Need help with Acoustat Model 4 speakers

Need a little more info. What amp are you using? How many watts? How much gain on your source/preamp?
A lot of amps simply will not have an impedance synergy with the interface box thus a weakened sort of drive. In deference to the last poster yes the speaker interface needs a bit of time to charge the diaphragms, however all the Acoustat interfaces I have charge up fully within 30 seconds so if the high voltage DC charging section of the box is weak then the diaphragm will not charge fully and there again weaken the sound output.
If your so inclined remove the top cover plates of the interface boxes. Just don't go poking around in there with your bare mits and electrocute yourself. There are a few things that are in the box that you need to familiarize yourself with.

Are you OK with looking inside?

I have disassembled several pair of these speakers including model 3 and model 4 and they are incredibly simple in concept and parts count. Brilliantly complex yet simple I should say.
Don't attempt to poke around the interface box with it plugged in or after it has just been turned off. I regularly do operate them and disconnect the 5000 DC voltage tap to the diaphragms and discharge the panels to ground while the box is on but I've familiarize myself over time with how to do this. I have gotten zapped several times but it is more or less a static spark that startles you but not harmful. That is not to say it can't harm you because many say that it can. I'm more concerned with staying away from the 120V AC that is powering the box than the low current 5000 volts.
I assume no responsibility for anyone hurting themselves with live electronics however I've done it with the Mk121 Acoustat interface and it is no big issue to do so.
Hope we can help you truly hear these great sounding speakers.;)

PS: what's with the time stamps on the AK posts? The OP is stamped at 9:05 AM Today and my reply is stamped at 12:36 PM today. It's 7:40 AM Monday morning here in Ohio! What time is the AK forum referencing?
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The HV runs on a multiplier string.

It is likely defective.
Very common, after long years.

You need to replace all the diodes and all of the caps.
Unless you can measure up the string with a HV probe... then you can just replace the dead ones.

What you want are HV caps rated at 6kv or better, ceramic, the value is not super critical, but ought to be ~+100% -0% of the value printed on the caps now on the board.
The diodes need to be rated at 3kv or better, you can string two diodes in series to get a higher PIV rating no problem in this application. If you can find genuine HV diodes rated at 6kv or better that is even more betterer. :D

Of course you need to be able to solder and desolder, but this is an easy board.

Remove the AC power.
Open the top of the Mk121.
Using a wire or clip lead - there are three terminals that connect to the panels - short each of the terminals to each other a few times to get rid of the charge.
Remove the wires that go to the panels
Mark the wires that go to the board on the other end - desolder them.
Remove the board.
MARK the direction of the diodes (VERY IMPORTANT!!) on the board with a pencil or a thin
marker. You want to mark the cathode end (either the dot, or the line --->|--- end of the diode's symbol) on the board. Either that or you can replace them one at a time.
Remove all the caps.
Now either replace the diodes one at a time or all at once.
Place new caps in, solder everything... inspect twice...

Fire it up!! :D

A HV probe will help determine if it is running properly, but your ears will know quickly.

The Acoustat plays quickly to full volume, although the speakers do sound best after a day of charging...


PS. you can "check" the HV the "old fashioned way" if you are very careful. WITH THE UNIT OFF, and discharged, you can put a clip lead onto one of the terminals on either side of the board (the ones that go to the panels) - then plug the unit in, then even unplug the unit (the panels not connected is prob best), now you can go and discharge the HV board by touching the clip lead to the center pin area... you should get a smart sounding "tick" of the HV... with the unit on you can likely draw a little bit of a spark... but DO NOT HAVE URSELF (your hand, or anything else) in contact with the metal enclosure or ANYTHING ELSE. It's not a lethal voltage and current combo - but should be treated with CAUTION and CARE, it's a little unpleasant to get the full dose... THE MORE INSULATION YOU HAVE THE BETTER. SAFETY FIRST!! UNSURE? DO NOT DO THIS!!
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And there you go, from one of the best in the business.
Thanks for that Bear! I believe I'll undertake to inspect the health of my own Mk121 interfaces as well. I "snapped" the panels to ground many times on the fly and also got snapped myself. It was a shocking experience.

FWIW the panel center lead will spark when remove from the HV terminal and touched to the chassis. So the panels carry and hold enough charge to ding you pretty good without the power supply plugged in.
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Thanks for the kind words - I doubt that I am even "good"... as far as "in the buisiness" goes, ha! It's a tough road to take... I just try to avoid landmines and plod along, there are others on here who are true luminaries. Me, I'm just a hacker and artist... :D

But yes the panels themselves will hold a nice ZAPPP!

As far as I can tell the caps break down, that causes an over current in the diodes...

Well Bear maybe the business end of a soldering iron and a calculator, as well as all the other measuring devices. But business nonetheless. If I could afford it now your Symphony 1 might just be my pick amongst several amps that drive Acoustat well. But I'm certain since you "attacked" the problem as a purpose built solution that your amp is the cats a$$ for ESL's. I am one of the ones on this forum who is intrigued by full range Acoustat panels and homebuilts of that sort. Personally I have one single Acoustat nine inch panel per side running and while it certainly has limits it makes darned good bass down to lower 30's. All the other attributes are there that are known and loved yet a single panel gives much less beaming and "ladder" effect when moving across the listening plane.

Sorry to hijack the Thread skykomish. Since you have a set of Acoustat now though you can appreciate the banter.

Thank Bear! I think we may chat more about power plants for ESL's in the near future!
Quick question about the transformers themselves.
How easy/hard is it to damage or roast the large bass transformer or the treble transformer in the interface box via over driving? I've seen pictures of the damage but in my mind can't conceive how much amplifier power would it take to cause such a thing? The panel diaphragm would be smacking the stators at that point no?
I thought they could be be popped if someone put an overvalue fuse in, and then sent signal with the 5k unplugged. I know I've done it.... the first instinct is to turn it up more...and then I remember unplugging them.
Basically overdrive the transformers to a level you would never reach if they were making sound. Is this correct?

Dave that does sound like a plausible scenario. You hear a tiny sound and ramp up the volume and saturate the audio stepup trannies with all the amp can throw at them. It would be a more powerful amp than 200 watts I would think if it were a doubling down amp that double power into lower ohms then the high frequency transformer is more in danger since that is where the load lowers in ESL's. I can also see someone jamming out at high volumes and then for whatever reason the 5KV fails and the power has nowhere to go.
It's pretty darn simple, all you need to be able to do is to solder.
If you can't there are few places on earth that with a little effort you can't find someone who can or who can help. Any "TV" repair shop can do the job for you.

If you post an image of the inside of the interface unit I can tell you what is what...

Beyond that PM me if you want to run the "offer" past me. You'd be giving up a very very excellent speaker for not much reason...