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Forum 141

[Closed] A Marconi 707 added to my collection

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Katie Bush
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Sorry Stephen, I didn't see your post before I wrote this one! There must be another factor involved ............ Maybe there has to be some acidity or other pollutant as well as moisture.
:aab

If the sets in question were stored in older houses, or the cellars thereof, then lime mortar may be leaching an alkaline into the air, also, urea was used in some mortars, and urea (urine) is very corrosive.. Have you ever noticed the salt that leaches out of damp bricks? - Another potential source of corrosive elements in the air, as would also be the case in the atmosphere around coastal areas.

Marion

 
Posted : 01/08/2015 9:07 pm
Brian Cuff
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Hi Marion. The house we live in was originally a farm worker's cottage attached to a barn - converted in 1951 so no expensive architect type conversion! We had problems in the large sitting room with salts coming out of the walls (efflorescence) which was due, or so we were told, to the urine passed by presumably many thousands of cows (it is about 250 years old) as that room was most of the original barn. It is also built with lime mortar so I'd better be careful with any sets in the house!
I'm seriously thinking about stripping the set down to the bare chassis, sand blasting and plating it and re-building from the bottom up but as I said in an earlier post, it will have to wait, very definitely in a warm dry place. I suppose that I will have to replace all the tags in the tag strips etc. but as long as it looks original from the top of the chassis that will be OK.

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Posted : 01/08/2015 11:32 pm
Brian Cuff
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Cathy is right about the loss of zinc from the brass alloy - there is even a word for it "dezincification"! Both acidic and alkaline moisture will give rise to it and the result is a weak brittle metal (of which I seem to have quite a lot!). https://www.google.co.uk/search?q=dezin ... nlUuLzjJAI.

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Posted : 02/08/2015 1:58 pm
Rally coordinator
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I'm seriously thinking about stripping the set down to the bare chassis, sand blasting and plating it and re-building from the bottom up.......

Looking on the positive side, given the current chassis condition, this is a relatively easy decision as there is no original chassis plating or patina to preserve and all components require a degree of restoration.

As chassis conservation measures, which replace the current finish, will be necessary by whatever method is adopted, IMHO a sandblasted and re-plated chassis will retain the neatness and appearance of the original design.

Mike

 
Posted : 02/08/2015 2:51 pm
Brian Cuff
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Yes Mike. That's what I will do. Meantime, I'm trying to find some suitable eyelet tags to replace the brittle ones. If I'm going to re-build, I might as well go the whole hog (I said it was a pig in a poke :bba :bba )

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Posted : 02/08/2015 4:22 pm
Brian Cuff
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I didn't realise that I hadn't posted about the manufacture of new tags on this forum so here goes:
I needed to replace loads of the tags on the strips because of the aforementioned dezincification. Looking through the tag makers' websites, there was nothing even similar so I drew a development of the tag and asked a couple of pressing companies on the web to quote their price for a minimum order quantity of tags and the tooling charges.
I was pleasantly surprised at the offer and went ahead. I finally got the finished tags today (the makers forgot to tin-plate them so I got it dome locally, saving £27!).
I had already made a special square punch and punched some trial Tufnel strips so I was able to try the tags straght away. The picture shows the result - I'm pleased but they were quite expensive but the 707 is worth it.

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Posted : 03/12/2015 5:36 pm
Katie Bush
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I always am amazed by the ingenuity of forum members, and this is one such occasion.. I am well impressed with those results!

Just out of curiosity, does Tufnol have the same insulation characteristic as Paxolin? Or maybe even better?

Tufnol is an amazingly versatile material, and I can remember it being used by David Brown to manufacture the timing gear trains used in their mid 50's range of tractors (DB25, DB30, DB900 and DB950) 0 I have no idea how successful they were, but I do remember Tufnol couplings being used in the diesel injection pump drives on some tractor engines. :aad

Marion

 
Posted : 03/12/2015 7:15 pm
Doz
 Doz
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I always am amazed by the ingenuity of forum members, and this is one such occasion.. I am well impressed with those results!

Just out of curiosity, does Tufnol have the same insulation characteristic as Paxolin? Or maybe even better?

Tufnol is an amazingly versatile material, and I can remember it being used by David Brown to manufacture the timing gear trains used in their mid 50's range of tractors (DB25, DB30, DB900 and DB950) 0 I have no idea how successful they were, but I do remember Tufnol couplings being used in the diesel injection pump drives on some tractor engines. :aad

Marion

Used to maintain a modulator which had 60KV on tufnol insulators. Never saw one fail.

 
Posted : 03/12/2015 7:41 pm
Brian Cuff
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Thanks for your comments, Marion.
There are several grades of Tufnel with characteristics chosen to suit various tasks such as insulation, mechanical wear and good machining properties. Some grades are made from paper (Whale for example) but most are cloth based. I would recommend visiting the Tufnel website for interesting details of the various grades.

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Posted : 03/12/2015 7:57 pm
Rebel Rafter
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Hi, from RR. I thought I'd done some restorations in my time but nothing like this one. That's real restoration. Dezincification is a problem with some brass plumbing fittings which is why some are now made from a special blend of metal. Couldn't those old valves be suspended over an ultrasonic cleaner so that just the pins are in the solution? It looks like you'll have to take lots of careful pictures of the old chassis to see where all the connections go afterwards, as well as drawing various diagrams, have you got a circuit diagram? I've never had a chassis that rusty. Has anyone on here ever seen the book about electronic classics? I can't remember the exact title but I've got it somewhere, amongst all the others. There's lots of suggestions in there for restoring bits of really old stuff. I used to completely strip down old sets with plastic cabinets and back covers and then stick the cabinets and backs in the bath and drench them in foam cleanser and get them looking like new, I thought that was restoration but it's nothing like stripping out all those hardwired bits and then trying to put them all back exactly as they were. And I thought rebuilding a 1000 watt stage PA amplifier was a big job. It had 20 TO3 o/p transistors which all had to be replaced. I might write a post about that some time in another section. Where do you get your little tags? RR.

 
Posted : 03/12/2015 8:47 pm
Brian Cuff
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Hi RR
I will try replacing the whole base on the valves that are past cleaning. I have tried my ultrasonic bath with this sort of corrosion and it didn't touch it so it will be replace or scrape!
I have the full manual for the 707 and I have already restored my 706 which is almost identical but you're right about lots of notes and pictures!

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Posted : 03/12/2015 9:29 pm
Brian Cuff
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I have constructed a replica EMI tagboard for someone on another forum and I might well reconstruct all the boards and strips on my 707. The picture shows the new tagboard.

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Posted : 05/12/2015 8:27 pm
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