Modifying an energised speaker circuit - UK Vintage Radio Repair and Restoration Discussion Forum

Calmore · 25th Jan 2020, 9:04 pm

Hi, I'm currently fixing an old pilot Radio. The speaker is badly torn and I'm not sure how long it will last. If I can't find a replacement I'm thinking of making digging the output to take a permanent magnet speaker. The current circuit is attached.

I don't particularly wish to keep an external speaker function, so could I just wire a 4R speaker to the output of T1 and simply omit the speaker assembly L17/L18/L19 and switch S1?

I realise that the existing includes hum reduction, but all of these components can be switched out by S1. All I'd really be doing is fitting the external speaker internally and removing the components that aren't active when S1 is open. L19 is still in circuit, but does this actually serve a purpose if L18 is disconnected via S1?

TrevorG3VLF · 25th Jan 2020, 10:20 pm

L19 is there to reduce hum as well as provide a magnetic field for the speaker.
It may be possible to replace L19 with a resistor, then the old speaker can be removed to provide space for the new speaker. Increase C26 if necessary.
Open S21 and connect to the external speaker sockets.

Simon Gittins · 25th Jan 2020, 10:23 pm

L19 is still serving a purpose as it functions as a choke for the HT supply.

L18 is the hum-bucking coil to reduce hum in the internal speaker only.

If you wanted to remove the whole speaker assembly you would have to include a suitable resistor (in place of L19) that gave the correct value of HT; the hum level would still be higher than with the original component. Some experimenting with a slightly higher value of smoothing capacitor may be required.

Cobaltblue · 25th Jan 2020, 10:32 pm

In some cases if field coil not open circuit the basket of faulty speakers was removed to use the field coil as a choke.

It was then to be found hanging in some spare corner of the cabinet.

You lost some of the hum reduction but this was easily resolved in most cases.

I must admit unless it was impossible I would keep the original speaker if at all possible.

Cheers

Mike T

cathoderay57 · 25th Jan 2020, 10:36 pm

L19 certainly does - the radio will not work without it because as well as acting as an electromagnet to magnetise the existing speaker pole piece, it is part of the HT smoothing circuit that links the rectifier's high voltage output to the second smoothing capacitor and all of the valves. If you fit a permanent magnet speaker you could:
Remove the field coil L19 leaving it connected and attached somehow to the inside of the cabinet, or:
Obtain and fit an HT choke in place of L19 .
The problem with using a choke is to find one capable of taking some 80mA and of having a high enough internal resistance. Typical field coils such as L19 have a DC resistance of between 800 And 2000 Ohms (I recommend you measure it) whereas modern chokes could be as little as 200 Ohms. That means the HT voltage at the valves would be too high. You could supplement with a 1k 10W resistor in series with the choke or just use the resistor on its own but you might experience more hum than before. I think by far the best thing to do is to repair the speaker cone and try to retain the set in as original condition as possible. If the speaker sounds OK then nobody is going to see the cone once the back is on. Cheers, Jerry

cathoderay57 · 25th Jan 2020, 10:51 pm

Here are some tips on repairing speaker cones https://www.vintage-radio.net/forum/...d.php?t=127846

Calmore · 26th Jan 2020, 12:33 pm

L19 is rated at 1,400 ohms.

The situation at the moment is that the original energised speaker has been unscrewed from the case and is merely lying in the base of the cabinet. A permanent magnet speaker has been wired up as an external one and attached in place of the original and link S1 removed. Obviously a less than ideal set
Up!

I have repaired the speaker but there's a lot of repair to the speaker cone. On particular worry is a split where the cone meets the coil and also the flexible bit round the edge. It's also crumbly in places It will probably work but I'm not convinced it'll last very long at all, before falling apart again.

