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 Post subject: VGA to scope converter build
PostPosted: Jun Tue 30, 2020 1:16 pm 
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I am going to build the following VGA to scope converter.

http://www.electronixandmore.com/projec ... index.html

I recently got a Leitch FR-684 that has 7 amp boards. It also has 2 delay boards which have a daughter board.

I plan on just using the ramp generators and building each on one of the two boards by replacing the daughter boards with perfboard and a single OP-AMP. The stock values of timing cap and resistors are for 800 X 600 75Hz refresh rate so I'll try those to see if the text on the Tektronix 606A X-Y monitor is large enough to be readable. I'll put those components in sockets so that I can change them if necessary.

The idea is for the sync signals to come into the board the normal way then be fed to the new daughter board the same way the delay board is fed. The output of the OP-AMP will be fed through a pot (if the board's gain control cannot drop the level enough or is on the input side of the circuit) and connect to where the delay circuit connected so that I have a proper output.

I need some help for the video amp portion.

I have seven video amp boards at my disposal. They are AC coupled on the input, but I can fix that easily by shorting the caps which the manual does say can be done if DC coupling is needed.

Here's the resistor circuit used for making a monochrome video signal.

Attachment:
VGA monochrome.png
VGA monochrome.png [ 3.66 KiB | Viewed 887 times ]


Now those resistors were intended to feed the + input of an OP-AMP which is high impedance.

The video amp is 75 ohms input impedance.

Will those resistors work properly and not cause the signal to be too low?

If not what values will I need or is it entirely possible to just use the green video signal?

What I am thinking of doing is mounting a VGA connector in a metal box with the necessary resistors and mounting three BNC jacks on the other side for connecting to the distribution amp. That way I can use any VGA cable and am not limited to a VGA to BNC cable.

Here's the basic circuit. The 470K resistors were copied from the TV to scope circuit so that the ramp generators keep running when there's no video signal which protects the X/Y display since a raster is always present.

Attachment:
VGA to scope.png
VGA to scope.png [ 14.23 KiB | Viewed 887 times ]


Here's a TV to scope circuit which I only show because I have a question about the ramp generators.

Attachment:
tvscope7.png
tvscope7.png [ 16.03 KiB | Viewed 887 times ]


My question is this.

The VGA to scope converter has a .01uF cap to ground on pin 5.

The TV to scope converter uses a diode from pin 3 to pin 5.

Now is the diode being used as part of the loop to keep the ramp generators running when there's no sync signal?

Also is the refresh rate simply the frequency the vertical ramp generator runs at?


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 Post subject: Re: VGA to scope converter build
PostPosted: Jun Tue 30, 2020 7:40 pm 
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Tube Radio wrote:
The VGA to scope converter has a .01uF cap to ground on pin 5.

The TV to scope converter uses a diode from pin 3 to pin 5.

Now is the diode being used as part of the loop to keep the ramp generators running when there's no sync signal?

Pin 3 is the output. Pin 5 goes directly to the threshold sensor which detects when the ramp capacitor has reached 2/3 Vcc.

When Pin 3 goes high it starts the charge cycle all over again. When Pin 3 goes low because of the diode there is no effect on the circuit.

The timer without a sync pulse would then oscillate at its defined RC time constant rate.


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 Post subject: Re: VGA to scope converter build
PostPosted: Jun Tue 30, 2020 8:34 pm 
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Location: Warner Robins, GA
OK thanks for the explanation. I'll modify the circuit to include the diode and remove the capacitor.

I suspect this circuit will be better than what I could have ever achieved building the whole thing from scratch.

Basically all I have to worry about is two ramp generators and two OP-AMP buffers.

The board with the daughter board has a + and - 5 volt regulator already so the power supply is taken care of. That gives me the +5 for the ramp generator and +/- 5 for the buffer.

I need to find a schematic for the board with the daughter board on it so I can see where in the circuit the daughter board is connected.

EDIT:

Here's the diagram of the board with daughter board. The module in the dotted square is a delay module on mine. I was afraid the gain control was on the input with the delay module somewhere in the middle. That complicates things I think IF the feedback loop with the clamp circuit in it is still a feedback loop with it set to no clamp. If set to no clamp makes it to where there is no feedback loop at all then I can do what I want.

