HI guys, I have attempted a repair on an FM transmitter. I replaced TR2,(shorted) VR1,(10.7 V output, VR1 is a SMD L78L09), and R4, it looked overheated, measured OK. I fired it up and then found the output Mosfet (F1)shorted and VR1 with an output of 11.2V. So I replaced the Mosfet and the L78L09. The unit works, output is good. However, the 9 volt regulator again has 11.2 v output and R4 100 ohm resistor is dissipating .76 watts, in 1/2 watt part. The resistance measured with no power from the 9v regulator output to ground is 6.1k ohms. 12.2v input, 11.2v output, ground pin 0v.
I drew a schematic.
A picture of the board.
I don't see why the 9V regulator isn't regulating and the 100 resistor gets very hot, it has 8.6V across it. This is the low power mode, the
100 ohm supplies power to the output MOSFET. In high power mode Q2 is turned on to supply power thru the 10 ohm resistor. Any hints are appreciated. I have ran the unit a few hours, and it survived, but I need to find out what else is wrong with it.Thanks, Mikek