KB 402

[Crackle]

Mike, your idea is a sound one, but it is easy to mix positive and negative if you're not careful, so lets take it in stages:

First find the frequency corresponding to 1600m:

300,000/1,600= 187.5 kHz

Now calculate LO frequency for R4/198kHz and 120kHz IF:

198 + 120 = 318 kHz

Now subtract the 'new' 130kHz IF to find the new signal frequency:

318 - 130 = 188 kHz

That's good enough for me!

Thanks Terry, but I am still confused, I appreciate the maths but there are too many numbers for me, I tend to think in practical things now days, are you saying there will be no difference, or yes it will help move R4 onto the correct mark.
thanks
Mike

[pwdrive]

The circuit looks unusual to the norm, the resonant frequency of the IFT's primary appears to vary with the main tuning, I suspect that it has something to do with the fact that the reaction control is "pre set"

Lawrence.

[Terry]

Mike, without the numbers, this is what I did:

You say that the tuning indicates 1600m when tuned to R4 so I calculated the local oscillator frequency for R4.

Then without touching the RF tuning, calculated what the signal frequency would be if you changed the IF from 120kHz to 130kHz, as you proposed.

This would be 188kHz/1600m which is exactly what the tuning scale says.

Therefore, if you retune so that the scale says 1500m, you should find R4, which is what you wanted.

Does that help?

[Crackle]

Yes thanks Terry, understood now.
cheers

Mike

[Crackle]

I had a go at doing a full realignment according to the instructions in the service manual.
It was not easy, mainly made difficult by the drawing of the layout above chassis being wrong, well it is for the KB 444 so it may be correct for that radio but it is definitely wrong for this one.
I can confirm all the component values of the KB 402 and layout positions under the chassis are identical to the KB 402 and the above chassis drawing is correct except they have labelled the trimmers on the tuning capacitor incorrectly.

If you look at the drawing and the layout of the aerial, bandpass and oscillator coils the layout dictates which is the obvious way to connect to each section of the tuning capacitor.
   
e.e. The Aerial coil is on the bottom left of the drawing, and this connects to the tuning cap section immediately to the right, labelled incorrectly as the oscillator. The bandpass coil connects to the middle section of the tuning cap, labelled correctly. The Oscillator coil, top centre of the drawing, connects to the rear section of the tuning cap, labelled incorrectly as Aerial.
Well done KB! I cant see it being any different on the actual KB 444 radio as it is so logical and natural to wire it this way.

Anyway I did the full realignment setting first the IF to 130kHz then doing the MW and LW RF alignment. It was difficult as performance had deteriorated on MW and LW, so I went back and peaked the IF trimmers for best reception on MW. Low and behold for best performance on MW and LW the IF frequency had moved back to 120kHz, so I wonder if that is another mistake in the service manual.
It was worth a try, thanks for all your help and advice.

Lawrence, I can confirm this circuit is weird and different, but the radio does work well, at least from the point of view of sensitivity. But there is a little distortion on some stations which improves if the Local-Distant switch is switched to local. This adds a very small value (twisted enamelled wire) capacitor in series with the aerial.
This circuit appears not to have any proper AGC.

Mike

[Crackle]

I have just had a quick check and the above chassis layout drawing is also wrong for the KB 444.
   

Mike

[Crackle]

I should have checked the corrections document I have, this error was corrected in March 1934 and again in 1936. It also appears that the 10k volume pot was correct after all, oh well the 22k temporary substitute one is also working well, the replacement will be here shortly, hopefully.
   

If anybody is interested in correcting their own copies of these early KB service sheets, the correction document is here. http://www.kbmuseum.org.uk/kb_service_ma...936_kb.pdf
Mike