Attachment:
1 Diagram.png
1 Diagram.png [ 42.1 KiB | Viewed 844 times ]



Now fore the video it looks like I may want a VPD-683-CLP as it has an adjustable luminance clip and hard white clip which will allow me to possibly get an even better display as with the VPD-683-DLY and VEA-681 board if I use Windows photo viewer to view a darker photo and move it to the display the photo gets darker and perhaps the two adjustments will allow me to eliminate that.

So I'll only be needing three boards.

I'll have seven empty slots and perhaps I can find a use for the other boards later on.

EDIT:

Now each output is 75 ohms impedance.

I currently use a pair of VEA-681 boards to buffer the X/Y ramp signals coming from my composite to X/Y adapter.

The only resistors I have are 150 ohm and 100 ohm resistors.

For the boards used to buffer the X/Y ramp signals should I use two 150 ohm resistors in parallel so I have a 75 ohm impedance or will it be ok to use a 100 ohm resistor?

The resistors will go in the Tektronix 606A X/Y monitor on its inputs.

Also concerning the video. If I use a 75 ohm load I need the card used to buffer the video with its gain set to max and I want to say I don't need to adjust the brightness control to provide the same picture. Currently the Z axis input impedance of the X/Y monitor is 1 meg which is how it came stock along with all the other inputs being 1 meg.

I get a good picture as is, but should I use two 150 ohm resistors in parallel since it is a video signal or can I simply use a 100 ohm resistor and not lose any video quality?

I also found a VEA-684 on eBay which is the base model of the card that would normally have a daughter board. Seems like that board is better for the video.

So I will not need any of the VEA-681 cards for this project provided the card I ordered actually works correctly. If it doesn't I might ditch the original circuit and make it be the ramp generator board which would then feed a pair of VEA-681 boards to buffer the ramp signals. I could then use a dual OP-AMP.

Might do that anyway though as it would greatly simplify things given I would not need to attempt to add it to an existing VPD-683-DLY card which based on the block diagram most likely wouldn't have worked anyways. Plus given how I would do it, the board can still be used only it would be driving 6 outputs instead of 8.

With the original plan the sync signals would have gone to differential inputs which would have helped ensure no ground loops. With the modified board I would not have the differential inputs for the sync signals which may or may not be a problem.

Plus given the whole circuit would be on one board I can simply breadboard it to get the circuit working correctly before putting it on a piece of perfboard.

So for the VGA connector I can either use

This one which is small.

https://www.mouser.com/ProductDetail/Am ... VQYg%3D%3D

OR

This one which will make soldering the resistors and wires much easier.

https://www.mouser.com/ProductDetail/Gr ... TiLg%3D%3D


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 Post subject: Re: VGA to scope converter build
PostPosted: Jul Thu 02, 2020 3:49 pm 
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Location: Warner Robins, GA
So doing some thinking here it looks like maybe I can use the VEA-684 for the video then add the daughter card which contains the ramp generators to it.

The card will then only be able to drive four outputs though and I would still only need three cards, but I only need one output though as I only plan on driving one display.

I may still look into doing the separate ramp generators though as I'll check the cards to see if I can insert the ramp generator circuit where I want it to be.

Here's some photos

These two photos hopefully show one reason why I am not wanting to use the original ramp generator which uses transistors. If you'll notice the text in the second photo is taller due to the scan lines being spaced farther apart which is a vertical linearity issue.

Attachment:
1 normal.jpg
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Attachment:
1 inverted.jpg
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This is a mostly white display.

Attachment:
2.jpg
2.jpg [ 634.76 KiB | Viewed 778 times ]


This is a mostly dark display.

Attachment:
3.jpg
3.jpg [ 589.03 KiB | Viewed 778 times ]


This one shows why I want to make it VGA to X/Y and not composite to X/Y as the display isn't too good at 800 X 600 with a composite video signal.

Attachment:
4 800 X 600.jpg
4 800 X 600.jpg [ 605.02 KiB | Viewed 778 times ]


Also I installed 75 ohm resistors on all the inputs.

Thought that might be an issue with the video amp and that it might not have enough output, but it worked just fine as I did not need the gain control set to full for the brightest display.

