Presto 1-D and 1-C cutter heads are frequently encountered on our lab bench for repair and restoration. Some are just regular repairs to original spec, others are rewound to a different impedance, but some undergo more comprehensive and exciting modifications!
We have previously examined modifications where a feedback system was added to a Neumann MS-52H and an Audax R-56 cutter head. This time, we shall discuss adding feedback to a Presto 1-D, which arrived with damaged coils.
The Presto 1-D consists of a horseshoe magnet, laminated pole pieces, two drive coils, the armature pivoted on a knife-edge bearing, a set of springs and a large damper.
The first step was to machine new coil formers, to accept the new coil assembly with feedback.
This takes some delicate machining on our 1930’s Lorch precision optical lathe, as the wall thickness of the formers is only a small fraction of a millimeter! Thin-wall micromachining has been the subject of several research projects in the fields of mechanical engineering and industrial manufacturing processes, over several decades now, due to the challenges it presents. Features with a wall thickness under 1 mm and a height to thickness ratio larger than 5 are generally considered particularly difficult to machine. In these coil formers, the height to thickness ratio is around 30, while tight geometric and dimensional tolerances must be maintained!
The machined formers are then taken to our precision milling machine to drill the tiny holes for the wires to enter and exit the former.
After cleaning and inspecting, a pair of tiny coil formers are selected and brought to our coil winding machine for the winding operations.
The completed coils are then varnished for stability and assembled into position.
This modified version shall now be called the Presto 1-DA. It is currently in active service at Epos Laboratories (https://www.instagram.com/eposlab/) in Greece.
Further information, measurements and a mathematical analysis will appear as part of a research paper, to be published in due course.
Many of these feedback techniques originate from the research and development efforts at Agnew Analog over the past couple of years, towards the design of a new stereophonic cutter head.
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