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Wanted Seas 25TFFN/X or similar tweeter for T17 coax H0723

Dheck

Member
2020-02-01 11:03 pm
Hi all,

I am hoping to repair a Seas G17 coax... marked as H0693, which has some tweeter damage (shorted coil). It seems to be a version of the H0723 T17REX COAX/F but with the cone in fibreglass.

The specs describe the tweeter as a 25mm, 6ohm part based on the H0615 25TFFN/G but with different housing for coax mounting. There are some references on the internet to a 25TFFN/X which looks like it may be the right part.

I'm looking for any suitable parts or donor speakers that I may be able to use for the repair. Information or advice also gratefully received!

I think that the suitable parts may be:
25TFFN/X. Seas 25TFFN/X (6Ohm)

or whole donor coax:H0489 P17REX/XF
Perhaps this also uses the same tweeter?
H0487 MP14RE/XF


Thanks!
Daniel
 
Hi Daniel,

The H693 is an OEM driver, but it most likely uses the same tweeter assembly as the standard models of that time (H-numbers are chronological). Have you tried contacting the good people at Seas, to find out?

I have a pair of H719 MP14RE/XFTV, which I could check for comparison. That is; if I can get the shielding cups off.

Tweeter assemblies for some of the more recent coax models are available as spare parts, e.g. from Madisound.

- H -
 

Dheck

Member
2020-02-01 11:03 pm
Thanks for this helpful reply and sorry that I didn't see it until now.

I did actually find a NOS H719 and indeed the tweeter was suitable for my repair.

Fwiw, you can remove the tweeter without removing the magnetic shield cup... But I would also be interested to know how to remove the magnetic shielding on Seas drivers as it's quite bulky and not needed on these post CRT days.

If you would consider selling the H719s that you have, please let me know. I have a project in mind that they would be good for!

Cheers
Daniel
 
I got rid of my last CRT a decade ago, so the shielding cups no longer made any sense. My main reason for removing them, however, was that the outer part resonated like a bell. Also, the large (half a centimeter or so) gap between the cup and ferrite rings creates quite a resonant cavity, almost directly behind the cone. As a side benefit, the resulting smaller magnet profile should improve the termination of rearward radiation into the enclosure.

I kind of assumed that the shielding cups would be glued in place, so I never attempted to pry them loose. Instead, I took the quick-and-dirty route, and cut the outer part off with an angle grinder. I covered the chassis and soft parts carefully with a thick plastic bag, to keep metal debris out (and then spent an hour or three cleaning it off the magnet). The bucking magnets are still in place, as well as the remaining circular bottom part of the cups. Not the most esthetically pleasing job in the world, but it solved the problems. :)

With regards to the H719's, I've already made plans for them, I'm afraid. The Seas MP15RE/XACG might be an interesting alternative.

- H -
 
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Dheck

Member
2020-02-01 11:03 pm
Hi Geir,

The shielding cup removal sounds messy! Having done the cutting, does it seem that the cups / bucking magnets are actually glued to the pole piece?

Thanks for the suggestion about the MP15 coax. I think that one uses the AL dome tweeter, like the EP14 coax that HifiTalo also sell. I have not liked the sound of these compared to the standard seas coax silk dome tweeters.

I have actually made a speaker using the H719 MP14, with an Al dome tweeter to replace the swapped out silk dome tweeter. It's a 4.5L ported box and measures quite well, although there seems to be some internal resonance at ~1.2kHz. I'm not sure whether this is pipe resonance or something else (shielding cup?). See the attached FR plots for the overall response and a measurement from inside the port. The FR bump at 1.2kHz still occurs when the port is completely blocked (stuffed).

Cheers,
Daniel
 

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Hi Daniel,

I think the cup must be glued to the bucking magnet, as it seems to require a lot of force just trying to get something sharp in between them (I've just managed to chip the bucking magnet, although I decided not to escalate the use of tools beyond a flathead screwdriver). The latter, I'm quite sure, is glued to the rear pole plate of the magnet system. If you want the cup and bucking magnet off, I really can't see a way to force them off, other than from the side (which means you'd have to cut the cup open first anyway). For that, you'd need a hammer and a chisel, alternatively a hammer and a knife blade you won't mind damaging. I suppose it's also possible to just break the bucking magnet into pieces, and take it all off in bits, although that would for sure be a heavy-handed and messy approach.

I doubt that the measured resonance is related to the shielding cup itself, although it should be quite easy to dampen that area, just to make sure: Stuff the gap between the cup and the magnets (both ferrites are 90 mm diameter) tightly with some damping material, and possibly put some bitumen padding on the outside of the cup, and it should take care of any potential issus.

Since you've already eliminated port modes, I think the resonance has to be caused by a strong standing wave related to the box dimensions. If the driver is in fact mounted on the symmetrical half-way point of a column, like you detailed in your PM, this is very likely the cause. Dampening out this resonance mode will probably require excessive stuffing, so if you can instead move the driver away from the middle of the column, that would the preferred way to go. The Golden Ratio can be a good starting point on where to place the driver: Make the column to one side of the driver ≈1.62 times the length of the other side, and the longitudinal mode should be significantly reduced, hopefully to a point where stuffing can take out of the remains.

- H -
 

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Dheck

Member
2020-02-01 11:03 pm
Hi Geir,

Thanks again for the very helpful reply. The picture is informative...

I do note that some of the more recent shielded Seas drivers (e.g. H1333) do have open cell foam inside the shielding cup to provide some damping.

And, I agree with you about the standing wave diagnosis... I'll experiment with some variations on internal stuffing and brace / diffusion structures with the current cabinet before taking the big step of rebuilding with different enclosure dimensions or driver placement. As this is a centre speaker, I'm still hoping to be able to have a symmetrical appearance.

Cheers,
Daniel

Cheers,
Daniel