To Flatten Warped or Dished Discs
By Mark Gray
The straightening of dished and warped discs is a seasonal occupation and in my experience, satisfactory results can be obtained with vinyl and shellac discs irrespective of size and thickness, provided you go about it with the preparation and care of a surgeon performing an operation. All students make mistakes, so from my few early failures, I recommend you don't put the patient in an oven or under a grill, no matter how low you set the temperature!
For this simple operation you will need patience, because you must wait for the midday sun in Summer, and a perfectly clear blue sky with no clouds around which could block the rays of the sun.
It is always best to operate indoors where temperatures are under greater control. I have a south-facing oriel window and with the sun high in the sky in the Summer, I get perfect results by working on the floor, taking care to anticipate the movement of the sun, so that no shadow creeps over the work area.
I am of course, referring to the method employing two sheets of plate glass placed horizontally with the disc positioned between them, and as Paul Baker mentions, they must be slightly larger than the diameter of the particular disc about to be resuscitated. I recommend that the top glass be at least 3mm. thick as the force of gravity alone supplies enough downward pressure to assist the collapse of the disc. You want to avoid putting any additional pressure on the playing surfaces as this could alter the profile of the grooves, with dire results.
You also require a thermometer placed on the top glass in the work position and reading no less than 40°C before you place the disc between the sheets of glass. Unlike the surgeon, you can dispense with antiseptics and a face mask, but make sure all contact surfaces are perfectly clean and free of impurities such as minute particles of grit.
Another of my early attempts which was less than perfect is clearly imprinted on my memory and will serve as a cautionary tale. On attempting to straighten HMV DA 901, on lifting the top glass, to my dismay I saw a perfectly embossed wavy line across the finely cut playing surface of Joseph Hislop's recording of Mary. This irreversible flaw proved to have been caused by a stray hair from my dog who was in the vicinity!
Having commenced the exposure, keep a watchful eye on proceedings. Be patient as the disc warms up because imperceptibly it will return to its original flat form and you will see this. By gently removing the top glass you can lift the bottom glass, which is supporting the now plastic disc, to eye level and rotate it slowly to check that the centre and outer edge are lying flat against the glass. On no account touch the playing area and never attempt to lift or turn the disc over or you will stretch it horizontally and this, I assure you from early experience, cannot be rectified. At this point it should be borne in mind that the procedure outlined above assumes that the disc to be straightened is not already stretched and was originally flat and not double-sided with raised outer and inner rims.
It would be invidious of me to state a precise time of exposure to the sun because there are several obvious variables such as thickness, composition, etc., but you cannot go far wrong if you remove the glass supporting the disc in its plastic state to a cool place immediately you have checked that it is perfectly flat.
Again, be patient and give it plenty of time to cool and harden before the ultimate test!
Stick to this procedure and I'm sure your 20" monster [or any other size - Ed.] will live to play again.
For this simple operation you will need patience, because you must wait for the midday sun in Summer, and a perfectly clear blue sky with no clouds around which could block the rays of the sun.
It is always best to operate indoors where temperatures are under greater control. I have a south-facing oriel window and with the sun high in the sky in the Summer, I get perfect results by working on the floor, taking care to anticipate the movement of the sun, so that no shadow creeps over the work area.
I am of course, referring to the method employing two sheets of plate glass placed horizontally with the disc positioned between them, and as Paul Baker mentions, they must be slightly larger than the diameter of the particular disc about to be resuscitated. I recommend that the top glass be at least 3mm. thick as the force of gravity alone supplies enough downward pressure to assist the collapse of the disc. You want to avoid putting any additional pressure on the playing surfaces as this could alter the profile of the grooves, with dire results.
You also require a thermometer placed on the top glass in the work position and reading no less than 40°C before you place the disc between the sheets of glass. Unlike the surgeon, you can dispense with antiseptics and a face mask, but make sure all contact surfaces are perfectly clean and free of impurities such as minute particles of grit.
Another of my early attempts which was less than perfect is clearly imprinted on my memory and will serve as a cautionary tale. On attempting to straighten HMV DA 901, on lifting the top glass, to my dismay I saw a perfectly embossed wavy line across the finely cut playing surface of Joseph Hislop's recording of Mary. This irreversible flaw proved to have been caused by a stray hair from my dog who was in the vicinity!
Having commenced the exposure, keep a watchful eye on proceedings. Be patient as the disc warms up because imperceptibly it will return to its original flat form and you will see this. By gently removing the top glass you can lift the bottom glass, which is supporting the now plastic disc, to eye level and rotate it slowly to check that the centre and outer edge are lying flat against the glass. On no account touch the playing area and never attempt to lift or turn the disc over or you will stretch it horizontally and this, I assure you from early experience, cannot be rectified. At this point it should be borne in mind that the procedure outlined above assumes that the disc to be straightened is not already stretched and was originally flat and not double-sided with raised outer and inner rims.
It would be invidious of me to state a precise time of exposure to the sun because there are several obvious variables such as thickness, composition, etc., but you cannot go far wrong if you remove the glass supporting the disc in its plastic state to a cool place immediately you have checked that it is perfectly flat.
Again, be patient and give it plenty of time to cool and harden before the ultimate test!
Stick to this procedure and I'm sure your 20" monster [or any other size - Ed.] will live to play again.