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Old 9th Jan 2019, 5:49 pm   #1
boxdoctor
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Default KB FB10 - I.F. frequency query.

A friend has presented me with his FB10 to fix for him. It has been out of use for well over 20 years, and, to start with, the dial drive cord had decayed to the point where it had come apart.
I replaced this with some fishing line as recommended in the service sheet, and upon trying the set, I had faint audio with some distortion. Cutting a long story short, the output valve holder was carbonised and leaky. I carefully drilled it out and fitted a ceramic one in the paxolin "factory bodge" panel which incorporated the original holder. Somewhat better results ensued, but the tuning seemed broad compared with the FB10 which I have in the kitchen. Sensitivity is below par as well. All waxies etc. have been replaced.
I thought to check the I.F.s, and found that they were aligned on 465 Khz rather than the service manual's 422Khz. recommendation.
Has anybody come across this before, i.e. was there a known production batch with 465Khz I.F.s? Or has the phantom twiddler been at work. The audio coupling cap has been replaced at some time.
I dont want to disturb the I.F.s unless necessary, as they have the dreadful paxolin threads to hold the cores in position, which I know from experience are very brittle and delicate. Tony.
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Old 9th Jan 2019, 6:27 pm   #2
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Default Re: KB FB10 - I.F. frequency query.

All of the KB service manuals I have for these models give 422kHz for the IF frequency.
Can I ask how you determined the IF was 465kHz and what equipment you used.

Mike
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Old 9th Jan 2019, 10:08 pm   #3
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Default Re: KB FB10 - I.F. frequency query.

Hi Mike,
Thanks for the quick and helpful reply.
I injected the sig generator (Gould-Advance SG200, accurate) into the mixer grid circuit via a 50pf mica cap. A quick sweep across the likely frequency band showed a sharp peaked response at 465Khz, with a bandwidth (estimated) of about 6 Khz or so. There was no other response around that frequency. I specifically checked on the low frequency side, as that was where I was expecting the I.F. amplifier to be tuned to.
Looks like the previous "repairer" decided that the I.F.s were "off", and needed re-aligning. "465 must be right, they're all 465 ain't they, innit?"
I am reluctant to do any adjustment due to the woeful screw thread arrangement with the cores referred to in my previous post. Tony.
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Old 10th Jan 2019, 9:28 am   #4
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Default Re: KB FB10 - I.F. frequency query.

Sounds like the measurement test was OK, strange, they may well have been twiddled.
Sorry I don't know enough technically to be able to say if it would make any difference to the response of the radio.

Mike
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Old 10th Jan 2019, 10:54 am   #5
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Default Re: KB FB10 - I.F. frequency query.

Thanks for the help, Mike.
I think I may leave things as they are, although the tracking may not be as good as it could be due to the wrong I.F.. I suspect it will be used only on Radio 5, or Radio 4 long wave, in the kitchen, as he has an enviable Hi Fi set-up in his living room.
Cheers, Tony.
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Old 10th Jan 2019, 10:56 am   #6
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Default Re: KB FB10 - I.F. frequency query.

Have you checked the local oscillator is properly adjusted to the 'new' IF and tracking correctly?

Frank C.
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Old 10th Jan 2019, 11:06 am   #7
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Default Re: KB FB10 - I.F. frequency query.

If the alignment is too far out you may get whistles on some stations. I seem to recall that early Japanese radios with a 450Khz IF would whistle on the BBC Home service (Radio 4 as it is now) when they used 330 metres due to second harmonic interference (900Khz). We used to slightly alter the IF to 455Khz which took the 2nd harmonic just out of range of R4 but didn't effect the sensitivity or tuning of the radio to any great extent.
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Old 10th Jan 2019, 11:18 am   #8
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Default Re: KB FB10 - I.F. frequency query.

Quote:
Originally Posted by boxdoctor View Post
Thanks for the help, Mike.
I think I may leave things as they are, although the tracking may not be as good as it could be due to the wrong I.F..

You might find that part of the waveband is 'bunched up' at one end or the other. Oddly enough I had an FB10 that had it's IF tuned to 465K but had also been heavily 'got at'. I attempted to return it to original and partially succeeded and retuned the IF's to 422 but I could never get it to track properly and sensitivity and selectivity was very poor. I even had the transformers apart to see if any dastardly deeds had been done inside them but couldn't find anything. In the end the FB10 in question was used as spares to fix a much better one which I now have.
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Old 10th Jan 2019, 1:07 pm   #9
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Default Re: KB FB10 - I.F. frequency query.

