One for the boffins!

[murphyv310]

No!!!!!! A Pussy Cat. Ha ha no wonder its called the "Cat Queens Site! He He

[Dave Moll]

John - you're nearly four weeks late with that one!

[murphyv310]

Hi.
Now I am really messing about! Here are some pics using the Tek scope. They are different line standards US 441 lines and some pictures of the frame rate, No wonder the image is not linear!

Hi.
I will have a look and see if it is possible to create an image from the 180 & 240 line standards, certainly 405, 625 & 819 work, 525 & US 441 are not as clear so this backs up my theory that the bend in the waveform is due to loading of the the video signal on the power supply.
I forgot to post the waveform of the supply rail but I will do that tomorrow!
If anyone wants the circuit of my little 3 transistor video amp let me know and I will draw it out so someone else can have a go!

[HamishBoxer]

If i didnt know better,i would say this was April 1st!

David

PS All very intresting Trevor.

[neon indicator]

I think it can be maybe done with just passive components ...

[murphyv310]

You could well be right about using passive components, even a diode across the video waveform may clip it enough, not too sure though if you could set up some "locked" ripple.
I don't know about you Neon' but this has been interesting!

Pity David its not the 1st of April!

[neon indicator]

Originally Posted by neon indicator

I think it can be maybe done with just passive components ...

Well maybe 2 x diodes needed.

[murphyv310]

Hi.
What do you think "Neon" about the theory?

[neon indicator]

The only reasonable explanation is a discontinuity in frequency response or incorrect biasing such that a coupling capacitor is forming an integrator (ramp generator) and getting reset by the field sync pulses. It's the only kind of composite signal that can give a pseudo raster image on the Scope. I'm thinking the intensity variation is due to edges, not the absolute luminance (i.e. vertical writing speed vs horizontal sweep). Thus certain logos and test cards will be clear, but "natural" images are converted to an "embossed" effect.

I've been connecting 'Scopes to video signals since before my late 1970s BBC days and I never saw this effect before. It's interesting that to generate it and see it you only need a single channel scope Y input. If you generate a ramp from the field signal and add it to video, and trigger at line rate you get a lovely embossed effect. You can offset it from "main" trace by DC offset of ramp and making the "base" part brighter according to flat part of ramp past the normal blanking I think...

[murphyv310]

I certainly agree that a fault condition in the video amp or associated circuitry is causing the effect. What got me in the Relay Station all those years ago was the clarity of the image, the caption was on screen for 10 to 20 seconds, so it was a mad dash to get the polaroid scope camera and snap it! I doubt the photo would have survived 40 years anyway. Without a doubt it was a freak occurrence but with better skills now and time I have thought about it carefully and have re-created it.
I was only 16 at the time and a first year apprentice but it has niggled away for years.
I am sending off the photos to my old District Engineer (76 now) with a little explanation and some of the replies from here, it will be interesting to hear from him as he too was baffled at the time!

[brianc]

As mentioned before, I have seen this effect while working at the BBC. This was when working on Marconi MkIII Image Orthicon camera CCUs or racks. It appeared when there was severe field shading so that successive line waveforms were written, each one slightly displaced from the previous trace. The apparent Z modulation is, as neon indicator says, due to the actual beam writing speed, the slower the speed (no vertical movement) the brighter the trace. Any circuitry that can introduce a field rate level or brightness distortion has a very good chance of causing this effect. Of course, clipping the video means that there are more "straight" portions of the waveform giving more bright areas. For interest's sake, the basic theory could be checked by adding the distortion so that the shading was from black to white ie starting the field at a lower level. This would give an inverted "picture" if it was beam writing speed which gives the visual effect

[neon indicator]

yes.
https://www.vintage-radio.net/forum/...0&postcount=30



Attachment 50164

Black case with white writing, plain white meters with black scales.

you can see that it creates an embossed effect with negative AND positive at edges.

[murphyv310]

Hi Brian.
Many thanks for the input, I am certainly going to play about with this some more as practical things like this I find makes you think about what is happening, I agree it is certainly velocity modulation of the beam, there is no connection to the CRT's "Z" input.
I wonder though how I can invert the signal? I could feed in the video, negative going and see the difference if that is what you mean.
All this though means I am not getting on with the V114 now I have the parts, (thanks).

[murphyv310]

Originally Posted by neon indicator

yes.
https://www.vintage-radio.net/forum/...0&postcount=30



Attachment 50164

Black case with white writing, plain white meters with black scales.

Can you drive your video amp into overload and make the video clip? I think then the image will pull up and it will be a lot clearer!

[neon indicator]

Here is original object

[murphyv310]

Hi.
Here is your PSU in a new light!

[murphyv310]

Hi.
I got a really nice call from my old "district Engineer" today (see post 50) And he still has the old polaroid photo from the early 70's along with some photo's of myself, Oh Dear!
He is going to get them to me so I will post the original if it is clear enough.