Acoustat 1+1 weak output

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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
 
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?
 
Hv bias failure to diaphram

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.
 
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
 
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?
 
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
 
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.
 
danielm said:
I'm assuming the panels are not repairable if this connection is bad???
If you use a conductive soap you can repair the boundary between the hv bias metal strip and the diaphragm without having to remove the outer stator. The trick here is to let the liquid soup make contact with diaphragm and soak up to the metal hv bias connection by using an eye dropper or a small tube filled with the liquid soap. The solution will never really dry or flake off and continue to conduct static current to the rest of the diaphragm.

Naturally there could be problems with the rest of the diaphragm as well, but you will never know until the hv bias connection has been restored.

BTW. The conductive liquid soap I used was called TERSASEPTIC but you can always do a search for other suitable products.
 
I've yet to come across an Acoustat cell that lost the HV connection.

The likelyhood of having 4 cells with the same problem seems even lower. I suppose there could have been a problem with a short run of them, but again unlikely.

The HV supply is the usual culprit.
You need to replace the CAPs as well as the diodes, if you are going to "shotgun" the repair. The caps usually go bad before the diodes, although both do fail.

The other culprit might be "leakage" as SY mentioned. But that is rare too, and unlikely to happen on both spkeakers - although it doesn't take much to drag down the bias voltage.

Start with the bias voltage.

Oh, fwiw, I had one interface where the megohm resistor was open! That's hard to test for, since it is already in the 100meg or higher range! I had to simply substitue in another, then it worked.

_-_-bear
 
I really don't plan on the "shotgun" repair. I have quite a collection of test equipment being I have designed numerous RF filters, antenna systems as well as a large pro audio line array.

The one thing I do not have is a means of measuring >2K DC until I whip up a voltage divider. Might as well R&R the caps and diodes in the HV PS while I have the soldering station hot.

I just can't see the diode stack going bad during storage but I suspect a cap would as well as any component in the system where two dissimilar conductors could become a semiconductor with time.

BTW, the large line array sounded like a really big pr of 1+1s averaging 100+db at 200 feet FOH. The 1+1s were a nice reference at one time.

Bear,
What do you normally see at the end of the diode stack ? I recall reading 5KV somewhere.

I've been out of touch with the consumer audio stuff for a few yrs. Back years ago I had my share of Quad ESLs, ProAcs, LS3/5s, Maggies...
I had a C22 pre driving a pr of 200watt 811A monoblocks. Ahhh, the days of vinyl spinning on the Thorens TT with an Alpha One MC. Sweet sound to my younger ears.
That system sure gave the AC a workout in the summer.

Thanks for all your assistance.

Cheers,

Daniel
 
Fyi, you can't use a "voltage divider".

You need a HV probe, which consists mainly of one very high Meg ohm resistor in series with the probe.

A divider of the usual sort will simply load down the supply.

Maybe if you use a 100meg and a 10 meg, you could do the job. That would be a 10:1, of course.

Edit: 90meg and 10meg...

The target voltage is 5kv.

And yes the caps can and do go bad with age. Once they short or get very leaky, they can take the HV diodes... doesn't take much.

For the cost of the caps and diodes vs. the hassle of a HV divider, I'd shotgun it. If you had ordered it when I posted, you'd probably be listening to music now... :rolleyes: :D

_-_-bear
 
Component Value?

How important are the value of the caps and diodes?
I got from The Audio Circuit that the caps are 3kv, 3300pf. I can get 3kv, 3900pf - Is that close enough?
As to the diodes I have a wiring diagram which states 25G10 as the part #, which as far as I can find out was 100 volt, 60 amp but the actual parts in my interfaces are 100G10 which was a 10kv, 100ma - are these parts interchangeable?
 
I found 25G10s in my MK121. Replaced each with two 1n4007s in series.
The 3900pf cap will be fine.

After the "shotgun" repair, I finally found the problem. The bundled stator and bias wires were arcing behind the bottom panel. The area hidden by the wood brace with the hole.
My bias supply was OK but I replaced the diodes, caps while I had it open.
 
Re: Component Value?

bodran said:
How important are the value of the caps and diodes?
I got from The Audio Circuit that the caps are 3kv, 3300pf. I can get 3kv, 3900pf - Is that close enough?
As to the diodes I have a wiring diagram which states 25G10 as the part #, which as far as I can find out was 100 volt, 60 amp but the actual parts in my interfaces are 100G10 which was a 10kv, 100ma - are these parts interchangeable?



The caps your mentioned are fine.

The diodes MUST be high voltage (PIV).
Actually, a few thousand volts per diode is likely sufficient - although it is nice to have 10kv diodes IF you can find them. Each section in the ladder has about 1kv across it under normal operating conditions...

_-_-bear
 
I got the caps 3300pf, 3kv from mouser, I am waiting on 10kv, 100ma diodes ordered off ebay. I will shotgun the bias supply.
In the meantime I have been comparing the interfaces to troubleshoot any other problems. I have noticed a difference on the high frequency step up transformers, measuring resistance across the blue to the yellow wire (in circuit) right side is 87 ohm whilst the left side reads infinite. Basically my problem is much less output in the right channel. I am getting 4 kV left and 3 kV right. So to sum up it looks like I have at least two problems:-
1) Uneven bias voltage.
2) A broken high freq. transformer.
Any suggestions on the transformer?

Regards and thanks for the help so far,
bodran
 
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