One for the boffins!

[murphyv310]

Hi Neon, Give me some credit please regarding the probe earth!

The field rate is some what bent and I think this is where the answer lies.
The output from the RT Russell test card generator is being fed to a two transistor amp with an emitter follower, nothing fancy three BC108's
When the amp is fed from 12v I can not get the effect but if I feed the amp from 12v via a 1k resistor I get the bending on field rate and the image on the scope, if fed direct from a 3v battery no bending or image just a crushed video waveform as I get with 3V fed from the 12v supply via the resistor!
So is it due to the load in the amp varying with the 50hz frame rate of the signal and modulating the scope, and is the good image due to the near inversion of the whites and deep black level?
One thing though why is the image so far up?
When this happened in the 70's it is possible there was a fault on the monitor giving me the image remember too this was on broadcast quality equipment.

The result with hum induced resulted in a indistinct image, the result now is very clear.
What do you think?


PS, Neon. The Scope I am using is a Hitachi V-212 but I will dig out the Tek later and do the same, I would assume that the result would be the same if not better!

[ppppenguin]

Here's what I'm getting.

Test card C to input 1. Video is about half screen height.
Line rate sawtooth to input 2. Sawtooth is full screen height.
Traces added.
2ms/cm, locked to TV field.
Nothing is overloading anywhere.

The photo does no justice to the display which is a lot clearer. Reducing the gain of the TCC channel reduces the "ghosting" effect. Changing the gain of the sawtooth changes the vertical (wrt scope screen) magnification.

[murphyv310]

Hi Jeffrey.
Thanks for the picture! Do you think then there is some modulating effect from the amp I am using as the supply rather than from a regulated 12 v is now one that "floats" via the resistor and the varying load of the video signal is causing the effect?

[Red to black]

Hi Trevor,

I have been following this thread with interest.

Why not try scoping (with the second input of your scope) the supply at the resistor/amp junction ?, then you will see if there is any modulation of the supply causing this interesting and unusual effect.

Just a thought.

Cheers,
Baz

[neon indicator]

Originally Posted by murphyv310

Hi Neon, Give me some credit please regarding the probe earth!

The field rate is some what bent and I think this is where the answer lies.

I can't duplicate on my Hameg 203e, though if I have mains hum on "earth" connection and scope at line rate I can nearly get a picture.

[murphyv310]

Hi Baz.
Well done! I must admit I actually stumbled on getting such a clear image by being lazy, I had fed my little amp via the 1k resistor from 12v. I thought when I got the image that I had hum getting in so Scoped the supply, perfect fromm the 12 source and it should be as my 12v supply here is two 12 leisure batteries but with a video input to the amp there was a bad ripple, easily smoothed with a 470uf cap though or a direct 3volt supply. I am now sure this is the reason why it works, that along with the clipping enhances the image.

David Boynes reminded me that Clive James had his name on a scope on his show a few years ago and I think this was the same effect.
I really think now this is the same effect as I saw years ago and it is most likely that the monitors video amp would have been fed from a supply that couldn't keep up with the massive increase of gain that happened when I turned up the gain in the monitor, in this case possibly the anode load resistor was high value. If my memory goes correctly it was a cathode follower stage then it was fed to the actual video output stage.
Its 40 years ago and no doubt I will be wrong.

Thats my theory though, would you be agreeing with it?

[murphyv310]

Originally Posted by neon indicator

I can't duplicate on my Hameg 203e, though if I have mains hum on "earth" connection and scope at line rate I can nearly get a picture.

Why not cobble a little video amp together and feed like I have done.
I am going into the loft after tea and will try it with the Tek scope!

Remember I have been trying to do this for 40 years!!

[ppppenguin]

I don't have a field rate sawtooth conveniently available so I tried some mains hum (British Stanard Finger) into channel 2. At 10us/cm and locked to TV line I get a picture.

[Red to black]

Hi Trevor,

I think with the ripple/video waveform on the supply line to the amp,we are getting a sort of crude self locking synchronous demodulation effect (long words for this time of the afternoon), as the amplifier is driven into non-linearity, but Iam just a mender, and cannot fully explain it properly, others are far cleverer and will no doubt explain it much better than I can.
Cheers,
Baz

[neon indicator]

Well, my theory is the 25Hz/50Hz gain is high and at low voltages the Frame signal Modulates the Line signal, or PSU on amp is ramping at Frame rate.?



Colour security camera pointed at the Bench PSU,

Scope Y1 is video at Line Sync rate (width of screen is one line)

Scope Y2 is scan sawtooth from my HP141T at 1ms per division.

Y2 inverted and scope set to Y1+ Y2

It's an interesting Embossed effect.

My ramp is flat for about 9ms and then ramps up for about 11ms, hence partial image and "1ms per div" range on HP141T

So on an video amp, if the frame pulse is overloading the amp (o/p hard off or inverted stage hard on if amp is two inverting amps., it's bias or DC point is ramping down on output(hence your image is correct way up) and this is similar effect to feeding a ramp to Y2 and adding inputs

[neon indicator]

Maybe the original was a valved and had a leaky inter-stage coupling capacitor...

[kalee20]

Originally Posted by neon indicator

...it's bias or DC point is ramping down on output(hence your image is correct way up) and this is similar effect to feeding a ramp to Y2 and adding inputs

That's what I was getting at - something is upsetting the DC level and it is drifting downwards throughout the frame. Something is clearly resetting it each frame (actually each field) and you could well be right that the frame sync pulse is somehow doing this - rather than frame timebase sawtooth getting into the video.

[Ian E G7OLT]

Ironic seeing this after changing my Avatar recently.

Moving sequence on YouTube.

http://www.youtube.com/watch?v=ldxdgWfobTs

--
Ian

[neon indicator]

yes,

but what's interesting is the apparent "intensity" modulation on scope with no Z-axis drive. But interestingly it's edges rather than actual level I think...

A large amount of field tilt and black/white images would give this effect. Each white bit of each line would be displayed a bit further up (or down), the black bits sinking below the bottom of the screen.

[murphyv310]

Originally Posted by Ian E G7OLT

Ironic seeing this after changing my Avatar recently.

Moving sequence on YouTube.

http://www.youtube.com/watch?v=ldxdgWfobTs

--
Ian

Excellent Ian.
But why is "mine" well out of the "busy" part of the waveform?

[neon indicator]

That one on youtube is completely different.

[malvision]

Very intresting thread take a look at this

http://tubetime.us/?p=96

[murphyv310]

Hi.
This is well known and actually you are using the scope as a TV monitor, hence the good pictures. The key is "Z" modulation which if you like is brightness modulation of the scopes beam.
In this instance what I am doing is velocity modulation.

[Heatercathodeshort]

This really is a very strange happening. I pulled my TV22 off the pile and fed it with around a volt of signal input. Advancing the vision limiter gave this image on the scope.
Amazing! J.