electric bedroom heated mattress vs danger
I am assuming that the heating device is an "electric blanket" used on top of a mattress.
Such blankets MUST NOT be used underneath a mattress as they can then cause fires.
If this is NOT an electric blanket, please advise. 

The "controller" has applied too much energy and allowed the blanket to overheat. The "repairs" are poorly done, the blanket is dangerously damaged and the controller is not reasonably able to be repaired. 


The repairman is not a competent repairer.
This blanket could kill you or burn your home 
While it is sad to have to buy another blanket, you are risking your life by continuing to use this blanket or this controller. Very sadly, both should be destroyed and discarded.
While the repairer may have genuinely been trying to help they lack the knowledge or the repair skills to do an adequate or safe
job.




I admire your enthusiasm for attempting to deal with this circuit. Without great effort attempting to see what the 'repairer' has done it's completely unknown what the finished circuit is or whether it in fact "controls" the heating level at all. I do not know with certainty what the diodes did or what the circuit now does.

It is usual for electric blankets to NOT have a power supply that is isolated from the mains. The whole of the wiring should be considered to always be at mains voltage when plugged in. Depending on the design the wiring may be "live" when plugged in but not operating!. 

Note that "isolated" usually means using a transformer with electrically separated primary and secondary windings. Use of diodes alone does not provide isolation.