Replacing an electrodynamic speaker with a fixed magnet?

[geofy]

Possibly holding the speaker so the cone is pointing downwards and spraying the coil with switch cleaner would wash out bits of grit, having it move at all is encouraging.

The OT should be mounted away from the mains input, at right angles to any other transformer to minimise stray fields.

g

[Jimmyhaflinger]

i have successfully repaired some speakers with scratchy coils this way: first hook the speaker to a stereo/cd player then play some disco music loud enough to bounce the cone almost to the limits of its excursion (be careful not to exagerate and burn the coil) then while the coil bounces back and forth, blow compressed air all around the gap with the air gun's nozzle inside the coil's cavity and at an angle to it's axis as to create a sort of tornado inside the gap.

You may have to repeat this procedure several times and blow air at different opposite angles

If this still doesn't work, another more drastic way is to manually pull the coil all the way out of the gap by pushing up the cone at its base (be gentle and careful, it's thin 60 years old paper!!) then with the coil out, blast the gap with compressed air.
This method is almost always successful as it allows you to blow the gap without the coil blocking the outside surface of the gap, but this may prove impossible if your speaker has a small or stiff spider that doesn't allow enough excursion to pull the coil out of the gap without applying hazardous forces to the fragile paper cone.

Since your speaker has spider centering screw (one on either side of the magnet) you can also remove these screws to make it easier to lift the coil out of the gap to blow it clean, then you can slide four paper shims into the gap to keep the coil centered while you tighten the spider screws

[murphyv310]

Hi
As Jimmyhaflinger says you can repair scratchy coils. Before using any fluids a really good blast with compressed air and driving the speaker hard with low frequency audio will normally cure all ills. My recent Ferranti 146 restoration had a seized speech coil and after a good blast works 100% perfectly.
Good luck
Trevor

[geofy]

This does sound drastic and could burn the coil or damage a fragile old cone, what type of compressed air, from a can? An air line could be too vicious, though a good last resort if the speaker is going to be scrapped otherwise.

I have removed magnets to clean dirty coils, though obviously some can't be taken apart, it can be a time consuming thing getting the magnet gap correct, (as an aside, old Philips speakers have a large flat disc before the magnet which has to be held central before the bolts are tightened, or it claps to the magnet and traps the coil as the magnet is lowered in place pushing out the cone ) Tapping the cone with a finger tells when the gap is correct by the resonant sound. A squirt of WD on the coil as it is reassembled does no harm and keeps corrosion at bay.

g

[Jimmyhaflinger]

i used compressed air @ 8 bar (about 110 psi) from my 1-1/2 hp compressor, while driving the speaker with a 1970s Orion 8-track receiver (about 10 wpc) anything less won't do the job, and no, compressed air won't damage the coil former, unless it's already rotten.

Of course compressed air should be kept away from the paper cone, especially from the thin fragile surround

Alnico magnets should not be taken out of their magnetic circuits as this will severely demagnetize them, also, this kind of "open box" magnetic structure often used in Italian and American made speaker (but rarely used in UK made speakers) makes it extremely difficult to reassemble the magnetic circuit once taken apart

[geofy]

Originally Posted by Jimmyhaflinger

I used compressed air @ 8 bar (about 110 psi)

Of course compressed air should be kept away from the paper cone, especially from the thin fragile surround

(but rarely used in UK made speakers) makes it extremely difficult to reassemble the magnetic circuit once taken apart

Compressed air at 8 bar should only be used by a trained person, this is a genuine 'elf and safety issue, it is the type of pressure I used with air lines to clean metalwork, lathe and mill beds during my mechanical instrument making days.

But old speakers have nothing to loose by taking the magnets off if they are of the bolt on type, I have no real trouble putting them back together and the permanency of the magnet does not seem to be affected to any great extent. I have also used big mag and demag coils when I worked at a microwave place but that is getting OT.

g

[Jimmyhaflinger]

8 bar is the standard cutoff pressure for consumer-grade air compressors, at least it is here in Italy.

a few years ago, i learned the hard way how Alnico magnets suffer from being removed from their circuits,
I had a pair of really nice and rare Siemens 10" fullrange speakers from the 60s, they were extremely sensitive (100+ db) and had an amazingly extended frequency range, the magnet was a massive Alnico ring held together by bolts
One speaker worked fine, the other had a rubbing coil due to a shifted center pole (its cabinet was dropped from ceiling height by workers renovating a church)

To make a long story short, i took the magnet apart to put the center pole back in center, i finally reassembled it and the coil no longer rubbed, but the speaker lost a lot of sensitivity due to the weakened magnet, this was most obvious when compared to the other intact speaker.

[geofy]

Originally Posted by Jimmyhaflinger

8 bar is the standard cutoff pressure for consumer-grade air compressors, at least it is here in Italy.

the other had a rubbing coil due to a shifted center pole (its cabinet was dropped from ceiling height by workers renovating a church)

The safety factor is that at these pressures a misdirected airgun can penetrate skin and cause embolisms or air bubbles in the blood, hence the need for training in their use.

