KB 402

[pwdrive]

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!)

You need to look at the whole picture (schematic)

Lawrence.

[Terry]

Thank you, Lawrence - I will cancel the optician's appointment!

[Crackle]

Hi Terry
Post #3 has the full picture, switch 5 is in the screen circuit of V1, I didnt think it relevant to include it. Probably to limit V1 use when the radio is switched to gram. Switch 2 is only closed on MW so maybe coil I E is LW coupling and D A is the MW coupling coil.

I have extricated the oscillator coil, dismantled it and found a the break in the wire on the LW coil, it is now repaired. The coil is fitted back into the chassis and now I have the job of replacing the switches, trimmers and the rest of the wiring I removed to get it out.
I'll post some photos later.

Mike

[Crackle]

LW is fully functional now, although I cant get the tracking quite right, but never mind.
The coil consists of an outer former and an inner former, the break was on the inner former and thankfully was easy to get at and repair.
On re-assembly a new fault happened, the radio was dead on both bands.
Then it came to life and then it faded again.
I checked the valve pins and re-soldered the tips of the pins on the RF and IF valves. This seems to have fixed it and it is now playing reliably.
The tuning drive is behaving itself reasonably well at the moment, but I may try and cut a lump off a fishing weight, cut a grove in it and fix it to the rim of the tuning dial, if there is enough room for it to swing round in the cabinet.
I have bought a 22k lin switched pot off ebay, not exactly vintage, probably more 80's than 30's but it will do for now, at least it looks like it is made of metal and Bakelite.
Next are the worm holes on the side of the cabinet and a scrape on the top edge to dress.

Mike

[Crackle]

I forgot to add the photo.
Tinned and ready to solder.
   

[Crackle]

I wonder if somebody could help me understand something. I need to visualise things in simple terms.
The IF frequency is governed by the tuned reaction of the IF transformers, and normally all the IF transformers have to be within a certain tolerance tuned to the same IF frequency. But if you only have the one IF transformer such as in this KB 402 then if I altered the tuning of the IF transformer from 120 to 130 kHz would this change the RF alignment, and what effect would it have? Would it tend to cause the HF end of the band to be lost, or would it increase the HF coverage at the expense of the LF coverage.
At the moment both bands are working very well, but R4 LW is coming up at 1600m on the scale, and the LW tracker trimmer seems to have no effect.
Would the increase of the IF to 130 bring R4 onto the 1500 mark (approximately).
Tracking on MW seems to be easily adjusted so I am hoping that the change in IF can be compensated for.

thanks
Mike

[boater sam]

Changing what is just the resonant frequency of the IF transformer should not alter the RF frequency.
Tracking is getting the aerial in step with the RF.
If the RF is low i.e.1600m instead of 1500m then you need to lower the oscillator, try a cap across the trimmer to alter its range. I think I have that the right way round, waiting for the corrections to pour in.

[Crackle]

Hi Sam
Thanks for your reply, I not sure if I can explain my thinking very well, but if the IF resonant frequency is higher and the oscillator frequency is the same then wont it "in effect" search for a higher RF frequency in order for the resonant circuits to work. At the moment R4 is showing at a lower frequency on the dial 1600 so it will tend to be in tune at a 10kHz higher frequency marking on the dial 1500.

Am I nuts, or does this make sense to anyone else.
Thanks Mike

p.s.
Metal tools and spanners upset the trimmers in this radio so I have made myself a little trimming tool. I found a piece of nylon and drilled a hole in the centre the size of the "flats" across a 4 BA nut. I then heated the nut and pushed it into the end of the nylon.
   

[Terry]

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!

[pwdrive]

Changing what is just the resonant frequency of the IF transformer should not alter the RF frequency.
Tracking is getting the aerial in step with the RF.
If the RF is  low i.e.1600m instead of 1500m then you need to lower the oscillator, try a cap across the trimmer to alter its range. I think I have that the right way round, waiting for the corrections to pour in.

Tracking generally refers to maintaining the same (or near as damn it) difference frequency (the IF) between the resonant frequency of the RF tuned circuit(s) and the oscillator frequency when tuning across the band.

Lawrence.