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

[rabruner at aol.com]

Another joy in the Early VPR-2s was the steady business replacing the 2N3055s in the output of the scanner servos I think these failures may have been aggravated by the same sort of problem in the scanner motor, dust from the brushes shorting out things. 
     Yes, when they came out with the new transports for the D2 machines, and later the DCT machines, with the voice coil in the AST heads, it was a significant improvement. No high voltage required and the mechanism inherently moved the head parallel to the tape surface, so that problem went away too.  
    We still operate two VPR-80s for the occasional legacy tape that comes up.  Since we have no need to still frame these, we have standard record heads in the scanners for those, which solves all the AST problems nicely. Generally get cleaner pictures and of course, much lower maintenance costs.  


Bob 
W9TAJ








Right Bob. I fought that battle with the VPR-80s. It was easy to short out the AST circuit. The encapsulated slip rings made it much better. I remember changing out that in all my machines.


Great improvements to the AST when the digital VPR-3000 came out. The AST articulation was now a small electromagnet that moved the head. You could even play music through it! Solved many problems.





Could the same technology be applied to Quad? 

Best,
Park



C. Park Seward
2" Quad and 1" "C" transfers
Visit us: http://www.videopark.com







On May 8, 2010, at 10:56 PM, rabruner at aol.com wrote:


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.  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 not 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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