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Old 19th May 2019, 2:43 pm   #1
PaulR
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Default Clarke and Smith 8812 FM IF transformer

The first IF transformer on my C&S is badly damaged, partly to a previous owner pushing the core too far into the former and leaving it stuck there and partly because of my ham fisted attempts to retrieve the remains.

I once successfully replaced an IF transformer in an AM set by using a resistor and a capacitor to bypass it. A kind forum member later sent me a replacement and I removed the by pass. I assume that the C&S transformer is unobtainable, even from others on the forum so I was wondering about bypassing it to get the set working at least.

Is this likely to be successful, and if so what value of capacitor would be recommended?
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Old 19th May 2019, 3:01 pm   #2
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Default Re: Clarke and Smith 8812 FM IF transformer

What is the IF frequency?
If 10.7Mhz then is this a possible replacement?
http://kambing.ui.ac.id/onnopurbo/or...ifications.htm

If there is HT on the filter you will need a capacitor to isolate them.
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Old 19th May 2019, 3:08 pm   #3
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Default Re: Clarke and Smith 8812 FM IF transformer

If all else fails you could have a go at making your own using stock inductors, trimmer capacitors etc, this shows one for AM but FM should be doable:

https://www.antiqueradios.com/forums...?f=12&t=320861

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Old 19th May 2019, 9:50 pm   #4
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Default Re: Clarke and Smith 8812 FM IF transformer

Thanks for the replies. The filter idea seems a possibility, I am not sure about building my own. In my ignorance of these things I am not sure why they have three leads. How would these be connected?

The transformer in question is L16 and 17 on the attached photograph of the circuit diagram
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Old 19th May 2019, 10:22 pm   #5
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Default Re: Clarke and Smith 8812 FM IF transformer

Cannot help with the mod but the three terminals are ”in common out”.
Lawrence’s suggestion may be easier or you may well find an IF transformer from another valve radio would just work.
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Old 19th May 2019, 11:19 pm   #6
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Default Re: Clarke and Smith 8812 FM IF transformer

What is wrong with the transformer? If it is just chewed up cores, then it would be possible to replace these. If the coils are open circuit, then a new transformer is required.
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Old 20th May 2019, 8:59 am   #7
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Default Re: Clarke and Smith 8812 FM IF transformer

The transformer consists of two coils formed round some sort of plastic or (I suspect) cellulose tubes. The tubes sit on two pegs within the can - I am not sure how they couple like that but it must have done

The core in the offending coil had been pushed right down to the peg and the slot in the top of it was a bit chipped. The core was stuck like that. I tried some switch cleaner to soften any wax but when it freed the core just turned round without moving up. Eventually the former split apart and the coil is now o/c.
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Old 20th May 2019, 9:35 am   #8
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Default Re: Clarke and Smith 8812 FM IF transformer

If one of the coils is still tuneable, use that as the anode feed for the mixer. Add a grid leak resistor to the following valve, 100k or so, and couple the anode of the mixer to the grid of the IF with ceramic 50pf or so.

Worth a try, at least.

If the set does AM too, dual frequency IF, this probably won't work.
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Old 20th May 2019, 10:06 am   #9
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Default Re: Clarke and Smith 8812 FM IF transformer

Thanks Chris, that is certainly worth a try. It is AM/FM but I would rather have FM working than AM if there is a choice.
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Old 20th May 2019, 10:21 am   #10
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Default Re: Clarke and Smith 8812 FM IF transformer

Ahh...., disconnect the AM IF secondary from the grid of the IF valve too and wire the VHF IFT working coil in series with the AM IFT primary (if it's the original VHF primary, it'll be connected that way to start with). Neither AM nor FM will be as selective as intended, but with a bit of luck there shouldn't be much difference otherwise.

Another thought.... if the AGC line feeds the IF grid via the IFT secondaries, you'll need to take the grid leak to the AGC line, rather than ground.
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Old 20th May 2019, 2:35 pm   #11
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Default Re: Clarke and Smith 8812 FM IF transformer

I think I might possibly have repaired it! I took it off again and looked very carefully through a jeweller's loupe and saw an end to the coil. I managed to unwrap enough to connect it to the tag. I have tried to repair the mangled end of the former using the insulation from a crimp forked connector and araldited it to the remains of the former and to the stub on the "chassis" of the transformer.

I now get an ohm or two between the tags. I have put some metal lacquer over the unwrapped bit of coil to insulate it and hold it to the new support. I am now letting everything dry and set. It will be a miracle if it works but there is some hope.

Can I align the transformer before refitting it using the signal gen set to 10.7 mHz and the oscilloscope to avoid putting any unnecessary strain on my repairs?
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Old 20th May 2019, 6:02 pm   #12
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Default Re: Clarke and Smith 8812 FM IF transformer

It's unlikely that tuning up the IFT out of the set will work very well- circuit strays etc will be very different.
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Old 20th May 2019, 7:09 pm   #13
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Default Re: Clarke and Smith 8812 FM IF transformer

Ok, thanks. I will reassemble it then.
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Old 22nd May 2019, 2:29 pm   #14
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Default Re: Clarke and Smith 8812 FM IF transformer

It lives again, amazingly. I haven't dared to touch the primary which was the coil in question but I have got a good signal by adjusting the secondary and I will leave well very much alone now. Thank you for the support, all.

Moral,the IF transformers are very fragile on these sets
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Old 22nd May 2019, 2:46 pm   #15
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Default Re: Clarke and Smith 8812 FM IF transformer




Congratulations!
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Old 23rd May 2019, 9:26 am   #16
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Default Re: Clarke and Smith 8812 FM IF transformer

That's a great job and in many ways is what we are all about on this forum.
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Old 23rd May 2019, 12:51 pm   #17
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Default Re: Clarke and Smith 8812 FM IF transformer

Thanks Simon. I have to say I am now very tempted to have a go at peaking the damaged core. It is a but stuck due to the shellac based lacquer I used to stabilise the coil but I imagine that warming with a hair dryer would free it.

FM has always sounded sibilant so maybe it could do with a proper alignment. I have a Philips PM6455 FM signal generator that I bought on a whim and have never used.
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