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DIY KLH Nine Diagnostics and Repair

I aim to be adding information here from time to time about how to do some basic diagnostics and servicing of the KLH Nine.

Hum

There are two cases where one might hear hum due to a speaker problem. One affects the entire speaker, and the other only smaller area.

Single or small number of woofers:

If the hum is caused by one or more woofer panel failures, it can be just one or several, to which the hum will be isolated. This is usually caused by warpage of the stators, which brings the membrane too close to one of the stators. Repair requires removal of the woofer(s) and flattening of the stators.
  • Separate the individual stators of the affected panel(s)
  • If the membrane(s) are intact, you can leave them in place. Remove any damaged membranes for later replacement. A power-washer with the finest, highest force nozzle works well, but test the black coating on the fronts first to make sure it isn't water-soluble, which some versions were. If it is, then vigorously scrape the membrane(s) away with a single-edged razor blade held vertically to the surface, taking care not scrape but not gouge the stator.
  •  Place the individual stators between alternating 12" x 12" [30 cm x 30 cm] sheets of a flat material, such as 1" [25 mm] MDF. The sheets of MDF will have to have holes or recesses put in them to accommodate the terminals.
  • Top the stack with a weight, such as a 10 lb [5 kg] barbell disc
  • Place the stack in an oven preheated to 170°F [75°C] for an hour
  • Allow the stack to cool for a couple of hours to room temperature before separating it. Stack no more than four stators, or there will be uneven heating and long baking and cool-down times.

    • Entire speaker:

    Hum from an entire KLH Nine speaker is usually caused by a bias leakage path that demands current from the DC bias supply, The supply is designed to simply maintain charge on the membranes, and is unable to produce enough current to power a leakage path without superimposing hum on the DC bias. When asked to supply even small amounts of current, some AC hum will ride on the DC and this will be audible. If the leakage is severe, the speaker will also be quieter than it should be.

    Coincidence with the arrival of humid weather could be a clue that this is the problem.

    This is not the same thing as an arcing path or carbon track. (But if you find a carbon track, do sand it away!)

    Leakage paths will not be visible, except possibly as a build-up of soot that's been precipitated from the air, such as in a polluted city or smoker's home. Such paths can form on the supply, the terminal block on the supply or speaker, or inside the speaker. The terminal blocks are easy to access, so . . .

    A quick thing to try after removing the supplies is cleaning the terminal boards on the speakers and the supplies, as well as the wax between and around the terminal strips on the supplies.

    • ​Remove the seven banana plugs and lift the wires from the screws.
    • Wipe the block with a paper towel dampened with ammonia (non-detergent type) until no more residue appears.
    • After thorough drying, wipe with denatured alcohol (preferably from a hardware store, not rubbing alcohol).
    • In addition, the terminal block could be sanded lightly, especially importantly around each terminal, with very fine grit sandpaper. This will make sure that any invisible conductive path is disrupted.
    • ​A final step that I consider optional is to spray the terminal block with a light coat of Krylon Crystal Clear acrylic spray paint. Do not use any other type of paint, especially not anything labeled "lacquer." They are almost all slightly conductive and will make things worse.
    • After allowing the surface to fully dry, reattach the wires and banana plugs, then mount the supply. Hopefully, the hum will be gone.