Doing that eliminated pretty much all of the issue I thought was due to a ground loop.

I'm going to perhaps breadoard the circuit first and experiment with 800 X 600 and 640 X 480 resolutions to see which is the best. I'd like 800 X 600 as more can fit on the screen. Just gotta see how small it makes the text.

EDIT:

Now the big issue is this.

The X/Y monitor has an aspect ratio of 5:4 which means I technically can only use 1280 X 1024 and a couple much higher resolutions.

The only reason I can get away with 800 X 600 and 640 X 480 is because the VGA to composite converter converts most resolutions to SD NTSC video.

EDIT:

Thinking about it I cannot really get away with those resolutions as NTSC composite is 4:3 but my display is 5:4.

All I can do is try one of the resolutions and see what works best.

At first I thought finding the horizontal sync frequency for the various resolutions was going to be hard (the vertical is easier as the refresh rate I believe is the vertical sync frequency), but I can simply get a VGA splitter and a VGA to BNC cable then connect the H sync to my frequency counter which should give me the frequency.

I then have to select a cap and resistor value that will make the oscillator run at the correct frequency with it being able to be adjusted properly by the pot.

Now if I knew how to program microcontrollers I could make a simple program that takes the sync signals and adjusts the ramp generators to the proper frequency.

That said is there any sort of simple circuit that would take the sync pulses of any VGA H and V sync frequency and turn it into a proper ramp? If so that would simplify things as I would only need to select the resolution from within Windows.

One option I thought of was to find a function generator kit that can accept a trigger signal, but that was before getting the video distribution box and that basically rules out any sort of function generator kit as I want everything to be kept in the box.

EDIT:

Would a circuit like this work for a ramp generator or will component values have to be changed based on the desired operating frequency? B+ will be 5 Vdc.

https://circuitdigest.com/tutorial/boot ... ransistors

Image

Also what component values should I use? I can breadboard it Tuesday at work and use a function generator set to square wave or pulse to trigger the circuit and see if it indeed does work over a wide range of frequencies without changing any components.


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 Post subject: Re: VGA to scope converter build
PostPosted: Jul Tue 07, 2020 9:07 pm 
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Location: Warner Robins, GA
The VEA-684 came in today and it works.

Only issue is the board by itself doesn't have quite enough gain so I cannot use it as the video amp which means I cannot have both ramp generators and the video amp on one board.

So it will become one of the ramp generators provided setting the clamp mode jumper to no clamp removes the feedback loop.

An alternative is to take one of the inputs and rewire it to the ramp generator input then wire its output to the board's other input which can be done by soldering wires to the terminals where the coupling jumpers are. I then lose the differential input though.

I may hold off until I get a schematic of the board, although I could use a dual OP-AMP and wire one section up as a differential input buffer with the other section being the ramp generator buffer. A 75 ohm resistor on the input buffer would keep the impedance at 75 ohms.

Only issue is that the ramp signals will be inverted if I go with the 555 timer circuit, but that will be fine as I can just swap the cables to the - X and Y inputs of my X/Y monitor.


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 Post subject: Re: VGA to scope converter build
PostPosted: Jul Wed 15, 2020 4:19 am 
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Location: Warner Robins, GA
I've been trying to think of a way to control the brightness while keeping the contrast the same.

The distribution amp does have +469mVdc on its output and I can buy a multi turn 75 ohm attenuator so perhaps I could use that on the output feeding the Z axis input of the X/Y monitor or would that not work like I'm thinking it will?

Now if I can get some values for the circuit I posted I can breadboard it to see if it will work.

I could either use the actual VGA sync signals or a function generator set to output pulses at the pulse width of the VGA sync signals then vary the frequency to see if it will work properly.

I'd still need the OP-AMP buffer though.

I'd also need some sort of way to have the circuit generate a ramp in the absence of sync pulses so that the raster is still present when there's no VGA input to keep the CRT from being damaged.

EDIT:

Found this dual OP-AMP board which I could maybe use as the input and output buffer or maybe figure how to make it a ramp generator.

https://learn.sparkfun.com/tutorials/ts ... e-overview

Whatever ramp generator I make I need to put a 75 ohm load on the output so that I can be sure it will still work properly.


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