As with the vast majority of disk recording lathes of US manufacture, the 1956 Rek-O-Kut Model V I took upon the task of restoring was configured to spin at the correct speeds when powered by 60 Hz mains. However, one of our customers imported it from the US and wanted to run it at 230 VAC/50 Hz European mains power. We have encountered this situation countless times by now. We had already converted a Presto MRC16 to 50 Hz operation by making new rubber rollers to different dimensions, while some years back, we developed the first version of our Type 191, a stable electronic frequency converter for professional disk mastering lathes, which are often impractical or costly to convert by other means (such as Lyrec direct-drive motors on Neumann lathes, belt-driven Scully lathes, gear-driven Fairchild systems, and so on).
The nameplate of the Rek-O-Kut lathe under restoration.
However, this Rek-O-Kut came with its original rubber rollers in excellent condition and the Type 191 would probably be overkill for a Model V, so we decided to machine a new capstan instead.
The original rubber rollers were still in great shape.
Before starting, we plugged the motor into the 110 VAC/60 Hz supply in our lab, to measure the basic parameters. We have our own clean audio-grade power generation system, and as we often supply customers in the US, we run both 50 and 60 Hz systems. The first problem appeared: It wouldn't even spin! With a bit of convincing, it started up, but would not hold synchronous speed and dropped out. We tried a new capacitor, but the problem was still there.
Apparently, someone had previously opened it up and messed up the reassembly. This explains the low price our customer paid for this lathe. So, we had to spend some time aligning everything properly, so the magnetic circuit could work as intended. A thorough clean-up and lubrication with our Type 1201 oil and it soon ran like new!
The original capstan in place.
So, back to the capstan. Rek-O-Kut lathes use resin-type capstan materials, this particular one being a bit rough. It is a press-fit onto the motor shaft, so it needed a custom capstan extractor setup to remove it without damaging anything.
The custom-made capstan extractor.
Turning a custom drift pin (a tool used to extract press-fit components without damage) on the lathe was the first step. This would bear against the motor shaft, while a special puller plate assembly pulled on the old capstan.
The special puller plate used to remove the capstan.
Fortunately, the capstan had a light interference fit onto the shaft, so it came off easily.
The next step was to take some proper measurements of the shaft, at different points along its length.
The new capstan was designed based on the shaft diameter and the required value of the external diameter, so that the linear velocity would be the same at 50 Hz as it was at 60 Hz with the old capstan.
The material of choice for the new capstan was a brass alloy, which lends itself to a good surface finish and dimensional accuracy (once the heat generated during machining is factored in and compensated for), while being durable in service for this particular application.
We like making things that will last longer than us!
For the level of accuracy required, the use of a quality 4-jaw chuck was called for, since the stock diameter exceeds the collet capacity of the super-precision headstock spindle. The stock was dialed in and center-drilled.
Machining the new capstan.
The tailstock, with a special custom-made precision center, was moved in for maintaining rigidity and the turning began, stopping at regular intervals to measure our progress. The desired dimension was achieved to better than 0.00003" accuracy, and our attention shifted to the accurate dimensioning of the center hole for the calculated interference fit on the motor shaft, without causing distortion or altering the external diameter, which would affect the speed accuracy of the platter.
We added some finishing touches (it might as well also look good after all this effort) and eventually cut off the completed capstan to clean the chips and cutting oil and carefully inspect our work.
The finished 50 Hz capstan was now ready!
The new, finished capstan.
Comparing the new part with the old 60 Hz capstan, the difference in external diameter is clearly visible. The synchronous AC motor spins more slowly at 50 Hz, so the capstan must be bigger to ensure the exact same linear velocity for the friction drive system to work as intended.
Now it was time to press the new capstan onto the motor shaft.
It worked exactly as calculated. An engineering background certainly comes in handy for such projects, as things get done right the first time, with no unpleasant surprises.
The new capstan mounted on the motor shaft.
But we were still far from being ready.
First of all, the original capacitor was no longer of the appropriate value, since the motor would now be operated at a different voltage and frequency. The new value was theoretically calculated and the nearest practical value selected and tested on our motor test rig, to ensure correct operation.
But wait, what do you mean, different voltage...?
Well, this motor could not be run at 230 VAC, but what many people do not fully appreciate is that you cannot run it at 115 VAC either, at 50 Hz, as it would overheat. The approximate voltage value for most 60 Hz motors to run at 50 Hz has been implemented in our Type 1760 step-down transformers, but we prefer, whenever possible, to actually test the particular motor on our test rig and find the exact value. Then we can make a custom Type 1760 transformer for a particular motor, for best results.
What is the difference?
Well, lower noise, less vibration and lower operating temperature, which means better performance of the lathe and longer service life.
This article originally appeared on the Agnew Analog Reference Instruments blog and is used by permission.
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