BG6G.pdf

Lawrence.

[Crackle]

I had a PM on UKVRR from Andrewausfa, who says a Mullard data book has given the IW4-350 max capacitor load as 12uF, just for the record.

Anyway the radio sounds good with a 10uF cap in that position.
Tomorrow I will be trying to make the little wave change flag which slides up and down in a window in the tuning dial.
I have an example in the 880 which I can photo to use as a pattern, it looks like it is made from a cellulose type of sheet.
Oddly KB put the frequency range on the flag rather than Long, Medium & Short.
Mike

[Nowhere-Man]

Glad to hear you got it up and running. Yes, I got one of those pots from Maplins and they are OK. Also agree waveband slat letters come off easily - someone had wiped a whole slat clean and I have to redo it.