Spectra 2200, Output, decrease in volume

Read more on the Acoustat Spectra 2200 in our home audio section

Bill Hartwig2004-02-23 22:21

Dear Andy, I am not sure if this is a unique problem or not. Over the 10 years that I have owned my Spectra 2200\'S (Serial #s 25040013 and 25040014) I have noticed a decrease in volume with the left speaker. After interchanging panels and discovering the volume decrease continued within the left side, I concluded it may be the power supply (not wall transformer as these were producing the required voltage).

Following your excellent instructions on the ESL website, I used Method 2 (Alternate) as I only had access to a 10-megohom digital voltmeter and discovered that the right high voltage bias power supply was able to be adjusted to obtain the recommended 75 volts DC. I made multiple attempts to adjust the left high voltage bias power supply and found that on all occasions the voltage began below 75 volts and continued to rise consistently over about 45 seconds when it overloaded my voltmeter. All attempts to adjust to trimmer potentiometer did not change to slow increase in voltage. Do you have any suggestions about what I should do at this point? Thank you very much for your assistance.

I also read that you suggest upgrading the electrolytic capacitors to polypropylene, as I am using the Acoustant subwoofer. Are there any you would recommend and where may they be purchased?


Andy Szabo2004-02-23 22:21

You may have a shorted 500-megohm resistor. This resistor rarely fails, and it usually fails in the ”open” state, but a short (or partial short) is possible. If you are connecting the meter first, and then applying power, you may be seeing the power supply rising to its rated voltage. Your meter overloads because there is no longer 500-megohms in series with the meter, and hence your meter is seeing the full output of the supply.

Your description of reduced volume may be consistent with a shorted resistor. The bias supply of an ESL produces very high voltage, but has very low current output. It doesn\'t need much ”muscle” since the 500-megohms limits the current. However, when limited by a much lower resistance, the supply is not able to develop full output voltage. A reduced voltage on the diaphragm will result in reduced volume.

A resistor that high in value is not measurable with an ordinary ohmmeter, and/or it may be failing only under high voltage conditions. You\'ll need to carefully remove the 500-megohm from the ”good” interface, and substitute it in the ”bad” interface. If the problem goes away, you have found the problem.

A possible source for this resistor (Newark Electronics) is listed under ”Refurbishment”. If you cannot find exactly 500-megohms, you may substitute a lower value, as low as 100-megohms. Whatever value you use, it is best to replace the resistors in both speakers to ensure a matched set.

The high-pass crossover (active only when the switch is set to ”Above 100-Hz”) is made up of three capacitors: a 100-uF electrolytic, a 47-uF electrolytic, and a 10-uF polypropylene, all in parallel. This yields a total value of 157-uF. You may notice some improvement if the 100 and 47-uF capacitors are replaced with polypropylene. Be aware that such capacitors will be quite large, and rather expensive. It may be best to use three 47-uF polypropylenes in parallel, plus the existing 10-uF. This will yield a total of 151-uF, which is close enough for this application.

I don\'t have any specific brand names or sources for these capacitors. Try some of the dealers who specialize in audio parts. You want a capacitor rated for at least 50 volts (capacitors with higher voltage ratings will be proportionally larger in size).

Post a reply

Your name will appear on the website next to your contribution. Your email address will only be used to contact you if something is wrong with your contribution. It will not be shared with others.