The machine should regularly go through an automatic defrost cycle. The timer is often in a white box, bottom LHS, looking from rear. With a stub handle screwdriver you can fast forward the mechanical clock (there is a hole for this in the side of the box which is facing the compressor) to defrost part of cycle. With power on you should be able to hear a constant whirring noise from this clock and its cogs (no noise means clock has probably died). However you will not be able to the noise if the compressor is running at the time. Be very careful as there are mains power leads very close by. I would not recommend you do this unless you are confident you will not kill yourself. You could switch off at plug, turn the clock till you hear a click, switch mains back on and watch to see if the coils defrost from front.
peter bennett, November 2007