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Connection problems in Old radios

Ernst Erb Martin Renz Vincent de Franco Miguel Bravo-Cos Alessandro De Poi Heribert Jung Bernhard Nagel Eilert Menke 
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Forum » In General » Connection problems in Old radios
Lars-G. Lundelin
FIN  Articles: 139
Schem.: 32
Pict.: 111
07.Mar.09 22:21

Count of Thanks: 3
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 Hello Radio Friends
When working with and repairing these old tube radios from 30th's and 40th's you probably have been confronted with any kind of peculiar faults and phenomenas.
In my latest project, an unknown RX yet, I did the usual work by changing all the C:s and necessary R:s. For the first time I got use of my new variable transformer equipment, when reforming the old smoothing caps. Yes, they'll still work after 70 (seventy) years!!.
But, I couldn't get a hiss from the loudspeaker . Although I knew that over 50 percent of all faults is due to bad connections, I couldn't expect this........
Please have a look at the attached picture and you'll understand the following...
The two tags (1 and 2)) are connected to the transformer, and the rivets (4 and 5) connected to the voice coil. The tags are made of steel (iron) and the rivets of brass.
Well, you see..after 70 years... there were no connections between the iron and brass anymore. Of some inscrutable reason they used iron and brass together in this very point. 
This again is a new experience and I hope it will be for all who do not know about these things too.

with kind regards




Todd Stackhouse
USA  Articles: 151
Schem.: 44
Pict.: 28
09.Mar.09 01:40

Count of Thanks: 2
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...over time a chemical reaction between the two different types of metal could have taken place.  'Dissimilar metal corrosion' occurs when different metals are brought together like that; an interaction takes place between the different metals and moisture, etc. in the air, actually generating an electric current from one metal to the other.  This causes a sort of electrolysis that eventually corrodes one or both of the metals.  I wonder if something like that happened here...