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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 10th Mar 2023, 4:35 pm   #1
RojDW48
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Default Field coil speaker?

This is the speaker in my Marconi C10A corner radio. It seems to have a lot of wires attached. Is it a field coil type - I have never seen one in the flesh.
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Old 10th Mar 2023, 4:48 pm   #2
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Default Re: Field coil speaker?

Yes, two wires to the usual speech coil and another two to the field coil (950 ohms).
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Old 10th Mar 2023, 5:43 pm   #3
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Default Re: Field coil speaker?

Before you do any restoration work it would be worth checking if the field coil is open circuit, as they often are.

If it is, you'd need to decide whether to have it rewound, which would be quite costly, or whether to replace it with an 8" 3.5/4 Ohm permanent magnet speaker. If you did that, as the field coil also acts as a smoothing choke, you'd either need to buy a suitable choke, rated to cope with the HT current, or use 1K resistor of suitable wattage.

The are two lots of service data - 4 pages of RER, and 2 pages of Radio Servicing. If you don't already have it, the RER one in more comprehensive in that it has above and below chassis component placements, and the two dial stringing arrangements (which look like the work of Satan.)

It went on the market in October 1947, rather late in the day for a field coil speaker, so was probably a design which dates from say 1939, paused for the war years. If the field coil is intact, I'd be cautious about running the radio for long before checking it out, (leaky capacitors etc) and only through a lamp limiter, as excess HT current might/would be 'curtains' for the coil.

Good luck with it.
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Old 10th Mar 2023, 6:06 pm   #4
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Default Re: Field coil speaker?

Quote:
Originally Posted by Simon Gittins View Post
Yes, two wires to the usual speech coil and another two to the field coil (950 ohms).
Thanks - I can see it on the circuit diagram now.
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Old 10th Mar 2023, 6:10 pm   #5
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Default Re: Field coil speaker?

I have no experience with this type of speaker, but the Odhams "Radio Television & Electrical Repairs" book describes a type where the field coil has an additional hum-bucking winding that was connected in series with the speech coil to neutralise residual hum induced by the field winding. So a field coil could have two sets of windings.

I think that Dad's late 1940's HMV table radio had a mains-energised speaker.
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Old 10th Mar 2023, 6:13 pm   #6
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Default Re: Field coil speaker?

Quote:
Originally Posted by David G4EBT View Post
Before you do any restoration work it would be worth checking if the field coil is open circuit, as they often are.

If it is, you'd need to decide whether to have it rewound, which would be quite costly, or whether to replace it with an 8" 3.5/4 Ohm permanent magnet speaker. If you did that, as the field coil also acts as a smoothing choke, you'd either need to buy a suitable choke, rated to cope with the HT current, or use 1K resistor of suitable wattage.

The are two lots of service data - 4 pages of RER, and 2 pages of Radio Servicing. If you don't already have it, the RER one in more comprehensive in that it has above and below chassis component placements, and the two dial stringing arrangements (which look like the work of Satan.)

It went on the market in October 1947, rather late in the day for a field coil speaker, so was probably a design which dates from say 1939, paused for the war years. If the field coil is intact, I'd be cautious about running the radio for long before checking it out, (leaky capacitors etc) and only through a lamp limiter, as excess HT current might/would be 'curtains' for the coil.

Good luck with it.
Thanks David - I have had it for about 3 years and all I did was replace the power supply caps. It has been running very nicely all that time till recently it made a kind of farting noise and died. So far I have replaced a dodgy looking 0.005 cap V3/vol to no avail. I'm getting a little hum (independent of vol) but nothing else. Any thoughts very welcome.

Roger
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Old 10th Mar 2023, 7:47 pm   #7
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Default Re: Field coil speaker?

Quote:
Originally Posted by emeritus View Post
I have no experience with this type of speaker, but the Odhams "Radio Television & Electrical Repairs" book describes a type where the field coil has an additional hum-bucking winding that was connected in series with the speech coil to neutralise residual hum induced by the field winding. So a field coil could have two sets of windings.
A shading ring was another method used instead of a hum-bucking winding.

Lawrence.
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Old 10th Mar 2023, 7:56 pm   #8
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Default Re: Field coil speaker?

That's a good point about a hum-bucking winding, most field coil speakers had one. In this case there isn't one shown, at least not as a separate winding. More about them in this thread: https://www.vintage-radio.net/forum/...ad.php?t=86359
Voltage checks are a good start or seeing if it works on the Gram position would show whether the amplifier stages are ok.
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Old 11th Mar 2023, 1:13 am   #9
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Default Re: Field coil speaker?

Quote:
Originally Posted by Simon Gittins View Post
That's a good point about a hum-bucking winding, most field coil speakers had one. In this case there isn't one shown, at least not as a separate winding. More about them in this thread: https://www.vintage-radio.net/forum/...ad.php?t=86359
Voltage checks are a good start or seeing if it works on the Gram position would show whether the amplifier stages are ok.
I am only using the gram position - the radio worked, but we use it as a CD player - and it was very nice too!
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Old 11th Mar 2023, 10:14 am   #10
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Default Re: Field coil speaker?

If you are solely using the radio as an amplifier for a CD player (interesting?) will not the line level output from the player be somewhat overloading the first AF stage?
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Old 11th Mar 2023, 10:40 am   #11
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Default Re: Field coil speaker?

I suggest following David's advice in Post 3. As you have slight hum the field coil may still be ok, if it is you need to protect it from excess current due to leaky capacitors.
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Old 11th Mar 2023, 12:08 pm   #12
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Default Re: Field coil speaker?

Quote:
Originally Posted by Edward Huggins View Post
If you are solely using the radio as an amplifier for a CD player (interesting?) will not the line level output from the player be somewhat overloading the first AF stage?
I haven't metered it, but the volume range seems perfectly normal and in line with the radio. It's actually a DVD player, because you can buy small DVD players which sit nicely on top of it, whereas all CD players seem to be standard hi-fi component size.
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Old 23rd Mar 2023, 8:50 pm   #13
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Default Re: Field coil speaker?

Guilty party found! This is C20 on the diagram and it was looking a bit worn out (only 75 years old - planned obsolescence I call it!). It appears to my poor brain to go from the HT line to ground. Does it have some sort of smoothing function? Isn't it a bit small at 0.1uF?

Also - in the last photo you can see the rear leg has a kind of scoop out of it with two rings top and bottom. Any ideas as to what they were for?
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Old 23rd Mar 2023, 8:54 pm   #14
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Default Re: Field coil speaker?

And another question. The old field coil speaker is working well, but there is a facility for attaching an ext speaker - only 2 sockets for it. Would that need to be FC or standard type?
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Old 23rd Mar 2023, 9:07 pm   #15
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Default Re: Field coil speaker?

That 0.1uF/100nF cap is a decoupling capacitor having a low reactance at RF and audio frequencies, most important.
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Old 23rd Mar 2023, 9:08 pm   #16
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Default Re: Field coil speaker?

Bulgin used to supply a 4-pin plug and socket for field coil extension speakers. This is from their 1936/37 catalogue.
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Old 23rd Mar 2023, 9:18 pm   #17
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Default Re: Field coil speaker?

Well done for finding that.
The extension speaker would be a standard type.
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Old 23rd Mar 2023, 11:44 pm   #18
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Default Re: Field coil speaker?

I assume you have replaced C25, the interstage coupling capacitor.
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Old 24th Mar 2023, 12:53 am   #19
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Default Re: Field coil speaker?

Quote:
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I assume you have replaced C25, the interstage coupling capacitor.
Yes I have. I am barely a journeyman repairer, but that was my first thought.
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