Bad news im afraid
The cooling unit is a sealed system on the back of the fridge. Heat is applied by a 12v/mains v element or a gas burner This has a reaction with the ammonia and hydrogen in the unit.
It rises in the unit and causes the refrigerant to cool the ice box first
Just like pipework with water in it, these small pipes can get furred up
Bits of this can break off causing blockages in the pipework, resulting in no cooling.
Alas it can be nothing else as it is the same on all 3 supplies. The unit is the only common part
Turning the fridge upside down, sometimes washes the blockage out, but it will still be in there and may cause it to block again
I am afraid that the cooling unit is very expensive, and on an appliance of this age, would not be a cost effective repair.
Please dont shoot the messenger!!
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