19/11/2025
Refoam of HPD 315:
I thought I'd post some pictures of this job to show the results of not refoaming these drivers in time to prevent voicecoil damage. Luckily for the customer, I was able to repair this one and undertake a successful refoam. The issue is that once the foams deteriorate beyond a certain point (and develop pin holes or tear) the result is a significant drop in acoustic suspension above the port tuning point which leads to over excursion of the drive unit which can result in the voice coil former bashing itself off the pole piece of the motor with the resultant damage as seen below. In this case it had deformed sufficiently to cause rubbing but the customer stopped using them when this happened which saved the driver, and I was able to repair it.
The two other points worth bearing in mind if you fancy a diy attempt (not recommended if they reach this point) is the many hours needed to painstakingly remove the gunge left by broken down foams and glue on the rear of the cone which is why recones can prove easier to do. This one took several days of painstaking work to refoam properly and effect the repairs needed.
Now for the word of warning: Cadmium plating was used by Tannoy on these and MG drivers. It's fine as long as it remains stable, but any yellowing of the horn or bolts, nuts and screws and of the quadrant gasket metal surfaces is a red flag as the Cadmium has oxidised to Cadmium Sulphate which is highly toxic. You can clearly see the tell tale sings on the photos.
Gloves MUST be worn and if the oxide layer has gone powdery, so must an FFP2 mask. This one was quite bad, so I had to carefully wipe down such surfaces with ACF50 to seal ad stabilise the surfaces rendering them much safer and the end result fine for normal use.
Such refoam work with all that this entails, including testing HF and LF voicecoil alignments and adjusting as required is skilled work needing specialist test kit, so the cost reflects the hours and work done, typically £200 to £300 per drive unit and that, for the hours worked, is cheap! This should be good for another 15 years service now. Final picture is of the refoamed unit back in its Cheviot cabinet.
The other point of note is that this one had an old recone kit fitted years ago by Lockwood. The spider suspensions used are not to the same spec as originals, and often test at lower Fs (this one was 15.5Hz instead of 25Hz). Whilst this won't affect cabinet tuning point or sound, it will alter LF response near tuning slightly but below tuning unless rumble filters are used with phonostages, or other means of bandwidth limiting at low frequencies is used you will experience excessive cone movement once below 30Hz in this cabinet, especially due to warped LPs and other sources of infrasonics.
