KB 402

[Pat Pending]

Have you thought of adding a fixed 10K on the low end of the volume control and see how it performs? If the volume still jumps and the RS pot is a log taper then try tacking in a linear 20K in to see how it goes. As the IF valve DC conditions are set by this component its value may be more important than a "normal" volume control.

I believe RS still has a range of 1/4" shaft switched pots, but they would not look very "period".

BTW the discussion on ultragainer and supergainer radios is here http://theradioboard.com/rb/viewtopic.php?f=3&t=3256&p=77918#p77918

[Crackle]

I have put in a temporary modern 22k linear pot and bypassed the switch, the volume control works very smoothly now, but I must get hold of a proper replacement switched pot.
Although MW is working exceptionally well there is hardly a peep out of LW, I can just, and I mean just hear R4 with the volume right up.
I cant think of what the problem can be other than a fault with the aerial or bandpass coils. All the information on the coil resistances is in the service manual but the coil connections are not identified. But I have found in some KB service notes a diagram for the coil base identifying the tags. So hopefully tomorrow armed with this and the details in the service manual for the KB 444 I may be able to identify the cause of no LW.
   

The KB service guidelines are here if anyone should want them. http://www.kbmuseum.org.uk/kb_service_ma...tes_kb.pdf
Mike

[Crackle]

This radio is proving to be very troublesome. What with the slipping tuning drive, which apart from the issue of the balance of the moving vanes is also caused by irregularities in the bearing cup bearing face, causing unwanted movement and sticking of the balls. I think It will require a new bearing cup to be made.

Apart from that I have the issue with LW. If I stop breathing then I can just about hear and tune in R4 but the tuning seems to wander.
My assumptions were that as I can JUST tune it in there must be a problem in the aerial circuits. There are very few fixed capacitors involved with the tuning of this radio they are all mostly trimmer capacitors. I have checked all the trimmer capacitors and they seem to measure OK and the capacitance changes slightly if I turn the nuts one way and then the other.
So it leaves me to think the fault lies in the aerial and or the bandpass coils. Resistance measurements confirm an possible issue with the aerial coil between tags I & E it is listed as 0 ohms but the reading is 8.4 ohms.

Question. Would you expect to see a resistance of 0 ohms on that part of the coil, is it not the LW section of the primary as it appears to be bypassed when switched for MW reception. Could the figure 0 be a misprint?
(switches 1 to 6 are all closed for MW, and only 4 & 5 are closed for LW)

Since measurements of the RF coils I decided to measure the oscillator coils, and found a problem there also on one of the LW windings. The resistance between tags K & H is infinity it is listed as 14 ohms. Could be the cause of the problem with LW, where it can tune R4 but only audible if I hold my breath.
   
   

It appears that the aluminium cans are fixed to the chassis somehow, possibly with a rolled rim, and the coil former assembly is removed from the can by pulling down though the chassis, so a lot of other parts will need to be removed in order to make a clear space for the cols to be pulled through with out risk of further damage.
   
I have had a look at quite a few other KB models schematics and there are not any others which use this particular aerial coil, most other designs use a 3 coil assembly.

I would really appreciate your thoughts please before I perform major surgery and try and extract the coils from the chassis.

Mike

[boater sam]

It is quite possibly the long wave oscillator coil, if it is reading open then there is definitely something amiss.
Can you be sure that the oscillator is running properly on long wave?

[Crackle]

I can never work this out, what sort of frequency would I search for if the IF is 130kHz. I'll go and have a check with a transistor radio.

Thanks
Mike

[pwdrive]

The osc frequency should be the frequency the receiver is tuned to plus the IF frequency, eg: signal frequency 200kHz, IF frequency 130kHz, osc frequency = 330kHz. The osc frequency is higher than the signal frequency except sometimes on the high band of some older shortwave receivers.

Lawrence.

[Pat Pending]

....If that coil is OC though, you are only confirming what you already know.... Time for surgery.

[Crackle]

Thanks, For your replies  
I have set my transistor radio to 330 kHz and there is nothing picked up as I tune the KB 402 across LW. So it does look like the fault is in the LW oscillator coil.

I tried more experiments on MW and if the KB radio is tuned into Lyca radio on 1458 then I get a strong signal on my transistor at 1577
And with Smooth on 1431 then I get a strong signal on 1550
And on BBC Essex north on 765 I get a strong signal on 886
All 3 give a difference of  about 120kHz so does that mean the IF tuning is out, or that this radio has a designed IF of 120 and not 130kHz. It is working very well on MW.

Mike

[Pat Pending]

If it's working well on the MW, don't worry too much about alignment. You may be fortunate enough that the coil tail has just come adrift from the tag, or not soldered properly as happened to me on a few occasions...

[Terry]

Post #23 has really got me confused, Mike!

the aerial coil between tags I & E it is listed as 0 ohms but the reading is 8.4 ohms.

Question. Would you expect to see a resistance of 0 ohms on that part of the coil, is it not the LW section of the primary
as it appears to be bypassed when switched for MW reception

No - if B-F is the MW coil, I-E must be the MW coupling coil.

I-E is not bypassed by switch II. Switch II connects LW coupling coil D-A in parallel with the MW coupling coil.

As for the DC resistance, it obviously cannot be as low as 0 ohms but nearly three times the value of the LW coupling coil does seem somewhat excessive!

(switches 1 to 6 are all closed for MW, and only 4 & 5 are closed for LW)

This doesn't make sense either because, from my comment above, Sw. II would have to be closed on LW and open for MW.

Also, try as I might, I can find no trace of Switch V. Should I get my eyes tested? Please tell me where it is! (I have no problem with the other switches in the I to VII range, but not switch V!)

If the LW coil is o/c as your measurements suggest, I can't see how a 198kHz signal would pass through the IF strip which must, of course. be working otherwise the set wouldn't work on MW.

198kHz + 120kHz = 318kHz = 943 metres. Does the radio you are using really tune above 300kHz/below 1,000 metres on LW?

Obviously, if you tune the  set to the 2,000 metre end of the band, the LO, if working, will definitely fall withing the LW frequency range.