cathoderay57 · 26th Jan 2020, 5:42 pm

Yeah, I see what you mean, the speaker's in pretty bad shape. The outer edge cone should be relatively easily repairable, the centre area less so. If you did manage a permanent repair, it would probably last indefinitely unless you are planning to play an electric guitar through it at full volume..... The problem that usually occurs is that the repair distorts cone alignment then the centre speech coil binds against the pole piece magnet and it sounds awful. It is possible to re-centre the cone by slipping 3 or 4 paper shims between the centre of the speech coil and the pole piece magnet then adjusting the screws that lock the rear diaphragm in position. However, if you want to try a resistor smoother in lieu of L19 then you can get a 1.2k 10W wire-wound resistor fairly cheaply on eBay here https://www.ebay.co.uk/itm/AG10-1-2K...IAAOSwb-Jcng5h . Worth a try I guess. Cheers, Jerry

Calmore · 26th Jan 2020, 7:00 pm

That's *after* repair; before repair it wouldn't have held together for more that 30 seconds.

The slopey bits of the cone will be fine, it's where the cone attaches to the voice coil that I'm really concerned about. The flexi bit around the perimiter still flexes very well, but it's still very fragile. I don't hold out much hope at anything above a moderate volume, TBH.

simpsons · 26th Jan 2020, 7:34 pm

Subject to finding a pm speaker the same size, why not remove that cone and and use it to replace the electromagnet one. Wedging the spider in position whilst using a scalpel to remove the old cone and then lifting the edge from the basket.

The edge on the replacement cone can be glued down with a PVA based glue after the centre/voice coil is superglued to the centre part of the cone.

Chris

Calmore · 27th Jan 2020, 4:04 pm

That is one thing to consider.

The other thought I had was to remove the coil assembly (l18 and l19) from the energised speaker and mount it on a piece of paxolin along with the audio transformer and tag strip and screw That to the bottom of the case. I could then connect a magnet speaker via some flying leads.

Downside of that is that the rest of the speaker becomes a paperweight, but is should be reliable.

25th Jan 2020, 9:04 pm	#1
Calmore Triode Join Date: Jan 2020 Location: Totton, Hampshire, UK. Posts: 20	Modifying an energised speaker circuit Hi, I'm currently fixing an old pilot Radio. The speaker is badly torn and I'm not sure how long it will last. If I can't find a replacement I'm thinking of making digging the output to take a permanent magnet speaker. The current circuit is attached. I don't particularly wish to keep an external speaker function, so could I just wire a 4R speaker to the output of T1 and simply omit the speaker assembly L17/L18/L19 and switch S1? I realise that the existing includes hum reduction, but all of these components can be switched out by S1. All I'd really be doing is fitting the external speaker internally and removing the components that aren't active when S1 is open. L19 is still in circuit, but does this actually serve a purpose if L18 is disconnected via S1? Attached Thumbnails

25th Jan 2020, 10:20 pm	#2
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Modifying an energised speaker circuit L19 is there to reduce hum as well as provide a magnetic field for the speaker. It may be possible to replace L19 with a resistor, then the old speaker can be removed to provide space for the new speaker. Increase C26 if necessary. Open S21 and connect to the external speaker sockets.

25th Jan 2020, 10:23 pm	#3
Simon Gittins Heptode Join Date: Jan 2006 Location: Cornwall, UK. Posts: 506	Re: Modifying an energised speaker circuit L19 is still serving a purpose as it functions as a choke for the HT supply. L18 is the hum-bucking coil to reduce hum in the internal speaker only. If you wanted to remove the whole speaker assembly you would have to include a suitable resistor (in place of L19) that gave the correct value of HT; the hum level would still be higher than with the original component. Some experimenting with a slightly higher value of smoothing capacitor may be required.

25th Jan 2020, 10:32 pm	#4
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,859	Re: Modifying an energised speaker circuit In some cases if field coil not open circuit the basket of faulty speakers was removed to use the field coil as a choke. It was then to be found hanging in some spare corner of the cabinet. You lost some of the hum reduction but this was easily resolved in most cases. I must admit unless it was impossible I would keep the original speaker if at all possible. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk Last edited by Cobaltblue; 25th Jan 2020 at 10:33 pm. Reason: spacing

