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Beovox M70 restoration

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johndon
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johndon Posted: Thu, Jun 22 2017 3:15 AM

I have owned my B&O system for 42 years. It comprises Beomaster 2000, Beogram 4002 and a pair of M70 speakers on trumpet stands.

 With help from contributors to this site, I have got the Beomaster going again so I decided to tidy up the speakers. There is nothing wrong electronically with the speakers, just the outside looking a bit tatty. The trumpet stands had 42 years of corrosion on the bases so that was the first job.

I knew they were not anodised ( otherwise there would be no corrosion) The round pillar connecting the base to the speaker support is a clear anodised extrusion but the trumpet flares are cast aluminium. So how to clean them? I had some liquid aluminium cleaner containing dilute sulphuric acid and sulfamic acid, so after testing some on the underside, I decided to give it a go.

 I used a Scotchbrite scourer pad ( attached to a sponge that can be purchased in a supermarket anywhere in the world) and wearing heavy duty rubber gloves I started. The corrosion began to come off and fairly soon I could see that it was going to work. It took 4 separate attempts with a good rinse off in between, but in the end the trumpet stands looked like new.

I wiped the grill cloth with a damp microfiber cloth, turning the cloth many times to get a clean face and when I rinsed the cloth, the water in the bucket was quite brown!

Next was the case. I used 0000 grade steel wool and Danish oil which I wiped the surplus off with a soft cloth ( an old T shirt) and set them aside to dry.

Now they are back in place looking ( and sounding ) like new     

Søren Mexico
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Good job, good looking items always sound better, and we love pics

Collecting Vintage B&O is not a hobby, its a lifestyle.

chartz
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chartz replied on Thu, Jun 22 2017 4:22 PM

That's right: no pics, it didn't happen!

Jacques

Saint Beogrowler
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I'd be interested in some pictures too. And maybe the name of the aluminum product you used too.

Peter-

johndon
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johndon replied on Thu, Jun 22 2017 10:49 PM

The product I used is made in Australia by a company called Septone. I posted that the product contained sulfamic acid. That is incorrect. When I rechecked the label it contains hydrofluoric acid and sulphuric acid. The label doesn't give any percentages. I am convinced the trumpet flares had some sort of clear coating to slow down the oxidisation process but it must degrade with time and allow air to come in contact with the aluminium. Mine had become dark grey. The clear anodizing on the vertical tube is still perfect. I am trying to get some photos to upload---when I succeed you'll see them!!

With the system back working again, I played a recording made in 1953 of the organ in Grace Cathedral, San Francisco. I think the original was made using Ampex open reel tapes but then the master was cut using a process that I think was called a western electric Harrison cutter . The recording is so good that you can almost the count the hertz on the 32' pipes and the M70's handle those notes with ease.

johndon
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Here are some photos

 


Saint Beogrowler
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That looks great! I will try to follow in your footsteps yet this summer. Thanks for sharing.

Peter-

Jeff
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Jeff replied on Sat, Jun 24 2017 6:52 AM

Are you sure you mean hydrofluoric acid and not hydrochloric acid? Hydrofluoric acid is highly toxic and also eats metal and glass and can only be stored in wax, rubber, or certain plastics.

Jeff

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chartz
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chartz replied on Sat, Jun 24 2017 7:19 AM
johndon:
With the system back working again, I played a recording made in 1953 of the organ in Grace Cathedral, San Francisco. I think the original was made using Ampex open reel tapes but then the master was cut using a process that I think was called a western electric Harrison cutter . The recording is so good that you can almost the count the hertz on the 32' pipes and the M70's handle those notes with ease.

The M70s do produce some serious low bass. I drive them with a Beomaster 4400 (linear mode) with the trumpet stands like you. They remind me of my old AR 38s, but with much better mids and treble!

Jacques

johndon
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johndon replied on Mon, Jun 26 2017 12:03 AM

Hi Jeff

The UN number is 2922 and the plastic bottle most definitely says Hydrofluoric acid and Sulfuric acid. I have just found the percentages: Hydrofluoric 9.8gr/L and Sulfuric 46g/L. Wearing heavy duty rubber gloves is a must and lots of serious rinsing after treatment is needed----but it works!

 

Jeff
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Jeff replied on Mon, Jun 26 2017 1:03 AM

johndon:

Hi Jeff

The UN number is 2922 and the plastic bottle most definitely says Hydrofluoric acid and Sulfuric acid. I have just found the percentages: Hydrofluoric 9.8gr/L and Sulfuric 46g/L. Wearing heavy duty rubber gloves is a must and lots of serious rinsing after treatment is needed----but it works!

 

Thanks for confirming! I'm just shocked actually, even though it is in a lot smaller concentration than the sulfuric acid. I remember my chemistry days reading how it was used for etching glass, and how most people who tried to isolate fluorine from HF acid wound up with fluorine poisoning.

Looking at your picture, it worked quite well, the bases look great! I have always admired those kinds of B&O speaker stands. My dining table and chairs are the Saarinen tulip set with the same style bases, only in white.

Jeff

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joeyboygolf
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Jeff:

johndon:

Hi Jeff

The UN number is 2922 and the plastic bottle most definitely says Hydrofluoric acid and Sulfuric acid. I have just found the percentages: Hydrofluoric 9.8gr/L and Sulfuric 46g/L. Wearing heavy duty rubber gloves is a must and lots of serious rinsing after treatment is needed----but it works!

 

Thanks for confirming! I'm just shocked actually, even though it is in a lot smaller concentration than the sulfuric acid. I remember my chemistry days reading how it was used for etching glass, and how most people who tried to isolate fluorine from HF acid wound up with fluorine poisoning.

Looking at your picture, it worked quite well, the bases look great! I have always admired those kinds of B&O speaker stands. My dining table and chairs are the Saarinen tulip set with the same style bases, only in white.

I'm a qualified chemist and I agree with you , Jeff. I am surprised that you can buy HF and H2SO4 over the counter at all, let alone in those concentrations.

Regards Graham

Jeff
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Jeff replied on Mon, Jun 26 2017 4:06 PM

Graham, if my memory serves me right (and it's been quite a while since I did chemistry, I was a physics type and systems engineer) it seems like the stuff contains about half a mole of each per liter!

Jeff

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joeyboygolf
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I agree

Regards Graham

Saint Beogrowler
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Some sacrifices are are worth it for my B&O.

Peter-

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