Klaas's magnetic deflection amplifier circuit

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Re: Klaas's magnetic deflection amplifier circuit

Postby Klaas Robers » 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.....
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Re: Klaas's magnetic deflection amplifier circuit

Postby Harry Dalek » Thu Dec 07, 2017 1:32 pm

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.
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Re: Klaas's magnetic deflection amplifier circuit

Postby Robonz » Thu Dec 07, 2017 4:05 pm

Hi Harry

Have you scoped your 12 volts rails? You might want something more like 470uf caps on both your power rails. It sound like your power supply is sagging under load. Post up a scope of the 12V rails and try some caps so we can see.

Cheers
Keith
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Re: Klaas's magnetic deflection amplifier circuit

Postby Harry Dalek » Thu Dec 07, 2017 8:45 pm

Robonz wrote:Hi Harry

Have you scoped your 12 volts rails? You might want something more like 470uf caps on both your power rails. It sound like your power supply is sagging under load. Post up a scope of the 12V rails and try some caps so we can see.

Cheers
Keith

Hi Keith
OK i tried it on the dual 12 volt supply for testing the supply
With positive there no change in voltage 11.9 on circuit 11.9 no load .. i will check that again tomorrow seems to good to me .
With Negative its 11.5 volts on circuit and 12.2 no load
Yes looking at the power rails on circuit off its the same just a line as in picture tried on the scopes ac and dc settings switch .
I noticed switching off i can see a larger poorer raster each time doing it quickly as well ,,might be scanning off screen perhaps ,on the lower voltage i can just get it in shot .
As in video file below
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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.
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Harry Dalek
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Re: Klaas's magnetic deflection amplifier circuit

Postby Robonz » Thu Dec 07, 2017 9:45 pm

Hi Harry

When you are scoping the 12 volts you should see your sawtooth wave coming through on the power supply. e.g. somthing like 0.2v peak to peak would be around normal. If your power supply is bad you will see 1 volt or more sawtooth wave on your 12 volt rail. If you see a straight line then your circuit is not running on a load or your volts per division on the scope is set too low.

Cheers
Keith
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Re: Klaas's magnetic deflection amplifier circuit

Postby Harry Dalek » Thu Dec 07, 2017 10:16 pm

Robonz wrote:Hi Harry

When you are scoping the 12 volts you should see your sawtooth wave coming through on the power supply. e.g. somthing like 0.2v peak to peak would be around normal. If your power supply is bad you will see 1 volt or more sawtooth wave on your 12 volt rail. If you see a straight line then your circuit is not running on a load or your volts per division on the scope is set too low.

Cheers
Keith


Thanks Keith for the advice here i will check this better tomorrow if i can see this on the power rails i may have had it set to low as you think as one scenario..
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.
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