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Old 11th Apr 2024, 1:21 am   #1
adeftereos
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Default Cossor 464ac faulty Oscillator coil

Hello,


I've been working on this set for a while and it currently performs well on MW and LW. Unfortunately SW is silent. The problem was traced to an open circuit oscillator reaction coil, which has almost completely rotted away.


The trouble is I have no idea how to rewind a replacement as the old wire has almost completely dissapeared.


As shown in the attached photo the SW reaction coil is wrapped around a hollow ferrite cylinder which fits inside a bakelite former. The SW tuning coil is wrapped around the former.


Does anyone have any experience in rewinding these? Which direction shoiuld I wrap the wire? Looking inside the former there are two channels, or grooves in either side of where the cylinder fits. I assume the wore goes around these grooves. It certainly woould not fit inside teh bakelite former if I would the wire around the diameter of the ferrite cylinder.


Also waht kind of wire should I use? According to the manual the coil should have a resistance of aboyt 29 Ohms, so some sort of resistance wire is indicated.


I'd be grateful for anyone's thoughts and experience.


Angelos
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Old 11th Apr 2024, 9:11 am   #2
vinrads
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Default Re: Cossor 464ac faulty Oscillator coil

I have read the post several times, can someone elucidate please, it would have to have a lot of turns to read 29ohms. Mick.
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Old 11th Apr 2024, 9:16 am   #3
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Default Re: Cossor 464ac faulty Oscillator coil

From memory I think it's about 8 turns with a wire the width of a human hair. You need to get the direction right. I'm sure theres a thread on this somewhere as it's Avery common fault in this set

I tried and failed and gave up as i don't listen to short wave

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Old 11th Apr 2024, 10:21 am   #4
adeftereos
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Default Re: Cossor 464ac faulty Oscillator coil

I suspect the wire may be resistance wire. But I don't have enough of the original to be able to tell. What is left just looks like green silk thread.
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Old 11th Apr 2024, 12:15 pm   #5
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Default Re: Cossor 464ac faulty Oscillator coil

Resistance wire was sometimes used in local oscillator circuits, as a way to even out the oscillator amplitude across the tuning range and to prevent the gain being too high at one or the other ends of the coverage, which could lead to the oscillator 'squegging'.

Other designers used a low value resistor in series with part of the oscillatory circuit, in your case it may be easier to rewind the reaction part of the coil with a few turns of ordinary enamelled wire, and to include a small resistor, 27 Ohms for example, in series with the winding to get the required effect.

Some experiments may be necessary....
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Old 11th Apr 2024, 2:13 pm   #6
adeftereos
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Default Re: Cossor 464ac faulty Oscillator coil

Thank you. I'll probably have to go the enamelled wire route.

Would anyone know what a correctly wound coil around the ferrite cylinder would look like?

For example, should the wire loop pass through the hole in the center of the cylinder?
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Old 13th Apr 2024, 11:36 pm   #7
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Default Re: Cossor 464ac faulty Oscillator coil

Good news! It works.

As per the received advice I wrapped 8 - 9 turns of enamelled 0.07mm wire around the middle ferrite cylinder. I thought that having the in parallel would maximise the signal transfer. I used clear nail polish every few turns, just to hold everything in place.

Initially it didn't work but reversing the coil connections brought SW to life.

Thank you to everyone for your thoughts and advice!
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Old 14th Apr 2024, 12:07 pm   #8
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Default Re: Cossor 464ac faulty Oscillator coil

Did you use the resistor?
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Old 15th Apr 2024, 1:24 pm   #9
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Default Re: Cossor 464ac faulty Oscillator coil

Yes. I added a 22 Ohm resistor in series. I thought it best to try and replicate the circuit characteristics as closely as I could.

I wonder why Cossor used resistance wire for the coil? Would it make much difference to the way the coil functioned?
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Old 15th Apr 2024, 4:32 pm   #10
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Default Re: Cossor 464ac faulty Oscillator coil

Hi, I believe that certain Wearite P coils also used resistance wire for the osc coupling winding. This probably reduced the Q of the coil and gave a flatter response across the band.
26 ohms seems a bit high, could it have been 2R6, which seems more likely, but if it is working just leave it as it is

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Old 15th Apr 2024, 5:08 pm   #11
adeftereos
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Default Re: Cossor 464ac faulty Oscillator coil

Hi Ed,

I've checked the Trader sheet and (just in case there was a misprint in either way) the Cossor manufacturer manual. Both list the SW oscillator reaction coil as having a resistance of 29.5 Ohm.

