[QuadList] Ampex AST heads and CMC DPT heads--what's different?

[rabruner at aol.com]

The earliest Ampex heads use a crystal plate with copper on either side. Applying voltage across the two plates flexed the crystal, which caused it to move  the head up and down in an arc, as someone has said.  In later versions, they cut the copper plates near the tip and cross connected the plates at the tip with the plates on the body.  This meant that a voltage that would deflect the head up, would deflect the end of the bimorph down, which more or less caused it to move parallel to the tape. 
    Both versions of the head required high voltages applied through brushes to make it work.  The early scanners used carbon rod brush assemblies that shed carbon dust and experienced many shorts.   They tried changing the order of the brushes so that adjacent brushes did carry not opposite polarity voltages and various other schemes. Nothing worked, the brush assemblies were noisy and shorted out, the brushes broke, etc.  Finally they threw in the towel and used wire brushes like Sony and greatly improved the operation and reliability of the AST.  Sony had their heads on a parallelogram, which kept it parallel to the tape without all the fuss of the double bimorph.


Bob Bruner
W9TAJ


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