Surround Replacement for the HPD-385A Dual Concentric

Left: Non-genuine suuround.
Right: Genuine Surround.

Replacement cones and coils are expensive for the HPD-385A. It is possible to repair many drive units at much less expense - if it is only the "Tanoplas" surround that has failed. These surrounds often decompose into a sticky mess over a period of years, as humidity affects the material the surround is made from. Life expectancies for the surrounds are often in the range of 10 to 20 years.

Provided the voice coil has not been damaged from heat (due to excessive power input), or has not been mechanically damaged from scraping on the magnet pole pieces (this can happen if the units is used for a with a completely failed surround), it is possible to replace the surround with a non-genuine component.

Unfortunately, Tannoy do not make surrounds available as a spare part, which is a pity. Advice from Tannoy indicated that the requisite skills required to replace a surround are not available in most of the service departments. However, both genuine ans non-genuine surrounds can be obtained and fitted satisfactorily. The genuine ones are prefereble as they have a high compliance, and thus maintain the low-frequency characteristics of the driver.

Surrounds

The Tannoy HPD385 dual concentric is nominally a 15-inch drive unit. However, when compared to other 15" speakers, the frame diameter is mariginally smaller, and the diameter of the actual cone is also a few millimeters smaller. This makes the location of a suitable surround more difficult. A suitable surround can be obtained from:

The inside and outside diameters of these suurounds is approximately 283mm and 373mm respectivley. The "active" portion of the roll-suuround has inside and outside diameters of 300 and 345mm (ie: the "roll" is 22.5mm wide).

Brief Procedure for Reconing

Remove the old cone by: undo all the scews at the back of the outside rim and remove the gasket, and the gasket metal rings that clamp the outside of the surround into position. Unsolder the low frequency voice coil leads, and then undo the for small nuts that clamp the spider into position. The cone should now be removed from the chassis. Immediatley seal the magnetic gap to prevent ingress of foreign matter into the gap Adhesive tape can do this job.

Inspect the coil for any damage, particularly if it is known that the coild has been scraping on the pole pieces, or has been overloaded electrically. If the coil is OK, and only the surround is faulty, then proceed with replacing the surround.

Now clean the outer rim. A solvent will be necessary if the old surround has decomposed. It is best to clean the rim, as it will make the alignment of the lf cone easier when it is re-assembled.

Remove old surround from the cone, and clean off any remnants adhereing to the cone.

The new surround can be glued with a high quality adhesive to the cone (for example Selley's Urethane Bond). A glue that doesn't set brittle is most suitable. The positioning and centering of the surround may take some some time. Place the cone face down on the bench, and raised off the bench, so that the surround is in contact only with the cone. Allow the glue to set overnight.

It is advantageous to temporarily place the cone into the chassis, and mark out the places on the surround for the retaining screw holes, and cut-outs for the mounting bolts (make sure the coild leads appear in the correct quadrant). Then cut these holes and cut-outs, with a punch and knife. To re-assemble the cone into the frame, remove the adhesive tape from the gap. Inspect the magnetic gap, and clean by using a non-magnetic probe, covered with adhesive tape, with the adhesive facing outwards. Once clean, replace the cone, and solder the coil leads into position. Loosely place the outer metal surround retaining rings into position, and insert just a few screws in each until the final position of the surround is determined. Loosley place two nuts onto the spider to allow for it to be adjusted too. If a signal generator is available, drive the coil with a small signal at a low frequency near the cone resonance (around 20Hz). The coil (spider) and outer surround should then be centered by ear - aligning so that coil scaping sounds are not present. If a signal generator is not available, this procedure will need to be done by tapping the cone repeatadly (and at various positions around the perimeter) by hand to excite it at its resonant frequency. Align as above for zero coil scraping.

Note: see also more photos taken during reconing of the Tannoy Monitor HPD-385.

 

 

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