My hotplate broke and the replacement one that they sent me has a temperature control knob. Is it ok to use the plata? Do I need to cover the knob?
Since your plata has a temperature control knob and the temperature can be adjusted it will need, to be covered in order to permit keeping food that is not fully cooked on it or to permit return food to it on shabbos, similar to your stovetop. The reason we may not keep uncooked food on the fire from before shabbos is because we might forget ourselves and make the fire larger. There are a number of things a person may to to permit this. Either to use a thick metal sheet (called a blech) that is placed over the hotplate, or you can use a few sheets of aluminum foil. (It shouldn’t be only one peice because then it doesn’t appear as being done to cover the hotplate but rather only to keep the hotplate clean.) It is preferable that the knobs also be covered, but merely covering the knob is not sufficient, because it is the actual fire that has to be covered.
You may however remove the knob and tape up the hole, the idea being that you would have to do two things in order to adjust the knob, and by that time you would remember that it is shabbos. According to some Poskim, removing the knobs is sufficient.
O:CH 253-1, 257-8, M:B 253-17, Igros Moshe O:CH 1-93, 4-74 (35), Orchos Shabbos chap. 2-12, Teshuva from R’ Vosner at the end of Orchos Shabbos.