Re: Klaas's magnetic deflection amplifier circuit
Posted: Wed Dec 06, 2017 10:24 pm
Sometimes things go wrong even here. Few days ago, when I was testing the Robot scanconverter on my pictuer generator built in the "Heathkit" SSTV monitor, I observed that the radar-type picture tube only showed the upper half of the picture. Once half way the line stopped moving and all lines were written at the same place.
Today I checked the circuits and it was obvious that the deflection amplifier worked only for positive input signals. However it oscillated too in that positive (upper) part. Then I measured the voltages and saw that the voltage on the emitter of the BD135 was nicely on -6 volt, while the base of that transistor was at about 0 volt. I measured on the pins of the transistor directly.! That cannot be of course. From base to emitter there is a diode, so no higher voltages than 0.7 volt.
I replaced the transistor for a BD137 and it worked again correctly. So the connection internally in the transistor was gone. And that for a circuit built in 1973.....
Today I checked the circuits and it was obvious that the deflection amplifier worked only for positive input signals. However it oscillated too in that positive (upper) part. Then I measured the voltages and saw that the voltage on the emitter of the BD135 was nicely on -6 volt, while the base of that transistor was at about 0 volt. I measured on the pins of the transistor directly.! That cannot be of course. From base to emitter there is a diode, so no higher voltages than 0.7 volt.
I replaced the transistor for a BD137 and it worked again correctly. So the connection internally in the transistor was gone. And that for a circuit built in 1973.....