Are you sure it wasn't the shock loading that realigned the magnetic structure if the two pole pieces met at high impact. I posted the speaker I repaired, and it still works loud and clear, I had it working just a while ago. It could be that the larger the magnet the more field it can initially loose and become less sensitive.

Geof

[jimmy74]

Well now I'm thinking of getting a tube voltage chart down for each tube just to make sure everything is fine. I started off pulling all the tubes out and measuring the vAC off the rectifier tube pins and was getting 244vAC and 115vAC off 2 different pins. Does anyone here have a pre written voltage chat that they've done so that I can compare the results I get?

thanks

[jimmy74]

I forgot to mention that I'm getting 6vAC off the chassis.... is this normal?

[dr peppers]

I concur with mr moose, I have a set where the ot has 3 windings, 1 the speaker, 1 the input and the other in series with the input is the 'choke', which also acts as a hum canceller, and it sounds like this is what you have.
I think the idea comes from older designs, my marconi has a electrodynamic speaker and that too has an energising coil, a voice coil, and the hum balance/choke.
If your not bothered about originality then another speaker will probably be fine, a later design might even improve the sound of the radio.
Try metering the voice coil connections to the chassis of the speaker, if the coil has delaminated and come into contact with the magnet the insulation will wear away giving you a short to the chassis.

[jimmy74]

this is probably a silly question, but according to the old paper caps the black line shows what side should be grounded, is there need of this with modern oil caps? If so how should I tell which side is to be grounded?

thanks

[geofy]

Originally Posted by jimmy74

this is probably a silly question, but according to the old paper caps the black line shows what side should be grounded, is there need of this with modern oil caps? If so how should I tell which side is to be grounded?

thanks

The black line is only there to show the outer foil of a paper capacitor, sometimes important in RF stages.

There are no modern oil filled types, at least I hope not for small work at least. If you are talking about Electrolytic types then yes it is essential that they are fitted with the correct polarity PLUS to PLUS and MINUS to MINUS. But if the are non polarised such as polyester then they can go either way round. A photo of what you have might be an idea before fitting if you are not sure.

Geof

[jimmy74]

Thanks Geof I am talking about these oil caps:

http://www.tubeampdoctor.com/index.php?cPath=35_120

I have already fitted them in as I was certain that only paper caps had the orientation to ground but I was suddenly struck with doubt just after I finished fitting them in

[jimmy74]

I've just noticed, that one of the copper wire coils, is moving around the shaft, it has clearly come apart from that white glue resin where it is fixed to the shaft? Could this cause any problems?

[geofy]

Originally Posted by jimmy74

copper wire coils, is moving around the shaft, it has clearly come apart from that white glue resin where it is fixed to the shaft?

Not too clear what you mean with this, a picture would help. Thanks for the link to the oil filled capacitors, not seen those before. With non polarised caps usually the outer case connection goes to the chassis side of the circuit.

Geof

[jimmy74]

here's a photo of the copper windings I'm talking about:

http://www.***********/photos/7383552...7622861186124/

so therefore these oil cap have to be mounted in a certain way?
thanks

[dr peppers]

I think what geoofy meant was that the caps can go either way round, but its good practice to put the outer foil connection side to chassis for various reasons including safety.
Not sure what part of the set the photo is looking at, if the socket on the right is the aerial then its the rf input stage, and those coils may be part of the input filtering, so long as they are reasonably central it should still work, at least enough to pick up a couple of stations, those blue plate like components look like mica capacitors, might be a good idea to meter those for correct value, theymight have the silver mica disease.
Those hellios 0.1u's dont look original, maybe the sets been repaired before.

[SEABAZ]

Re the Jensen speaker - I wouldn't use them in a radio because they're guitar speakers, I bought the very one you are on about for a guitar amp I made because all the normal speakers I had sounded kind of mushy and dull - the jensen speaker sounds great for a guitar. I did a little experiment and connected it to one of my favourite old wirelesses and it sounded ****** horrible - hardly any bass and very peaky aroud the voice sort of range, a bit like a loud telephone!. The cone suspension on these is very tight, you've got to use a fair bit of force to push the cone in, hence the poor bass output. They're not really designed for full range reproduction. If I were you I'd try to find an old speaker from another radio or use something meant for car use (3 or 4 ohm).You could even use a smaller speaker if you made a fretboard that fits behind the original hole. I realise the topic has moved on a bit but I googled Jensen and ended up here. Hope I am not making a tit of myself.

[jimmy74]

Ok speaker wise: I have replaced the speaker with the jensen and fitted in a new OT NOS for UL41 type tube and it works and sounds fine to me, the original speaker has gone through all the cleaning tests and air blasts but is still very very scratchy, I think it has to do with the paper on the coil itself which has decayed and rusted through and the tiny bits fo scratched paper are rubbing against the center magnet, so I've just opted for the new jensen speaker

thanks