Quote:
Originally Posted by G0HFCFrank View Post
Have you checked the local oscillator is properly adjusted to the 'new' IF and tracking correctly?
Frank C.
Hello Frank. No, I've not done anything with the front-end alignment, as that would be making things even more out of spec., and the dial would be more inaccurate if I track the front end to suit the different I.F. I was going to try to fix the I.F.T.s, but it will make for less hassle to leave it as is.
@Sideband. Thanks for the comments and observations. Moving the I.F. 10 Khz is, at the frequecy in question not so bigger percentage error as moving it 40Khz, so your little dodge to avoid the heterodyne is a neat answer to that particular problem.
I have some pin vices in the workshop, so I might see if it is feasible to align the cores using one of those to grip the adjusters, and when optimised, seal them with a blob of epoxy resin. I think the owner will be not critical about the ultimate results providing he gets Radios 4&5 reasonably. He is located in a good spot "out in the sticks" for reception. Tony.
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Old 10th Jan 2019, 5:27 pm   #10
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Default Re: KB FB10 - I.F. frequency query.

I doubt they need sealing, or locking, epoxy sounds like a permanent solution to never being able to adjust them again.

Mike
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Old 10th Jan 2019, 5:48 pm   #11
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Default Re: KB FB10 - I.F. frequency query.

I have been thinking about the issue of adjusting the IF transformers, I couldn't remember what they looked like so went and had a look at my FB10.
Here are some photos.
Click image for larger version

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Name:	KB FB10 Achassis.jpg
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My radio appears to be a MK1 model with the frame aerial and 6BW4 output valve and 6X5GT rectifier.
As far as I can see all models appear to have the same part number for their IF transformer.

In my radio it is easy, with a fine screwdriver, to turn the thin brass screws to adjust the IF, there is an adjuster above and one below each of the cans..

Mike
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Old 10th Jan 2019, 8:16 pm   #12
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Default Re: KB FB10 - I.F. frequency query.

Yes you don't need a special trimming tool and if I remember, the adjusting stud is threaded into bakelite so shouldn't be a problem to turn. All you need is something like nail varnish to lock them with (my wife gave me a pot of pink nail varnish so any that I have done over the years are instantly recognisable....)! Failing that, a dab of correcting fluid (Tippex) should do the job.
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Old 11th Jan 2019, 11:02 am   #13
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Default Re: KB FB10 - I.F. frequency query.

Thanks for the further comments and advice, Gents.
Mike: I have had the 2nd I.F. can out of the set to recover the core assembly which dropped down into the coil former when I tried to turn the adjusting screw. I can't see any terrible crime in fixing them with a resin glue once they are accurately setup. Re-adjusting I.F.s is not normally a regular operation, and if they are set correctly they shouldn't need to be disturbed again.
Sideband: The adjusters on this set are not set in Bakelite, if they were I doubt I would be having all this drama. They are going through a couple of layers of Paxolin sheet, into which I suspect that they cut their own threads when first fitted, in the manner of a self-tapping screw. The core (unintended pun) of the problem is that the paxolin has become friable due to it's age. So the threads of the adjusters no longer grip on the paxolin. I shall re-align them with the set in such a position that the I.F.s are horizontal, and then fix them before standing the set upright again, thus defeating the effects of gravity.
Other things press for my time, but I hope to resolve this problem quickly, and will report results, hopefully, very soon.
Thanks for the helpful tips.Tony.
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Old 11th Jan 2019, 11:49 am   #14
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Default Re: KB FB10 - I.F. frequency query.

If you feel adventurous, I have a set of IF transformers from a scrap set...…..cost of postage only.


Maybe I was thinking of another set where I had to change the parallel capacitor in the IF transformer. They were of Bakelite construction.
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Old 11th Jan 2019, 11:49 am   #15
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Default Re: KB FB10 - I.F. frequency query.

I see what you mean now. The paxolin gripping the brass adjuster threads can be seen in my first photo of the undersides and does look a delicate affair.

Yes the IF cans on other radios, not this one, which have a Bakelite former where they have a threaded ferrite slug with a small slot in the top and bottom can be a devil if the ferrite slug chips or cracks.

Mike
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Old 16th Jan 2019, 2:11 pm   #16
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Default Re: KB FB10 - I.F. frequency query.

Finally got the I.F.s to line up on 422Khz, and fixed them with liquid contact adhesive. This can be peeled off relatively easily in the unlikely event that further "twitching" is ever required.
Still found it necessary to put a 15pf cap across the primary of the second transformer to get it to peak spot on. Gain and selectivity now acceptable. Just the audio stages to sort out. I note that the O.P. valve has had a replacement bias resistor. Some other minor 'mods' to disentangle.
A big thanks to all for the help. A special thanks to Sideband for the offer of the pair of transformers. Tony.
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