Spectra 22, Output, reduction in sound level

Read more on the Acoustat Spectra 22 in our home audio section

Charle Cannon2004-02-23 22:21

Hi, Andy, my name is Charles Cannon and I\'m in urgent need of a schematic/service manual for my Acoustat Spectra 22\'s. I have owned them since 1986 (bought new) and I love the sound, but lately I am having a problem, i.e., While listening, the right channel suddenly drops in volume (output), significantly. It is an intermittent condition that does not seem to be thermal-related. (It happens sometimes when I first fire-up and sometimes after they have been on for a while.

I did a voltage check and although a point on the circuit board states 5 KV, I am only getting a high of 3700 KV, and in one instance the voltage dropped to 3100 KV. I don\'t know if this is caused by a bad capacitor (s), leaking diodes or the 500 megohm dropping resistor. Can you help?

Incidentally, both interfaces read a high of 3700 KV instead of 5000 KV, but as stated previously, only one of them shows a 600 KV intermittent drop.My interfaces are model number MK-2123.

I am looking forward to hearing from you. Respectfully Yours, Charles Cannon


Andy Szabo2004-02-23 22:21

There is no service manual for Acoustat loudspeakers. A schematic is available on this site, under Spectra 2200. The Spectra 22 and 2200 are different in cosmetics only, and are electrically and structurally identical.

If your speaker suddenly drops in volume, I doubt there is a problem with the bias power supply. If the bias supply stopped working, it would take a few moments for the panel\'s charge to leak off, so you would experience a gradual reduction in volume, rather than a sudden reduction. (Try unplugging the AC power to the ”good” speaker while playing music, to see what that\'s like.)

I suspect you have an electrical connection problem. The crossover board in the interface is mounted with screws to the input and output binding posts, and those screws also provide the electrical connections. If those screw(s) are loose, you could experience a sudden loss of sound. Check all other connections inside the interface, too.

Due to the very high output impedance of the bias power supply, you will never measure 5000 volts. Your reading of 3700 volts is typical for most high-voltage probes, especially if you are reading the voltage after the 500-megohm resistor. Depending on the probe, you\'ll measure a bit higher on the other side of the resistor.

Does your speaker use a wall transformer for the bias supply, or is it an early Spectra 22 with the direct AC line cord? If the bias supply is powered directly from the AC line, you will see some fluctuation of the bias voltage with varying line voltage.

However, if your speaker uses a wall transformer (for the so-called Ultrasonic Bias Power Supply), it has a regulated bias voltage, which should remain constant. Any variation in this voltage, therefore, would suggest a problem with the bias power supply. Some of the early versions of the Ultrasonic supply did have a problem on power-up, (they wouldn\'t always oscillate), but usually, once oscillating, they continued to run. So I am not sure if this could be the problem in your case. Do you leave your speakers plugged in at all times, as recommended?

There is an easy way to tell if you have the early Ultrasonic supply. Locate the squarish air-core transformer located on the bias supply board (the board with the wall transformer jack). If this transformer has a single red wire exiting the top, it is the early design. If that red wire has another wire wrapped around it, secured with heat shrink tubing, then it is the later design.

There is no reason to update this supply if it is not causing a problem (many of the early versions worked fine). But if your supply is acting up, doing the upgrade is easy. I don\'t have any instructions currently prepared, but I can write some it if you need it. The schematic shown on this website reflects the upgraded design (note the wire ”antenna” wrapped around the output lead of the bias transformer).

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