09-04-2015, 08:56 PM
The yellow cans will be OC81D and two OC81 transistors with sleeves over them like the red ones on the AF117s.
What is the component that is slightly obscured by the aerial between the coil connections? If this is a capacitor replace it before you do any other work as it is likely to be the power decoupling capacitor for the audio driver (OC81D) and will be causing the oscillation and overheating.
The method I use to deal with AF117s is to disconnect the screen lead by heating the solder and lifting the wire and cleaning the solder off the board so that there is no connection and then measure it again. Your meter will be reading the biasing resistors if the screen is connected to the earth track.
If there is now a reading when the isolated screen is measured it can be treated using a 12 volt power supply and a 100 ohm resistor and three silicone rectifier diodes.
First connect the diodes in series and connect the banded ends the same way round with the end one connected to the negative of the power supply and the resistor to the other end and connect it to the positive side of the power supply. You can if you want monitor the junction between the diodes and the resistor with a moving coil meter.
Now connect the junction between the diodes and the resistor to the screen and touch the negative of the meter and the banded end of the diodes to the electrode with the lowest reading when you tested with the meter. If you have the moving coil meter connected you should see a very short dip in the reading from a little under 2 volts for a fraction of a second. You should now find that the fault is gone.
In the data sheet for the AF117 the maximum applied voltage between any two pins is about 3 volts or so so the silicone diode shunt regulator is enough for you not to need to take the transistor out of the board.
What is the component that is slightly obscured by the aerial between the coil connections? If this is a capacitor replace it before you do any other work as it is likely to be the power decoupling capacitor for the audio driver (OC81D) and will be causing the oscillation and overheating.
The method I use to deal with AF117s is to disconnect the screen lead by heating the solder and lifting the wire and cleaning the solder off the board so that there is no connection and then measure it again. Your meter will be reading the biasing resistors if the screen is connected to the earth track.
If there is now a reading when the isolated screen is measured it can be treated using a 12 volt power supply and a 100 ohm resistor and three silicone rectifier diodes.
First connect the diodes in series and connect the banded ends the same way round with the end one connected to the negative of the power supply and the resistor to the other end and connect it to the positive side of the power supply. You can if you want monitor the junction between the diodes and the resistor with a moving coil meter.
Now connect the junction between the diodes and the resistor to the screen and touch the negative of the meter and the banded end of the diodes to the electrode with the lowest reading when you tested with the meter. If you have the moving coil meter connected you should see a very short dip in the reading from a little under 2 volts for a fraction of a second. You should now find that the fault is gone.
In the data sheet for the AF117 the maximum applied voltage between any two pins is about 3 volts or so so the silicone diode shunt regulator is enough for you not to need to take the transistor out of the board.
