2 plus 2, Interfaces, corosion

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Richard Kline2004-02-23 22:21

After owning (& loving) Monitor 3\'s for 20 years, I have finally realized a longstanding dream and acquired an immaculate pair of 2+2\'s (121-C interface). My large listening room is rather heavily damped with padded carpet, so I opened the interfaces, as illustrated in the instruction manual, to try and adjust the high frequencies up some. I immediately saw that all of the exposed wire coils on the potentiometer were covered with blue-green corrosion. Is this presently affecting the sound? How should I clean it off? Also, the instruction book says to use an ohmeter to make sure settings on each speaker are as similar as possible. Sorry, but I have no practical electronic experience - where exactly do I put the leads from the multimeter to measure the resistance?

By the way, I cannot thank you enough for making your expertise available to us. We all live in fear that one day our Acoustats won\'t work any more, and Lord knows what we will do then.

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Andy Szabo2004-02-23 22:21

The green corrosion on the high frequency resistor is copper oxide. This is unfortunately quite common on these resistors, caused by oxygen in the air reacting with the copper of the wire coils. The corrosion doesn\'t really cause any harm, except that it could interfere with proper contact of the slider portion of the adjustable resistor.

The corrosion should be cleaned off prior to making any adjustments of the High Frequency Balance Control. You can use a coarse pencil eraser, or some very fine (400-grit) emery paper. Apply only enough pressure to remove the oxide, without digging into the base copper. Make sure to move the slider and clean the spot where it was making contact.

Using an ohmmeter to balance the two channels is straightforward. Connect one lead of the meter to the center terminal, which corresponds to the slider. Connect the other lead of the meter to the ”high end” of the resistor, that is, the end of the resistor where the slider would be if adjusted to maximum. Make a note of the resistance, and then adjust the other speaker to read the same value (within 5% is close enough). This resistance will be in the range of 0-6 ohms. This procedure works for both the rotary and linear-slider types of High Frequency Balance Controls, as well as any versions of the MK-121 interface.

Although I have never tried it, you may want to coat the exposed metal parts of the resistor with Dielectric Grease to inhibit further corrosion. (Do this after you have made the final settings.) This material (silicon-based, I think) is often used on automotive sparkplug connections, and is a high temperature, high voltage grease. You can get a small tube at an auto parts store.

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