3, Output, one speaker plays 3-5db louder

Read more on the Acoustat 3 in our home audio section

Lee Sulka2004-02-23 22:21

Hi Andy, I have a pair of Model 3s with the MK-121-2 interfaces. One speaker plays 3-5db louder than the other. All the individual panels in each speaker are working properly and the rotary controls for the HF seem to work fine.

I swapped interfaces on the speakers and the problem followed the interface eliminating the problem being with the panels. By adjusting the balance control I can equalize the levels and there is no overt distortion in either speaker even when driven loudly with my Acoustat TNT-200 amp. I am unable to test the B+ on the interface but thought the voltage must be low, to produce the reduced output, so I rebuilt the voltage multiplier with microwave oven diodes rated at 15KV and also replaced the .0033mF caps in the with stacked .0022mF pairs to equal .0044mF per step rated at 10KV.

Same problem. I was able to locate replacement HV bias transformers and replaced the one in the low output interface.

Same problem. I replaced the Monster Cable, which had turned green, with Romex solid core wire. No change. All the resistors and cap in the HFcrossovers were replaced but this didn\'t help either.

I discovered a carbon trail from the terminal strip to ground and replaced the terminal strip and checked to make sure that the interface was configured for the correct number of panels-It is.

Still less output. The only original parts I have not replaced are the audio transformers, the .1mF 10K coupling caps, the large power resistors, and the 500Meg resistor in the bias supplies. I strongly suspect that the 500Meg resistor has gone out of spec but cannot test it nor locate a replacement.

I don\'t mind the extra parts as these speakers are ”keepers” but I would like to have the speakers matched as closely as possible. Any suggestions or comments would be greatly appreciated. I use these speakers daily and like the sound even better than the 2+2s I used to listen to. They are crossed -over at 80Hz and used with a Dahlquist sub with excellent results. Thank you for your consideration, Lee.


Andy Szabo2004-02-23 22:21

Well, Lee, you\'ve already covered a lot of ground with your various parts replacements and substitutions.

It is unlikely the 500 meg resistor is bad, but you could try swapping those resistors from one interface to the other, to see if the problem follows the resistor. If the resistor is bad, finding a replacement will be difficult (in fact, that resistor was one of the hardest parts to find even when Acoustat was buying thousands at a time). The good news is that the resistance value is not critical: a value as low as 100 meg is fine, as long as the two speakers have the same value. Just make sure, however, that the voltage rating is at least 5000 volts. You might try a repair shop specializing in televisions: the high voltage sections of TV\'s use similar high-meg high-voltage resistors.

The same technique of swapping parts from one interface to another could be used to test the .01 uF capacitors, the 50,000 ohm resistors, and the audio transformers. Just make sure you swap only one part at a time, to maintain a valid experiment.

The 50K resistors are commonly available. Try Digikey Corp. 1-800-344-4539, or www.digikey.com). It looks like Ohmite\'s type 270 will work. Make sure you get the 50 watt rating. Don\'t be concerned about the voltage rating.

The 0.01 uF capacitors are another story. These are high voltage types (minimum rating 6000 volts), and were also difficult to obtain even when Acoustat was in production. In later years, when Acoustat used only polypropylene capacitors in this application, they were custom made. Other dielectric types, such as oil-filled or mylar, might be available if you search hard enough. Again, try TV repair shops, and replace all four capacitors.

As I\'m sure you\'ve already seen elsewhere in this site, the audio transformers are not available. My only suggestion is to find a used pair of interfaces and use them for spare parts (a potentially expensive way to go!). Let\'s hope that is not the problem.

Please write back and let us know what you find. Sources for parts are always of interest, so if you find a source for a difficult-to-find item, let us know where you found them! Good Luck!

Gary Clausen2007-06-05 21:25

One of the sides of my 2+2\'s was lower in level and distorting. I found one of the output side wires of the large medallion tranny was very thinnned out and suspected broken internal wire. I spliced it and resoldered and used some teflon heat shrink tube to cover up the splice. I think I\'d definitely check the general condition of those HV side leads off the tranny\'s.

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