So back to the original post:
Last week I happened across a pair of rather unloved Ditton 33s. Upon hooking up to a source I immediately realised that both tweeters were blown. Darn! They are known as HF-1000, HD-1000, MD-1000 or T-2350. Modern day replacements are expensive but also tend to be more efficient and require the addition of an L-pad or resistor.
I found that both tweeters were open circuit & so removed the faceplates to find a brittle, cremated mess:
I did consider attempting to recreate the former & windings but I searched the internet and sourced some suitable silk dome diaphragms & 8 ohm voice coils:
I cut the original lead wires on the front of the tweeter as close as I could on the dome. Then the dome was removed and the rear of the faceplate cleaned. At this point I did set the faceplates up on the CNC mill to remove about 0.5mm from the inside diameter to allow a better fit for the new domes (the holes originally appear to have been trimmed with a knife in the factory). I also lasercut some 3M foam tape to attached the domes and also a simple centralising fixture from a sheet of thin card. Two trackways were Dremelled into the rear of the faceplate for the voice coil lead wires.
The voice coil was centralised in the faceplate and attached using the tape. There is very little room for error here & fixturing is essential. By my reckoning, positioning must be attained by +/-0.1mm to avoid voice coil rub!
Once soldered to the terminals, the faceplate needs to be trial fitted to the magnet & the fibre former adjusted to give adequate clearance to the rear of the dome. Once that is acheived the faceplate can be reattached to the magnet. At this point the four screws allow a bit of float, roughly 0.2mm, so initial alignment needs to be done by ear. By lifting the faceplate up & down a small amount (perpendicularly!), aim to tighten the screws when no voice coil rub can be heard. Once fitted to the speaker, it is easy to do listening tests & further refinement to the alignment if required.
Now you can see why I trimmed the original leads close to the dome. They are left as dummy leads & can be lightly bonded to the dome surround. As the 33s crossovers & cabinets are being worked on at the moment I tested the units in the 15XRs. The sound is reminiscent of the originals but improved by being more open & airy. They are also proving to reproduce clean, incredibly detailed highs.