Help wanted repairing a 1950s 10x6 PM speaker

[Nickthedentist]

Hi everyone,

I'm sorting out a tatty Cossor 524 Melody Maker at the moment - the one from 1955 with a Bakelite case, AM and FM, and a huge (10" x 6") elliptical speaker.

It looks like its been stored damp, and all the steel bits are somewhat rusty.

With some fettling, the chassis performs excellently, but although the speaker (Plessey?) "works", the voicecoil rubs leading to distortion especially at low volume levels. I suspect there is rust in the gap and/or the voicecoil former is distorted.

Apart from repairing the inner/rear suspension on ex-DAC90A Celestions, my experience with speakers is very limited.

Any suggestions, other than replacement (perhaps with a modern car stereo speaker)?

Many thanks,

Nick.

[GrimJosef]

Distortion of voicecoil formers is a well-known problem I'm afraid. It can be caused by heat (common in over-driven guitar speakers apparently) and certainly by damp. Repair normally involves fitting a replacement surround/cone/coil assembly but I'd be surprised if you could find one commercially for this speaker. Beyond that the only prospect is to see if you can extract the existing assembly and somehow restore the coil to its original circular cross-section, perhaps by dampening it, sliding it onto a close-fit cylindrical former, drying it there and then varnishing it ? If the speaker has been run for any length of time while rubbing on the magnet it may be that the varnish on the coil windings has worn away and there is a risk then of shorting between the turns.

As far as getting debris out of the magnet gap goes, the most common advice seems to be to use sticky tape. Obviously extracting anything magnetic may well be a challenge.

Cheers,

GJ

[Nickthedentist]

Thanks G, and nice to hear from you.

Yes, I suspect damp rather than excessive power and agree that a spare parts would be impossible to source (and prohibitively expensive - the whole set only cost £15).

As the magnet assembly is permanently attached to the chassis, I would presumably need to remove the cone/voicecoil to investigate. Any suggestions as to what might be useful as a solvent to free it?

Nick.

[Leon Crampin]

Nick, one thing to do before you dismantle the 'speaker would be to manipulate the cone axially whilst applying a sideways bias in each direction in turn with your fingers. You can manipulate the cone gently, using the "windows" in the frame.

If it rubs under all conditions, you have to either dismantle the 'speaker or scrap it. If, however you can get it to move freely with a radial bias in a given direction, there's a possible fix which worked for me.

The idea is to reset the cone suspension in a new position, for which I used a Bic biro, some cotton wool and a couple of rubber bands...

Determine which way the cone needs to be biased, and insert some cotton wool in the back through the two "windows" as appropriate. The idea of the wool is to spread the load which you are about to apply. Now insert the biro so that it bears on the wool and finally loop a couple of bands around the back of the magnet and hook the ends over the ends of the biro. The idea is that the bands and biro apply a steady radial pressure to the cone in the direction you need to shift it. Don't use too much pressure or you may distort the cone.

I than tried just leaving the 'speaker untouched for a month - with limited success - better, but not completely free. So, I left the 'speaker in the bathroom (humid atmosphere) for a week, manipulating the cone axially from time to time. I then gave the whole thing a month in the airing cupboard to dry out.

It worked, for a very nice 10 x 6" Goodmans unit. But I've only ever tried this once. I suspect dismantling a 'speaker is not a worthwhile project unless it's something very old or special. Best of luck.

Leon.

[Nickthedentist]

Hi Leon, great to hear from you too - the VHF64 is still playing wonderfully every day.

I think your no-dismantling suggestion is the way to go. You've jogged my memory and in fact I did something similar with a 5" round Celestion from a Roberts RMB a few years ago, except I did it by removing the felt dust cover from the front, placing a cylinder of acetate sheet between the inside of the voicecoil former and the central magnet, then dampening the entire cone and suspensions with water from a plant mist spray bottle, then leaving things to dry out for a few days. It worked well, so I might just try that and/or your method on this one, assuming the gap's not clogged-up with rusty debris.

I think I have a nice Goodmans unit to replace it with if all else fails, which came from the relatives of a member of this forum who passed away a few years ago.

Thanks,
Nick.

[ukcol]

Hi Nick

i repaired a speaker in an HMV Stereomaster back in 2010 using the same method of dampening the cone and it was successful. Be careful not to get any water in the magnetic gap though.

[Boater Sam]

Its not so difficult to remove the cone completely.
Disconnect the pigtails from the solder tags.
Pick the dust cover off the cone centre with a point or scalpel
Soak the cardboard surround by placing the speaker face down in a shallow dish of white spirit for a while. Pick the back diaphragm off the chassis with white spirit. The cone will come out complete. Let it dry out totally.
Clean it all up, clear the magnet gap of dirt with a sticky note. If there is metallic or rust bits, drag them out with a neodymium magnet.
Reassemble with three 6mm wide bits of thin plastic, water bottle, or marge carton cut up in the gap to centre the coil whilst you glue it all back together.
Always worth a go, done quite a few.
I'm still trying to make new elliptical cones, on going project.

[Nickthedentist]

Thanks Sam, much appreciated. I will try that if the more noninvasive methods don't work.

Like pots, switches, ceramic cartridges and so on, vintage speakers are a rarer commodity today than not so long ago, so it's worth trying to resurrect them.

Nick.

[stevedee]

Something I used to do with rubbing speaker cones was to slightly distort the speaker frame. This involves slackening off the 4 fixing nuts, then putting a small metal shim (like a washer) under one fixing point. Tighten down the other 3 fixing nuts with very little pressure, to see if the distortion gets better or worse. Move the shim to other fixing points as necessary and repeat.

Ideally, you don't want to bend the frame beyond its elastic limit (in other words, the frame should return to its original shape if you take the pressure off). If you over-tighten, you may make the problem much worse.

Also an idea to feed the amplifier with a sinewave (say 400Hz) to get a clear indication of whether its [still] rubbing as you carry out this adjustment.

[GeoffK]

This elliptical speaker had a similar fault and after finding a spot where the cone didn't rub, with a sharp scalpel cut a small section near the dust cap to relieve one side of the cone, then held it in the new position with a slither of epoxy adhesive. It saved the speaker from being scrapped as it would have been difficult to find a suitable replacement, as well as no longer being an original part of the radio.

[Nickthedentist]

Thanks for the tips, Colin and Geoff.

[Radio tirana]

Hi Nick
I have an identical speaker that I have no use for, it is a 3 Ohm unit I aquired with a job lot of parts several years ago. I connected it up and it sounds good, on closer inspection it does have a small tear near the edge
of the cone, should be an easy fix with some tissue paper and glue. If you would like it you would be most welcome. I also live near Oxford so could probably drop it round PM me if you are interested

Kind regards Radio Tirana

[Nickthedentist]

Wow, that's very kind! If I have no luck with mine, I'll definitely take you up on that. Thanks!

[Nickthedentist]

Originally Posted by stevedee

Something I used to do with rubbing speaker cones was to slightly distort the speaker frame. This involves slackening off the 4 fixing nuts, then putting a small metal shim (like a washer) under one fixing point.

Thanks Steve and welcome to the forums.

I had forgotten that "trick", thanks for reminding me. I will try that before delving deeper.