Oh how I lurve the Celestion HF1300

[spendorman]

To my mind your best bet is to replace the "re-coned" bass units with original DM2 or DM2a bass units.

I believe that the Radford Monitor was a good speaker, if you have this, best to keep standard.

[florin]

To my mind your best bet is to replace the "re-coned" bass units with original DM2 or DM2a bass units.

I believe that the Radford Monitor was a good speaker, if you have this, best to keep standard.

I'll see when they arrive, I might leave them as they are.

DM4 uses a Bextrene unit, I believe polypropylene coned bass units were pioneered after the DM4 and DM2 were designed.

Having had a look at DM4's specs I see that some Bextrene cones were laminated so they look similar to polypropylene, or at least they don't have the brush patterns DW200 of the DM2's have on the cones.
After I bought the DM2's in the video mentioned above I bought another pair with an upgraded crossover by a retired audio engineer. I took the drivers in the video mentioned above labeled "tested 1993" to him and he said they are polypropylene.

[spendorman]

Generally, all Bextrene cones will be coated (sometimes on the rear of the cone), early days this was brush painted, later some were sprayed. In my view the brush painted is better acoustically, because the irregular coating breaks up the unwanted resonances better.

Polypropylene cones are mainly translucent, though, of course they don't have to be. Polypropylene is a difficult material to glue, often dust caps fall off and surrounds may come unstuck. Although I have some Rogers LS7's and LS2's which have polyprop cones, and they are still fine.

I think your engineer maybe mistaken in his opinion that the cones are polyprop.

[florin]

Generally, all Bextrene cones will be coated (sometimes on the rear of the cone), early days this was brush painted, later some were sprayed. In my view the brush painted is better acoustically, because the irregular coating breaks up the unwanted resonances better.

Polypropylene cones are mainly translucent, though, of course they don't have to be. Polypropylene is a difficult material to glue, often dust caps fall off and surrounds may come unstuck. Although I have some Rogers LS7's and LS2's which have polyprop cones, and they are still fine.

I think your engineer maybe mistaken in his opinion that the cones are polyprop.

I think you might be right.
Just had a look at the driver and the cone looks very similar if not identical to the one in DM4 which I also have.
Poly cones were difficult to glue in the beginning from what I understand, later on they found methods and glue materials that were much better at bonding poly.
Here's an up-close video of the driver in question. https://youtu.be/MIXDwRNceNo
By the way, what is the powder on some metal magnetic parts on vintage drivers? Some form of corrosion due to humidity?

[walpurgis]

Still it doesn't explain as to why the "core tube" (thanks for that) hits the magnet

Cone travel (extension) reduces in reflex systems as the speaker reaches its Hemholtz resonance, so it should not bottom out onto the magnet backplate.

[walpurgis]

DM4 uses a Bextrene unit, I believe polypropylene coned bass units were pioneered after the DM4 and DM2 were designed.

The DM4 had bextrene (polystyrene) cones. Early DM4 cones were brush coated and later ones were sprayed (I've owned both), The coating was Plastiflex, a rubbery clear synthetic material used by several makers.

[florin]

Cone travel (extension) reduces in reflex systems as the speaker reaches its Hemholtz resonance, so it should not bottom out onto the magnet backplate.

I've used this on Sansui 881 and depending on how bass heavy the content might be it would hit the magnet making horrible sounds. Luckily I didnt just crancked the volume straight up. I don't listen too loud but sometimes I like it a bit louder. Will look for spmeone to have a look at them at some point.

[spendorman]

The close up - looks like Bextrene to me.

Plastiflex is very much like PVA wood glue.

[spendorman]

I've used this on Sansui 881 and depending on how bass heavy the content might be it would hit the magnet making horrible sounds. Luckily I didnt just crancked the volume straight up. I don't listen too loud but sometimes I like it a bit louder. Will look for spmeone to have a look at them at some point.

Still think this is a faulty unit, possibly coil/former is distorted and hitting some part of the magnetic circuit.

[spendorman]

"By the way, what is the powder on some metal magnetic parts on vintage drivers? Some form of corrosion due to humidity?"

Yes, corrosion.