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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 4th Nov 2018, 10:50 pm   #1
Paul JD
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Default Field Coil Speaker

Currently working on a Goblin Time Spot with a field coil speaker. The speaker cone was badly torn and the field coil was open circuit so my initial thought was to replace it with a permanent magnet speaker and a separate filter choke. However, curiosity got the better of me so I decided to start unwinding the coil to see if I could find the break. Of course once you have started.....! About 3 hours later found the break right at the start of the coil! Break repaired and coil re-wound (another 3 hours), patched up the torn cone and the speaker is now working perfectly. I'm a happy bunny

The coil is 1500 ohms of 35 awg which equates to just under a mile of wire all done by hand - I think I've earned a beer
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Old 5th Nov 2018, 12:08 am   #2
AC/HL
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Default Re: Field Coil Speaker

So do I
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Old 5th Nov 2018, 8:36 am   #3
Kevin Hoyland
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Default Re: Field Coil Speaker

Hello Paul.
Well done outstanding work. I have 3 Goblin Time spot radios all working, and I never get tired of looking at them.

Kevin.
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Old 5th Nov 2018, 9:12 am   #4
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Default Re: Field Coil Speaker

Yes they are nice looking radios. Mine needs quite a bit of work, the chassis is quite rusty and the cabinet needs re-finishing but I think it will be worth the effort in the end.
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Old 5th Nov 2018, 9:50 am   #5
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Default Re: Field Coil Speaker

Well done Paul, nice job.
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Old 5th Nov 2018, 9:54 am   #6
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Default Re: Field Coil Speaker

I was lucky with mine, the 'speaker is OK, but sometimes the clock starts backwards.
well done, hand winding, wow! RSI injury?
Sam.
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Old 5th Nov 2018, 11:33 am   #7
Paul JD
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Default Re: Field Coil Speaker

Funnily enough I've just been looking at the clock and surprise surprise that is open circuit as well. Time to think about building or buying a coil winder I think!
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Old 5th Nov 2018, 12:18 pm   #8
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Default Re: Field Coil Speaker

Clock coils can be rewound with fewer turns of thicker wire then run from a small transformer. A standalone clock can use a basic wallwart though a space in the case would be better for one that's part of another unit.
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Old 5th Nov 2018, 12:28 pm   #9
Paul JD
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Default Re: Field Coil Speaker

Quote:
Originally Posted by Herald1360 View Post
Clock coils can be rewound with fewer turns of thicker wire then run from a small transformer. A standalone clock can use a basic wallwart though a space in the case would be better for one that's part of another unit.
Thanks for the info - Yes I've read some other threads on the forum about this so that is one option to consider. There is plenty of room in the chassis so it would be easy enough to mount a small transformer from a cannibalised wall wart.
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Old 5th Nov 2018, 12:55 pm   #10
David G4EBT
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Default Re: Field Coil Speaker

Wow - not just unwinding the coil, but presumably, at the same time, winding the wire onto a spool to reuse without kinking it, then rewinding the coil again? Quite a task, and in the true traditions of 'make do and mend' & 'waste not - want not'. I've only re-wound one field coil, and as with most things in life, the only time you think you might do it twice, is just before you've done it once! In my case, I simply used new wire and a homebrew motor -powered coil winder and that was challenging enough (2.3kms of wire).

I've hand-wound a 1,000 Ohm coil on a set of HR headphones by hand, and that was a doddle by comparison.
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Old 5th Nov 2018, 1:15 pm   #11
Paul JD
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Default Re: Field Coil Speaker

Quote:
Originally Posted by David G4EBT View Post
Wow - not just unwinding the coil, but presumably, at the same time, winding the wire onto a spool to reuse without kinking it, then rewinding the coil again?
Yes that's pretty much exactly how I did it David. I didn't really intend to unwind it all when I started, it was a case of "I'll just unwind a bit to see if I find the break". Of course once you have started.....
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Old 12th Nov 2018, 12:07 am   #12
Phil G4SPZ
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Default Re: Field Coil Speaker

Fantastic work Paul, such tenacity and patience is commendable. Well done!
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Old 14th Nov 2018, 9:50 pm   #13
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Default Re: Field Coil Speaker

Nice work on the coil...murphy's law was applied in your case it was the last piece of wire that was broken

I usually patch my speakers with kitchen paper soaked in a solution of wood glue and water, it works very well and retains flexibility and resistance. how did you patch that one? sometimes i keep modern speakers to reuse the cone and the modern foam suspension when the old one is badly damaged. when i reuse the cone i keep the original spider and voice coil and glue the new cone on it, sometimes keeping a bit of the old cone in place because they tend to be deeper....it works very well, sounds good and doesnt rattle. not for audiophile grade of course but more than enough to keep a radio going
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Old 14th Nov 2018, 10:12 pm   #14
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Default Re: Field Coil Speaker

Yep - great work Paul ...... and no departure from originality. Any clue as to why it went open .... green spot?
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Old 15th Nov 2018, 1:57 pm   #15
Paul JD
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Default Re: Field Coil Speaker

Quote:
Originally Posted by PortugalTV View Post
I usually patch my speakers with kitchen paper soaked in a solution of wood glue and water, it works very well and retains flexibility and resistance. how did you patch that one?
Yes pretty much the same - I used PVA glue and tissue paper but put some fine gauze material over the top, it's the stuff sold at garden centres as weed control fabric.
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Old 15th Nov 2018, 2:00 pm   #16
Paul JD
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Default Re: Field Coil Speaker

Quote:
Originally Posted by Tractionist View Post
Yep - great work Paul ...... and no departure from originality. Any clue as to why it went open .... green spot?
It looked as though the wire had just become detached from the copper strip it was connected to, just re-soldered it! The radio looked like it had been stored in a damp location so corrosion may have had something to do with it.
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Old 16th Nov 2018, 10:25 pm   #17
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Default Re: Field Coil Speaker

Nice one well done Paul.
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Old 28th Nov 2018, 10:10 am   #18
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Default Re: Field Coil Speaker

Is that PVA glue covering the diaphragm and outer corrugations?
If it is the bass response is likely to be severely handicapped, I did that on my first speaker repair and the sound was very thin and somewhat quieter.

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Old 29th Nov 2018, 1:22 pm   #19
Paul JD
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Default Re: Field Coil Speaker

Yes it is, unfortunately the original paper was so fragile it seemed to be the only way of doing anything with it. The sound quality is OK but not brilliant and as you say I'm sure the PVA will be having a negative effect on it. I might look into the possibility of grafting a new cone onto the existing voice coil at some point in the future.
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Old 1st Dec 2018, 9:18 pm   #20
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Default Re: Field Coil Speaker

On my frequent visits to Thailand I found shops there that sell individual loudspeaker parts (sold with disco/PA type eqpt) parts include voice coils (imperial and metric sizes), rolled surrounds and cones. But alas they all use modern high flux ceramic permanent magnets, so don't give up on your field coil winding skills. They also sell the rigs for aligning and gluing the coils. You can barely buy basic components here in UK now except on-line. Backwards progress or what?

Mind you its a long way to go for a 50 Bhat voice coil...but a good excuse for a holiday.
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