Rebuilding the KLH 5

KLH Five Rebuild

In late November I bought a pair of KLH Five speakers. The cabinets were in reasonably good condition, the grilles were intact, not too discolored and with no holes, and the logos were present. The only problem noted was no tweeter output from either speaker.

I pulled each tweeter out and hooked it up to an amplifier through a 10mfd capacitor. Both units worked. Okay, the crossover was defective. I pulled the woofer from one cabinet in order to access the crossover. Troubleshooting by shunting the aforementioned 10mfd cap across the individual caps on the crossover, I found that a dual 4mfd non-polar electrolytic cap was defective. I pulled the crossover out of that cabinet and found that the caps were all 50 volt non-polar types. I ordered replacement 250 volt 5% polypropylene caps for both speakers from Speaker City in Burbank. While waiting for the parts to arrive, I drew up a schematic. The design is interesting: the frequency contouring controlled by the midrange and tweeter switches affects the signal to the tweeter only. The midrange drivers, two 4-inch high-compliance units operated in series, are fed a fixed bandpass signal.

After the caps arrived, I started the rebuild on one speaker. The original caps were soldered to terminal strips riveted to the aluminum base of the crossover, and physically secured to the base with cable ties. Since the polyprop caps were quite a bit larger than the non-polars, and I had to build out a couple of values with a parallel combination of larger and smaller values, I secured them to the aluminum base with hot-melt glue. The rivet attachment of the terminal strips to the base was used to supply the ground return for a number of lines. This was not a reliable method of connection. The metal was oxidized at some rivets and one rivet was not even tightly clamped. (A piece of cardboard had been sandwiched under the strip to provide tension. This can be seen in the photograph of the crossover. I hope that this was not a factory “fix”.) I consequently ran a separate ground wire from each rivet back to the return wire at the input terminals.

While the crossover was apart, I measured the values of the inductors and verified that the wirewound resistors were still good. I also cleaned the switches with some Cramolin contact cleaner. After reinstalling everything, I hooked the speaker up and listened. The bass end was definitely wrong. I wasn’t surprised at that because the woofer surround was no longer sealed very well. Cloth surrounds need some kind of flexible sealer coating to provide a reasonable acoustic seal between the air trapped inside the cabinet and the outside air in an acoustic suspension speaker. Pushing evenly and carefully on the woofer cone confirmed a very poor seal: the cone returned to rest almost immediately. I knew that Lord Industries supplies a water soluble polyurethane sealer, but it is available in gallon cans only for well upwards of $100. Layne Audio used to sell one- and two- ounce containers filled from the gallon can but Steven said that he had none available and would probably not be supplying it anymore because he never sold enough of the small containers before the large container started drying out. (He supplied the small ones as needed and did not fill all at once.) He suggested as an alternative to try “Aleene’s Tacky Glue”, available at art and craft stores (I found some at Michael’s). He said to mix it one part glue to 4 or 5 parts water and to apply to the surrounds. I needed two coats to get a reasonable seal. Now the woofer takes about a half-second to return to rest. This is more than adequate for an acoustic seal. An air-tight seal is not necessary (nor easily achieved) and would cause the woofer to respond to atmospheric pressure changes like an aneroid barometer. Evidently, the glue is PVA or something like that and will not last as long as polyurethane, but Steven said that it should be fine for a good long while. I specifically asked about using a diluted polyurethane caulk but he pooh-poohed the idea.

The midrange drivers are mounted in their own sub-enclosure and also needed their surrounds treated. The midrange, by the way, is the same as, or very similar to, the speaker used  in the model Eleven portable phonograph, the model Twenty-One solid state FM radio and possibly the model Eight tube FM radio (and probably other models). In these models, it is driven “full-range” and equalized.

Now the speaker sounded very good. I made all the same changes to the other speaker (the same dual-section cap was bad in that one as well) and hooked them up. I am extremely pleased with the sound. Definitely more forward than my AR4x’s, but I don’t think the midrange is peaky. Probably the differences in voicing philosophies between AR (Roy Allison?) and Henry Kloss. More listening, trying different amps, is in order, but as of right now, I am a happy camper.

By the way, my total investment is $60 for the speakers and about $60 for the parts. Not too bad, and very edifying.

Bob Van der Veen (mrdb)

If God wanted us to go to concerts, He would have given us tickets.  AR circa 1980.