The Fine Things are Always Hand Made
I have a Magnavox speaker from a Scott phantom to repair,
As the voice coil and field assembly are ok these will be left untouched but there are a few issues with the spider, there are several small breaks and a partial tear to be repaired.
First job is to measure the cone depth, this can be done by placing a straight edge across the speaker, measuring down to the voice coil edge then removing the thickness of the gasket,
The nearest cone I could find was a cone No.9853 (listed on Ebay) it’s 11.8 inches in diameter, has a 1.5 inch voice coil opening and has a depth of 2.8 inches, this is very close to being ideal.
The next step is the horrible part, removing the old cone, first thing is to shim the voice coil, this is done with strips of plastic, the stuff I’m using here is 19 thou thick, cut into thin strips to fit around the pole piece.
The cone is then cut out, leaving a small amount of paper around the voice coil,
the gasket is then removed by using a sharp blade between the speaker basket and gasket, using a downward twist on the blade the blade will cut the old paper metal joint and not the gasket so as this can be kept cleaned up and reused.
The remnants of the old cone can now be removed from the voice coil, with a speaker of this age the old glue has gone quite brittle, otherwise a solvent like acetone can be used to break the glue bond.
I found the as there is a garolite or similar ring fitted to the end of the voice coil on this speaker, this could be removed or left place and modify the cone to fit,
as this part of the speaker was in good condition I decided to leave alone,
note; small split to spider at top and bottom of photo
Now the cone is off the voice coil and gap can be cleaned, this can easily be done with a vacuum cleaner and a soft brush, once done the spider can be inspected and repaired,
photo of one of several spider splits
The splits and cracks can be fixed with epoxy, be sure to use a slow setting type as the rapid epoxies do not set hard and the bond is not as strong, apply the adhesive sparingly with a toothpick the idea is to get enough to bond the spider but leave it flexible.
The cone can now be resized, I have found a rolled up length of 80 grit paper is good for this,
the roll of abrasive is put through the voice coil hole and allowed to expand,
it is then twisted in the direction with the roll,
looking at the above photo twist clockwise.
This will cause the paper roll to open up sand away the edge of the cone without jamming,
keep the abrasive paper perpendicular to the cone and it will sand evenly,
the tip here is to stop and check often, as you can’t put the cone paper back on.
This is a photo of how much I had to remove compared to the original cone
The cone is then fitted to the speaker basket at the speaker edge, running a bead of rubber contact adhesive around the perimeter,
the cone is placed on this bead and clamped I use clothes pegs and toothpick spreaders as these will clamp the cone without touching any surplus glue,
once the glue is dry these can be removed.
Next the cone can be fitted to the voice coil, I use slow set epoxy for this,
applied to the cone coil junction with a toothpick, applied a small amount at a time until there is a continuous bead, once this is done the speaker should be put cone down to ensure any excess glue runs away from the voice coil.
After 24 hours or when the glue has set up the speaker can be turned over and the shims removed, if the cone is gently tapped with a finger it should sound like a drum with no rasping noises, the old speaker gasket can now be fitted to the edge of the speaker and the HF cone fitted
By applying 60v dc to the field coil and running the voice coil from a test amplifier the speaker can be cheked, I'ts a pity that I cannot reproduce sound her as the completed speaker gives very pleasing performance.
Awesome tutorial Mike! I have started to learn to re-cone speakers, having done three smaller ones so far. Two turned out well...one not so good. But I will keep trying, and use a couple of your tips. :)
Nice presentation. That same speaker is used for the Scott model Sixteen and the 14 tube Scott Masterpiece and 19 tube Phantom (NOT 20 tube Phantom DELUXE, nor FM Phantom Deluxe). It uses a curved cone. Octal plug. It has two field coils - one for B+ and one in the negative voltage bias circuit.
I have had several of this model Scott Magnavox speaker and running about 50% with bad output transformer. Due to the 38 ohm impedance voice coil (VTVM resistance about 31 ohms) and 6V6 output tubes, I found no suitable universal output transformer which max out at a 16 ohm speaker. But can modify the Hammond 15 watt universal 125-E by adding additional turns (18 gage magnet wire is about right) to lug 6 and the far end becomes "lug #7". Then the voice coil is attached to lug #1 and the new lug #7. It takes about 12 feet of wire to add 24 turns. The purpose is to further reduce the primary to secondary turns ratio to accommodate the 38 ohm voice coil.