Lowther refurbishment notes

External terminal replacement

The original paxolin-mounted external terminals were in relatively poor condition, so I replaced them with pairs of Monacor BP-420 terminals. Since these are mounted fitting each one in a 7.2 mm hole, I needed to block the holes they formerly occupied, I first used scrap wood to partly fill the holes, with rapid-setting (5 minute) epoxy, thickened with around 30% by volume of micro-balloons[1]. This mixture has a similar consistency to silicone filler or soft putty. It is easy to use as a gap filler, sets a bit faster than straight epoxy and, when set, sands easily.

1. micro-balloons are tiny, lightweight hollow glass spheres that look similar to coarse flour or plaster of paris but are much lighter.
   Most fibreglass or epoxy suppliers sell them.

External terminal protection

The Monacor BP-420 terminals I fitted to the enclosures are quite bulky, as shown by this dimensioned drawing:

Because of this bulk, I also glued and screwed strakes, made from three 20x20mm softwood strips glued together side by side, that extend across the full width of each enclosure's rear surface. The ends and horizontal edges are bevelled to minimise harm to other furniture. One end of the middle 20x20mm wood strip was left short so that the BP-420 terminals are partially enclosed by the two side strips and, since the terminals are only 19mm high, they don't project above the strakes. See pictures of the enclosures for more detail.

The terminals must be mounted by drilling 7.3mm holes in the enclosure's rear surface. These should be 165mm above the external base of the enclosure with the terminal nearest the side of the enclosure set 35mm in from the enclosure's external corner and the other one 25mm further in.

I calculated the vertical terminal position as the sum of:

• 15mm enclosure base thickness
• 140mm - this is the internal height of the 45 degree bevel panel between the enclosure base and back surface. It is used to reflect sound waves out through the front port.
• 10mm clearance between the top of the bevel panel and the terminal's threaded shaft. This protrudes into the enclosure and carries mounting nuts, lock washers and a tag which the internal speaker wiring is soldered to.

I calculated the horizontal terminal positions as:

• 15mm enclosure wall thickness plus 20mm working space between the terminal and inside of the wall.
• the second terminal should be 25mm further in than the first one to keep wiring, solder and solder tags clear of the other terminal.

Internal wiring

I replaced the enclosure's internal wiring because I wasn't sure that the original wiring would be long enough after I'd repositioned the enclosure's external terminals. B&Q sells reasonable quality copper audio cable in sensible short lengths, so I used that. The points to note are that:

• The internal cable is threaded through the enclosure's audio passages and its length is at least twice the height of the enclosure so I was careful to tape the ends of the cable to the internal walls because I realised that thr only way to get the new cable in place would be to use the old cable to pull it through the labyrinth.
• I taped the speaker end of the cable to the enclosure when I removed the old speaker and the external end to the enclosure when I unsoldered it from the old external terminals.
• Then I attched the new cable end to the old cable with masking tape and used the old cable to pull the new stuff into place, taping the end of the new cable to the same position at the speaker end as the old cable had occcupied. The old cable was then discarded.
• The new cable was cut off the reel where it reached the external pair of terminals, soldered to the supplied solder tags, the solder covered with heat-shrink insulation and the solder tags connected to the terminals.
• The speaker ends of the cable were now attached to the new speaker, making sure that the wired were not crossed, i.e. that the same wire is attached to the '+' terminal of both external terminal and speaker and the new speaker installed.