Acoustat 1+1 weak output

[danielm]

They have been unhooked for four yrs. I bought them new and always had plenty of output. Now I find them weak, very weak.
As if the PS has failed. But both of them ???

I pulled down one sock and had a look at the panels. No arcing, holes or loose film.

What now?

BTW, there is a place just up the road that makes mylar film. I see rolls of the stuff in the dumpster at times.
I may stop and check it out next time.
I always wanted to DIY some ESLs.

Regards,
Daniel

[SY]

I'd double check all the connections inside the interfaces and the fuses. You could also have corroded connections to the diaphragm.

[danielm]

I'm surprised I have this problem. Last yr I pulled some Maggies out of the closet to find the HF aluminum had... decomposed.
Now this.
I guess I can't complain since the're what 20+ yrs old now.

I'll open up the interface and see what I find.

Any suggested mods while I'm there?

[SY]

Mods to the interface have been discussed here before- do a search and you'll find all kinds of fun things to do. But... I'd get them working stone-stock first.

From your Maggie story, I'll bet that you find that the diaphragm contact has gone to meet its Maker.

[enilsen]

I came across a similar problem about 3 years ago when I took my Martin Logan CLS speakers out of 10 years of storage. The sound was very weak from both panels and I did all sorts of things to try and get them working again. I was told that new panels were the only way to get it working again.

I felt that the replacement with new panels was not the solution to the problem rather than a quick fix because nobody would give me a logical answer to what the problem was.

Anyhow to cut a long story short I believe problem is something similar to what happens to aging capacitors. The diaphragm coating and the high-voltage bias contact has had some chemical degrading and eventually the hv bias current no longer is conducted to the diaphragm.

The way I could determine this was to use a plant sprayer and spray a small amount of water onto the diaphragm near the hv bias connection. The sound would then return like magic and would vanish in a couple of days when the water had evaporated.

The final fix for this problem was to use an eye dropper with diluted liquid soap (conductive) and apply a very small amount on the diaphragm. This would run down the diaphragm and make contact with the hv bias metal strip. The hv bias connection was restored and the speakers have worked ever since.

[danielm]

Hmm... a lossy (resistive) connection to the panel...
I could understand that on the low impedance of the Maggies but for the esl panels to be this weak it must be damn near open.

I noticed acoustat has run all three panel wires together up one side. It seems to me that the diaphagm wire could go down the opposite side. The added capacitance of those wires bundled together coud be eliminated.

I'm sure my supplys are ok as the diaphragm slapped the stator when I plugged them in.

I'll start digging this weekend.

Thanks,
Daniel

[danielm]

I'm beginning to think it is a bias voltage issue on the diapragm.
Looking inside the MK121 interface I see no obvious problems. Bias voltage seems to be good to the "red" wire leading to both panels.
I fed a 400hz sine wave and made some measurements using a DVM.
With 8VAC input to the binding post I measured approx 600VAC on the stator wires. (the upper ac limit of my meter)
With a >250hz sine wave the sound appears loud.
There's audable distortion begining at 11VAC input below 200HZ. At 400HZ it is fine sounding up to 40VAC input. I am using a large QSC power amp so, this test was brief. I never saw arcing.
At 40hz the panel starts to rattle at an input of 11VAC.
I cannot figure out why they have little output with music but the 400hz tone sounds loud.
It sounds as if the panel's bias voltage is collapsing with bass transients.
I see no way to to check or fix the bias connection to the panels.
Is this a common problem for 20 yr old Acoustat panels?

[bear]

This problem has been discussed several times recently... do a search and look down the subject rows a bit for other Acoustat threads??

The cause of low output on the Acoustats is usually a bad diode or cap in the HV multiplier.

The way to test it is with a HV probe on a suitable meter.
The proper HV is ~5kv and not too much lower.

It is simple enough and cheap enough to pull the board and buy some high voltage diodes - higher is better, 2kv being ok in general, and some high voltage rated ceramic caps, take the old ones out, solder the new ones in. Done.

Nothing else will go bad on BOTH cells on BOTH channels, unless the panels have been somehow exposed to a highly corrosive environment for a long time... but then you should see some light through the black part of the mylar coating, AND see rust and corrosion elsewhere.

Sometimes it will take 24hrs to reach full charge on an Acoustat, although it should make decent sound right away.

Change the electrolytic coupling caps to FILM caps asap for best overall sound and sound stage. <--- free advice dept, unrelated to ur problem.

_-_-bear

[danielm]

OK, I'm going to pick up some parts this week to make a HV probe if I get a chance. Might as well get some 1N4007s as well.

But, I have this gut feeling the panels are loosing their conductivity. Mist from a plant sprayer was an easy enough test and resulted in a bit more output.

I don't see how to get to the bias connection on the film.

SY, is this the "corroded connection" you are refering to?

I'll make some spl checks at 1m with 2.83volts. Maybe the corroded connection is between my ears.

[SY]

SY, is this the "corroded connection" you are refering to?

Yes. Likewise, you could also snoop around for any leakage between the diaphragm and ground.