if the transformer is open its likely that somewhere in the winding of it
the wire is just burned out. to check a diode u need a multimeter that
will do resistance checks, check resistance over the diode, one way (the
right way) should be close to 0 ohms, the opposite way, should be very
high (kiloohms,megaohms) if they are close to the same or both really
high, it is bad. i dont know how well he checked it, transformers usually
last a good while, to check it, just make sure its putting out the correct
current right at the transformer. typically i usually find if the trans
is burned out you can see it and may be able to smell it also. if trans
good, look to the voltage regulator ic or transistor (assumeing it has
one) one leg should have an input voltage from the transformer, another
leg will have a much lower voltage that controls the load. the last leg
should have the output voltage (for a 3 leg transistor). measure all of
these to the ground and determin where you are loosing current