Presto MRC-16 Variable Pitch Modification

A rather optimistic gentleman arrived with a Presto MRC-16 disk recording lathe, inquiring about the possibility of repairing the broken suspension and making a cutter head bracket from scratch. He then asked about the possibility of figuring out some way in which lead-in, lead-out, and track mark spirals could be cut on this machine, which was originally intended as a very basic machine, with no provision for spirals, not even a hand crank. Then came the really ambitious part: He would ideally like to be able to adjust the pitch and if possible, control the lathe through his computer!

One glance at the steampunk aesthetic of the manufacturer’s name plate and the overall state of this primitive machine would have been enough to deter the average engineer, in fear that there might be something potentially radioactive hiding within…

But, at Agnew Analog Reference Instruments, we have a bit too much of a “can do” mentality for our own sake and we always like a challenge! “Sure, everything is possible…” came the answer, while three dimensional objects were being mentally rotated and shaped behind J. I. Agnew’s forehead. Never one to shy away from such a pile of pre-war bronze, he already had the basic concept figured out!

The leadscrew was originally powered by the platter via a flange and shaft, with a helical gear engaging a gear on the leadscrew to offer a reduction. This had to go. If we want total control of the leadscrew, it cannot be powered by the platter.

The first thought was to turn the existing shaft/flange assembly upside down.

Two issues here: The flange also acts as support for the entire overhead mechanism. Without it sitting on the platter, we need external support, as in the design of the Fairchild overhead mechanism. The second issue is that the flange is now too much in the way. Time for some radical changes.

Move the entire thing a bit further back, to make space for a proper suspension unit with oil dashpot and provision to hold any head we may want to use.

Machine a support bracket, to hold the overhead assembly over the platter.

Machine a suspension member with slots to allow precise alignment with the centerline.

Machine a set of brackets to hold the new motor which will power the leadscrew.

Machine a new, longer shaft for the helical gear.

Machine a pretty knurled knob for precise manual positioning.

Machine a couple of pulleys for driving the leadscrew via belt.

Machine the main suspension part out of a substantial block of a suitable aluminum alloy.

The major part of the support bracket was designed by Sabine Agnew, combining her artistic inspiration of a water droplet shape with the practical requirement for sturdy, solid support.

Assemble all of the above in a sensible manner.

The leadscrew is now driven by a high precision brushless DC motor, via belt to the new shaft which operates the original reduction gearing. The motor control electronics allow for manually variable pitch control, or fully automated, computer controlled variable pitch, to vary the groove spacing according to the music.

Off to finish turning the dashpot and then finish the platter drive system. More soon from the dark, infinite depths of the “strange stuff” sector of the lab…

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