Klaas Robers wrote: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.....
Hi Klass
Sorry to hear of the problem with your deflection circuit on the 73 sstv monitor it sounds like it was a easy fix .
I have not been able to increase the deflection size of the lines even with reducing the low ohm yoke resistors on that tiny raster i posted i am using now 2.5 and 4.7 ohm at the moment.
A slight increase in line size can happen with disconnecting the 10k feed back resistor ..
Some thing i also noticed was a resistor to the base of the power transistors other end to ground you can adjust the centering or position of the raster line ...more just experimenting nothing soldered in .
I know the power transistors can work as i have used these in the other circuit perhaps i should try some thing closer to the BD types you have .
i am trying to work out what factors are different mine to yours ...the diodes are the power transistors and the and the increase in resistors for 12 volt dual supply voltage .
I find i have to reduce this supply as i think it starts oscillating very distorted wave form .....pretty much viewing line on screen when the waveform is correct looking so is the line and when not theres line oscillation in the raster line you can see .
I wonder if giving the power transistors its own supply separate to the rest of the circuit ,you do notice a line size increase with an increase in supply voltage until it starts oscillating perhaps a test here might show up something.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.