Control Grid modulation

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Control Grid modulation

Postby Harry Dalek » Fri Feb 24, 2017 11:06 am

I tend to need and example on circuits very much so on hooking things up to a CRT so the only example i have for my CRT on grid modulation is the SSTV monitor in the August 1973 73 magazine on Ralph Taggart's.

My transistor is a ZRX458 rated at 400 v

So i put that circuit together to try .
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Re: Control Grid modulation

Postby Klaas Robers » Sat Feb 25, 2017 6:48 am

Harry, it depends on what you have for voltages available. I used modulation of the cathode and the control grid for brightness control. I had a voltage of +100 volts available and something as +300 volt for the first acceleration and focus control. What voltages have you now for the CRT? I think that - 300V is rather rare.
Image
This is the video filter, don't mind it. And the video amplifier with kathode modulation. The BD115 is an old transistor that can have at least 100 volt. You see too the brightness control on the right, controlling the G1.
In the middle is a help circuit. In fact it creates an adjustable voltage with the BC107 emitter follower, giving a voltage equal to the input voltage of black (1.15 volts). See the video wave form below the circuit. This makes it possible that the contrast potentiometer is not affecting black, and only controls the shades grey and white. You can adjust that with the trimmer potentiometer ZB. If set at a wrong voltage, black varies also when you adjust the contrast control. You don't want that.
If you can't see the diagram completely, download the attached .gif file.
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Re: Control Grid modulation

Postby Harry Dalek » Sat Feb 25, 2017 8:14 pm

Klaas Robers wrote:Harry, it depends on what you have for voltages available.


I have a dual 300 volt supply......but i find hooking it up to the focus and brigntness pots it drops this supply to 100 and 50 volts acoss those pots .
The I am pretty much using circuit as shown below but i changed the diodes from half wave to full wave rectifier on Steves advice for DC supply but i also have 2 load resistors still onit acoss the positive and negative supply ...i put them on to drop the voltage at touch to 300 volts and to bleed the voltage away when switching off power for safety ...may over kill now as i for got about the pots being in parallel with those Pot resistors .


I used modulation of the cathode and the control grid for brightness control.


Testing the control Grid today with a negative voltage -12 volts halfs the brightness of the raster line and i would guess say 50 would extinguish the beam .

It seems a touch less voltage hungry than this way ,i noticed when i was researching this on other sstv's most used your way cathode to modulate but i wasn't sure if i could use it on my tube as they were all your CRT type .

I had a voltage of +100 volts available and something as +300 volt for the first acceleration and focus control. What voltages have you now for the CRT? I think that - 300V is rather rare.

Yes as i mention above dual 300...it was easy as it uses the same transformer AC supply as the 3PB1 .

Image
This is the video filter, don't mind it. And the video amplifier with kathode modulation. The BD115 is an old transistor that can have at least 100 volt. You see too the brightness control on the right, controlling the G1.
In the middle is a help circuit. In fact it creates an adjustable voltage with the BC107 emitter follower, giving a voltage equal to the input voltage of black (1.15 volts). See the video wave form below the circuit. This makes it possible that the contrast potentiometer is not affecting black, and only controls the shades grey and white. You can adjust that with the trimmer potentiometer ZB. If set at a wrong voltage, black varies also when you adjust the contrast control. You don't want that.
If you can't see the diagram completely, download the attached .gif file.

prof[/quote]

Looking that your circuit it uses a dual 6 volt supply i have a dual 12 volt supply for the main circuits and + 5 volt for the inverter .
If you think your circuit with transistor changes will work on mine i am fine to give it a go and see it would mean just not using the negative 300 volt supply and dropping the positive 300 to 100 not hard to to doing it now without trying :roll:
(would it operate without a blanking signal just for testing ?)
I am sure i would have some thing handy for transistor substitutes.
i will have a look for the parts and get it together give it a go this week .
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Last edited by Harry Dalek on Sun Feb 26, 2017 9:46 am, edited 1 time in total.
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: Control Grid modulation

Postby Klaas Robers » Sat Feb 25, 2017 9:03 pm

Yes Harry, I think it will work as well on + and - 12 volt and without the blanking signal. Then connect the blanking input to ground. May be you double the values of the 5k6 and 3k3 into 10k and 6k8.

But first look at the video signal.! It should be about 1 volt peak to peak and have a mean level of between 0 and 6 volt. And the sync pulses should be more negative than the video levels. Use your scope to examen this first.
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Re: Control Grid modulation

Postby Harry Dalek » Sun Feb 26, 2017 6:40 pm

Klaas Robers wrote:Yes Harry, I think it will work as well on + and - 12 volt and without the blanking signal. Then connect the blanking input to ground. May be you double the values of the 5k6 and 3k3 into 10k and 6k8.


Yes theres a few ways to perhaps go there ...some ideas here to run by you what you think .
i was thinking it might be easier to drop the 12 volt supply with a 5 volt 7805 7905 regulator if a volt less is not a problem .
i tried also with a 8.3 meg resistor that drops the 12 volts to about 6.5 volts might be another way to try .

The main BD115 100 volt needs to be replaced as in the first data sheet ...as i don't have that one but i found a few transistors so far that might be ok i just want to run these by you see what you think be the best choice .

i am going to use my constructed Steves (demodulator circuit ..thats one working tested ) i should see a rather amplified video on the collector via my scope one way working out i got the transistor working ....

But first look at the video signal.! It should be about 1 volt peak to peak and have a mean level of between 0 and 6 volt. And the sync pulses should be more negative than the video levels. Use your scope to examen this first.


Are these any good Klass i think they are Steves reversing wavs i know you and Steve have posted a bit up on one of the other sstv topics ...if theres any other you rather me try fine to test .

I was just getting parts together today but rather wait to hear if any of my transistors for the 100 volt supply are worth trying .
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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: Control Grid modulation

Postby Klaas Robers » Sun Feb 26, 2017 11:51 pm

Harry, I see a video wave form on the bottom of the circuit diagram. 0V tot 1.1 volt. That suits to the BD115 video amp.

For the transistor:
1. I should not choose a darlington (double) transistor.
2. Further the transistor should be able to withstand the 100V supply voltage.
3. The max. current is 100V / 22k = 4.5 mA, so even the 2SC5201 (max 50 mA) will do.

Then if you want to use the +300V you may use a voltage divider in stead of the 22k collector resistor. Connect from the collector
- 33k to ground and
- 68k to +300V.
For the collector this equals to 22k to +100 volt.

But be aware that there is running a continuous current through this voltage divider. It dissipates 0.7 watt in the resistor of 68k and 0.3 watt in the resistor of 33k. Especially the resistor of 68k should be a 1 watt resistor (or two resistors of 33k 0.5 W in series).
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