It is currently working, so I'm minded to leave well enough alone. However, SW is not amazing. I know the set has been fiddled with so an alignment may improve the performance. That's the next job.

On the other hand, from reading other threads on the 464AC, my sense is that SW performance wasn't a strong point in these sets anyway.
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Old 15th Apr 2024, 6:25 pm   #12
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Default Re: Cossor 464ac faulty Oscillator coil

From memory, I believe that SW required an external aerial on this set
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Old 15th Apr 2024, 7:01 pm   #13
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Default Re: Cossor 464ac faulty Oscillator coil

This is a 1940's domestic set and relies on a long wire antennae as suggest by Gabe001.

SW was more of a marketing thing than actually much use that being said I picked up half a dozen overseas stations on a Roberts RMB using 20ft of wire.

some were decently strong but the content not so much

Cheers

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Old 15th Apr 2024, 10:09 pm   #14
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Default Re: Cossor 464ac faulty Oscillator coil

It seems OTT using resistance wire for the oscillator feedback winding on this (single-layer) coil, but it was not uncommon to use it for multilayer coils.

Why? Because multilayer coils have significant self-capacitance, and with the inductance of the feedback coil, there could be unwanted spurious resonances in the thing as a whole. It can give weird effects when trying to tune to frequencies in the vicinity of these unwanted resonances.

Using resistance wire kills the Q of these, leaving only the principal resonance, of the tuning capacitor and the main coil (which must of course be wound with copper wire!) with no awkward effects.
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Old 16th Apr 2024, 12:01 am   #15
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Default Re: Cossor 464ac faulty Oscillator coil

I know nothing about this coil but reading a post on another forum.

"This coil is only 8 turns of wire.
The poster "Afeftereos", is under the impression that this is resistance wire and he may well be correct. However he and some of the respondents think that it is 26 ohms when the Cossor manual gives the resistance as 2.6 ohms."

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Old 18th Apr 2024, 12:36 am   #16
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Default Re: Cossor 464ac faulty Oscillator coil

Not sure what to say. I’m happy to be proven wrong regarding the resistance of the SW reaction coil. The version of the manufacturers manual I have lists the coil as L10 and gives the resistance as 29.5 Ohms.

Perhaps the circuit was modified during production?
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Old 18th Apr 2024, 5:06 am   #17
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Default Re: Cossor 464ac faulty Oscillator coil

29.5 ohms does seem an awful lot, even for resistance wire!

Can you post a photograph of that section of the manufacturers manual for us?
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Old 18th Apr 2024, 7:59 am   #18
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Default Re: Cossor 464ac faulty Oscillator coil

Two snips attached

Cheers

Mike T
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Old 18th Apr 2024, 11:32 am   #19
kalee20
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Default Re: Cossor 464ac faulty Oscillator coil

Can't argue with that! Thanks Mike! It vindicates Adeftereos's assertion.

I have checked typical resistance figures of 40SWG wire (0.125mm). Copper is 1.4 ohms/metre; resistance wire depends on the alloy but 80% nickel alloy gives 88 ohms/metre.

So 29.5 ohms would require 335mm of wire; 8 turns would be 41mm per turn, or a diameter of the coil of 13mm. It's credible, at least, and a tweak to wire size would bring it to the Cossor figure.

(It's unlikely to be critical. Something like this, unlike the tuned winding, doesn't matter within wide limits and the Cossor designers would no doubt have used a stock size.)

Quote:
Originally Posted by adeftereos View Post
However, SW is not amazing. I know the set has been fiddled with so an alignment may improve the performance. That's the next job.

On the other hand, from reading other threads on the 464AC, my sense is that SW performance wasn't a strong point in these sets anyway.
I have a Cossor 500, which is very similar... and SW performance is mediocre: it's more a selling feature than of major entertainment use, as others have already pointed out, although a strong station will come through.
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Old 18th Apr 2024, 7:30 pm   #20
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Default Re: Cossor 464ac faulty Oscillator coil

Quote:
Originally Posted by adeftereos View Post
SW performance wasn't a strong point in these sets anyway.
It's not. Nothing more to add really. Very mediocre.
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