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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 25th Nov 2018, 8:08 am   #1
BulgingCap
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Default Corrosion inside speaker box

I have just acquired a pair of Acoustica Dynastatic speakers (I found them inside a book cabinet that I bought at an auction !)
They have 4mm sockets on the rear, and plugs with short leads were inserted as found. These plugs are of the old type with a 'leaf' wrapped around the pin as a contact. They were a real problem to remove as there was dry corrosion inside the sockets.
One unit was O/C so I removed the 10" bass driver to access inside. One connection fell off the speaker frame, and I could see that the wires to the 4mm sockets had also rotted off (the sockets have a clear hole through which is why the plugs had corroded too.)
The wires to the mains socket had also rotted off.
There is a small amount of corrosion in the area of the electrostatic driver but this is 'outside' the box in a recess at the front and open to the atmosphere through the cloth.
I wonder if this corrosion is caused by the HT voltage (unknown value at present), or some chemical leeching from the chipboard box, or something from the damping wadding. This wadding appears to be of the 'pink batts' roof insulation type so beloved by Australians (and nearly responsible for the downfall of a government...)
Does anyone have any ideas?
Regards, BC.
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Old 26th Nov 2018, 4:48 am   #2
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Default Re: Corrosion inside speaker box

I probably should clarify:
The corrosion is only internal but the sealed part of the cabinet that the bass driver is fitted into has no holes- the wires from the mains socket and the amp inputs pass through very tight holes to the HT unit and crossover which is external in the cavity with the electrostic unit.
These speakers are in excellent external condition, considering that they may be from 1971. I can find no info on these at all, but the manufacturer was 'Tasman Acoustics Pty Ltd' of Launceston, Tasmania.
The 10" bass drivers with AlNiCo magnets are made by SEAS, who I have not heard of before but are still in business and appear to make well-respected drivers. They are in Norway.
The electrostatic drivers are probably Alex Shackman designed units- he was a BBC engineer and worked with Peter Walker of 'Quad' fame to design the ESL57 units.
I will rewire the cabs and keep an eye on the corrosion issue.
BC
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Old 26th Nov 2018, 9:51 am   #3
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Default Re: Corrosion inside speaker box

Quote:
I will rewire the cabs and keep an eye on the corrosion issue.
That’s probably the best solution, it will be guess work not knowing how the speakers have been used and stored theses last 40 odd years to decide what caused the corrosion.
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Old 26th Nov 2018, 11:03 am   #4
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Default Re: Corrosion inside speaker box

I have repaired a few electric fence units and severe corrosion will occur if there is sparking and ozone production. I soldered the connections to the plug-in boards to eliminate the sparking. The ozone would also rot any plastic parts.

I did not think sparking would happen in a speaker until I saw a picture of the electrostatic driver.
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Old 26th Nov 2018, 11:22 am   #5
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Default Re: Corrosion inside speaker box

Where the corrosion has occurred on solder joints it’s possible that they used an acid flux that ran into the cable and has gradually corroded the copper wire/tags.
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Old 26th Nov 2018, 11:24 am   #6
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Default Re: Corrosion inside speaker box

I suppose the iron transformer is the signal voltage step-up for the electrostatic driver. So where does the high voltage polarising supply come from?

I have an awful suspicion it's the mains rectified directly. Maybe with a Cockroft-Walton multiplier?

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Old 26th Nov 2018, 11:41 am   #7
vidjoman
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Default Re: Corrosion inside speaker box

Just looked at that board and the soldering is very poor. Looks like a very amateurish attempt. At least the caps are Mullard/Philips types plus an unknown electrolytic.
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Old 26th Nov 2018, 5:41 pm   #8
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Default Re: Corrosion inside speaker box

There doesn't look to be anywhere near enough in there to drive the speakers and generate the HV supply ....
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Old 27th Nov 2018, 3:54 am   #9
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Default Re: Corrosion inside speaker box

I haven't had any time to have a good delve, and have not tried to power the units. Last night I drew out the position of components on the tag strip and will create a schematic later today, but it looks as if Radio Wrangler is correct. It seems there is a doubler, but with the fronts removed there is very easy finger access to the tag strip and the connections to the panel. I cannot see how this ever conformed to any regulations, and 8 dots of Velcro does not satisfy a "tool to remove", but Australia in 1971 was a very different place to what it is today (and then there is Tasmania...)
Trevor: I can't see any evidence of sparking, and if the HT is only 400V or so then this is not high enough for ozone production (?)
I always understood that this type of unit needed a polarising voltage in the kV region.
I cannot see how anything corrosive like ozone could get from the panel area into the infinite baffle box as the wires are in very tight holes, but the pulse action of the speaker would possibly encourage this. The corrosion has not been carried down the conductor as the wire is OK inside the insulation.
Vidjoman: The poor soldering seems to be only on the wires onto/from the tags. It seems that the tag strip must have been bought it assembled, and the cabinets assembled and wired by the company. Maybe they were audio enthusiasts with access to a cabinet maker.
The units were produced in quantity, I assume, because I have a sales brochure that was in the cupboard with the speakers.
Nuvistor: I guess that we have all seen uncared for equipment over the years, but these units are in perfect condition externally and don't seem to have been in a damp environment (but have probably lived their lives by the sea.) The SEAS drivers look as if they are new but are dated 1971, and the surrounds have not been replaced despite being of the dreaded foam type.
I will move the speakers from the dining table to the workshop today and rewire them, and will report back on how they sound if they sound at all !
Strangely excited by this particularly as they were an unexpected bonus.
My partner is very pleased with the cupboard to house her sheet music so Brownie points all round...
Thanks to all, BC
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Old 9th Dec 2018, 11:52 am   #10
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Default Re: Corrosion inside speaker box

I finally found time to sort out the wiring in the cabinets, but did a fairly temporary job just to check that the speakers worked and sounded good enough to rewire properly. I had to drill out the 4mm sockets on the rear as the corrosion was so bad that the nuts would not move. I have installed terminal blocks externally until I receive some new panel mount sockets.
One unit seemed as if it would work as received but the other was open circuit. The manual suggests that the unit impedance is '8 ohms' which is what the good unit measured externally. I checked the label on the 10" driver to discover its manufacture date- 1971- and type number and impedance, BUT did not read the other unit too closely. The second speaker had a totally corroded litz pigtail and terminal, so I acquired a donor driver from the dump and soldered on a new pigtail. This entailed removing the Presspahn strip to enable the new terminal to be riveted from the rear.
All was reassembled and then I made the discovery that the second unit had a 15 ohm driver fitted, made in 1973. All other components in the xover unit are identical.
I connected the speakers to my big Denon amp and had a listen to some clean music (Martin Simpson for a start): I was very impressed by the clear sound and the bass is decent too. I have no wadding in the boxes at present as I removed the glass fibre roof insulation to sort out the wiring (and I am very allergic to the darned stuff), and there is a slight 100hz hum from the panels.
I am not sure what to do about the SEAS drivers- the image is displaced slightly towards the 8 ohm side.
BC
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