25th Jan 2020, 10:36 pm	#5
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: Modifying an energised speaker circuit L19 certainly does - the radio will not work without it because as well as acting as an electromagnet to magnetise the existing speaker pole piece, it is part of the HT smoothing circuit that links the rectifier's high voltage output to the second smoothing capacitor and all of the valves. If you fit a permanent magnet speaker you could: Remove the field coil L19 leaving it connected and attached somehow to the inside of the cabinet, or: Obtain and fit an HT choke in place of L19 . The problem with using a choke is to find one capable of taking some 80mA and of having a high enough internal resistance. Typical field coils such as L19 have a DC resistance of between 800 And 2000 Ohms (I recommend you measure it) whereas modern chokes could be as little as 200 Ohms. That means the HT voltage at the valves would be too high. You could supplement with a 1k 10W resistor in series with the choke or just use the resistor on its own but you might experience more hum than before. I think by far the best thing to do is to repair the speaker cone and try to retain the set in as original condition as possible. If the speaker sounds OK then nobody is going to see the cone once the back is on. Cheers, Jerry

25th Jan 2020, 10:51 pm	#6
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: Modifying an energised speaker circuit Here are some tips on repairing speaker cones https://www.vintage-radio.net/forum/...d.php?t=127846

26th Jan 2020, 12:33 pm	#7
Calmore Triode Join Date: Jan 2020 Location: Totton, Hampshire, UK. Posts: 20	Re: Modifying an energised speaker circuit L19 is rated at 1,400 ohms. The situation at the moment is that the original energised speaker has been unscrewed from the case and is merely lying in the base of the cabinet. A permanent magnet speaker has been wired up as an external one and attached in place of the original and link S1 removed. Obviously a less than ideal set Up! I have repaired the speaker but there's a lot of repair to the speaker cone. On particular worry is a split where the cone meets the coil and also the flexible bit round the edge. It's also crumbly in places It will probably work but I'm not convinced it'll last very long at all, before falling apart again. Attached Thumbnails Last edited by Calmore; 26th Jan 2020 at 12:42 pm.

26th Jan 2020, 5:42 pm	#8
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: Modifying an energised speaker circuit Yeah, I see what you mean, the speaker's in pretty bad shape. The outer edge cone should be relatively easily repairable, the centre area less so. If you did manage a permanent repair, it would probably last indefinitely unless you are planning to play an electric guitar through it at full volume..... The problem that usually occurs is that the repair distorts cone alignment then the centre speech coil binds against the pole piece magnet and it sounds awful. It is possible to re-centre the cone by slipping 3 or 4 paper shims between the centre of the speech coil and the pole piece magnet then adjusting the screws that lock the rear diaphragm in position. However, if you want to try a resistor smoother in lieu of L19 then you can get a 1.2k 10W wire-wound resistor fairly cheaply on eBay here https://www.ebay.co.uk/itm/AG10-1-2K...IAAOSwb-Jcng5h . Worth a try I guess. Cheers, Jerry

26th Jan 2020, 7:00 pm	#9
Calmore Triode Join Date: Jan 2020 Location: Totton, Hampshire, UK. Posts: 20	Re: Modifying an energised speaker circuit That's after repair; before repair it wouldn't have held together for more that 30 seconds. The slopey bits of the cone will be fine, it's where the cone attaches to the voice coil that I'm really concerned about. The flexi bit around the perimiter still flexes very well, but it's still very fragile. I don't hold out much hope at anything above a moderate volume, TBH.

26th Jan 2020, 7:34 pm	#10
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: Modifying an energised speaker circuit Subject to finding a pm speaker the same size, why not remove that cone and and use it to replace the electromagnet one. Wedging the spider in position whilst using a scalpel to remove the old cone and then lifting the edge from the basket. The edge on the replacement cone can be glued down with a PVA based glue after the centre/voice coil is superglued to the centre part of the cone. Chris

27th Jan 2020, 4:04 pm	#11
Calmore Triode Join Date: Jan 2020 Location: Totton, Hampshire, UK. Posts: 20	Re: Modifying an energised speaker circuit That is one thing to consider. The other thought I had was to remove the coil assembly (l18 and l19) from the energised speaker and mount it on a piece of paxolin along with the audio transformer and tag strip and screw That to the bottom of the case. I could then connect a magnet speaker via some flying leads. Downside of that is that the rest of the speaker becomes a paperweight, but is should be reliable.