National WV-341N single-tube monochrome video camera

I'm pretty sure that this is the second video camera that I bought in the 1980s (secondhand, it is a 1970s era camera).  This one's a mains powered industrial studio camera.  “Industrial” being a grade of technology between home user, and broadcasting, both in signal and equipment build quality.  And “studio” generally refers to indoor use, and in combination with other equipment.  Originally it had a dull green/grey paint job, but it was badly worn, so it got resprayed with white car paint.

These cameras need a lot of light to get a decent image out of them, almost daylight levels.

A few of its features:

• Rear panel controlled “back focus”, which has a cable drive to move the video camera pick-up tube physically inside the cabinet.  This makes it easy to adjust the camera for different lenses.

• Runs from internal (random) or external sync.  In internal mode the camera is a completely stand-alone device, however random sync can be too crude for many video recorders and some monitors.  Random sync means that there's no correlation between horizontal and vertical sync.  Normally, vertical sync is divided down from horizontal sync with a precise relationship.

• A “line view” switch, which allows you to see a signal coming back from a mixer or recorder, or the signal going out from the camera.  Though this signal has to be in sync with the camera, because the viewfinder monitor doesn't have separate sync circuitry.  It's a bit crude, since the CCJ cable carries it on an unshielded wire to the camera.

• Side mounted camera tube beam focus, beam current, and target voltage, controls.

• It uses a ⅔″ vidicon tube, so produces a better picture than many other cameras which were typically using ½″ camera tubes.

Initial set up

The side controls are for setting up the camera tube.  Once set, should normally be left alone.  This focus adjustment is the internal electrical focus of the tube (the electron beam scanning the target), not optical focus (light going through the lens focussing on the target).

For initial set up, put the target control into the auto click-stop position.  Crudely speaking, it's using the target voltage as a video gain control, which can give you a terrible way to adjust the exposure if using a lens without an iris, especially if the scene is too bright for the camera tube.  Afterwards, you'd normally want to set it in one position that gives decent exposure when using the iris to actually control the exposure (note that this target control rotates backwards from the way you'd normally expect).  The auto target voltage position can change the video signal a bit too much with any movement in the picture (it samples the video output to modify the target voltage, made worse by the video signal is AC coupled throughout the entire video signal path).

Nothing is helped by the video signal being AC coupled everywhere, nor the camera tube blanking level being very different from lens-capped black video level.  The one DC clamp is in the middle of the video signal path, and only really does crude retrace blanking (squelching the noise during the blanking period, so to speak), it doesn't set video black level output to a normal spot.  The camera is probably relying on black level being too high so that it can't sink below blanking level in a very contrasty scene.  But removing D1 next to TR3, which is close to the NONCOMP/COMP sync switch on the left-hand circuit board (the one without the FOCUS, BEAM and TARGET pots), and replacing D1 with a trimpot, gives you way to way to crudely set a better (lower) black level (the diode was just setting a fixed voltage level, and at a bad level).  I unsoldered one leg of the diode and bent it out of the way, then soldered a plastic bodied multiturn trimpot on the track-side of the board.  There's enough of a gap between the board and the side-panel to fit it there.  You're still hamstrung by the AC coupling of the video signal whenever there's a sudden change in scene brightness (put your hand over the lens and take it away, to see), but I don't feel like redesigning the whole camera when it's little more than a novelty item these days.

Aim the camera at something and roughly set up the lens to see it (exposure, focus, zoom).  If you can't get a good image, don't worry about it until after this set-up process.

Bring up the beam current until a picture appears.  If it's too low, bright objects will flare badly.  If it's too high, the tube will wear out prematurely.  Adjust the current until bright objects stop flaring.  I'm talking about bright objects that are properly exposed, not over-exposed highlights (they'll always be a problem).  It's a balance between allowing enough beam current to recharge the target when bright objects hit it, and disallowing too much current when much too bright objects hit it.  You'll want to play around with exposure and lighting while adjusting this.

Adjust the electrical focus to get the sharpest possible picture.  Alternate between adjusting electrical and optical focus.  You'll notice that beam current also affects the electrical focus, and that the electrical focus twists the image.  The target control will also affect beam current and focus, but to a much smaller degree.

Now these controls are set, you should not need to adjust them again.  I used to fit a cardboard cover over these, slotted into the hole they protrude through the metal side panel, to prevent them getting accidentally maladjusted if something rubbed against the camera.

The back-panel focus changes the position of the camera tube inside the camera, racking it back and forth relative to the lens.  This allows you to match different lenses to the camera, or work with fixed lenses without focus controls, or do macro photography with something that's too close to the lens.  To set it once to match a zoom lens to the camera, zoom out wide and adjust the back-focus for a sharp image, zoom fully in and set the front focus (on the lens).  You may want to go through that process two or three times for more precision.  Ideally, to get it right, you want to aim at something very far away from the camera, and with the iris wide open (for minimum depth of field), as wide open as is possible considering the light levels the camera is working in (you want a normal picture to view, not an overblown overexposed shot).  After you've set back-focus correctly the focus on a zoom lens will work properly, and you shouldn't need to touch the back focus again.

Zoom into your target object, focus, zoom out to your desired show–the object will stay in focus throughout the whole zoom range, unless something moves and changes the distance between it and the camera (focus is distance-related).  If the image goes in and out of focus while adjusting the zoom on a zoom lens, you have the back-focus set wrong.  A varifocal lens will actually always work like that—its focus doesn't stay the same when you change its magnification—it's a much more basic type of lens.

Simple maintainence

Tube cameras are damaged by strong light, so maintaining normal exposure when in use, and keeping the lens capped when not, is required to avoid damaging the tube.  Or, if removing the lens, capping the opening that the lens is screwed into.  The tube is susceptible to damage whether the camera is powered on or off, and in a fraction of a second if bright enough.  Tube cameras should not be left unattended and unobserved when filming, especially outdoors.

The front of the tube is glass, and can be cleaned without needing any specialist liquids.  However, lens cleaning fluid may be the best choice (as it's designed to not leave a mess on optical surfaces).  Pure alcohol is another option, bearing in mind that many cleaning alcohols are deliberately contaminated with agents that can leave a mess behind.  Do not press hard.  Do this with the camera off (several sections of the camera tube are run at few hundred volts), and do not allow very bright light to hit the target.  Indoors under normal room lighting should be safe.

Under bright lighting, and small iris settings, dirt on the front of the camera tube can be seen as dark spots on the picture.

Connectors

On the back is a B-gauge ¼″ TRS (tip, ring & sleeve) socket for a carbon mike headset (tip is mike, ring is ear, sleeve is common).  This connects directly to two pins in the 10-pin CCJ connector underneath the camera (with a baristor across tip and sleeve).

Underneath is a UHF/SO-239 composite video out connector.

And a 10-pin CCJ connector underneath the camera.

1. Video out signal
2. Video out ground
3. Vertical sync in (50 Hz, 4 volts p-p)
4. Vertical & horizontal sync ground
5. Horizontal sync in (15,625 Hz, 4 volts p-p)
6. Tally lamp (24 volt bulbs wired directly to pins 6 & 10)
7. Intercom audio (to ¼″ socket tip)
8. Intercom audio (to ¼″ socket ring)
9. Line view video input (it's an unshielded wire in the CCJ cable)
10. Tally lamp (24 volt bulb wired directly to pins 6 & 10)

There is an internal switch for the pin 1 video out signal, it can either be composite video, or video without sync pulses.  Either way, the camera video is AC coupled internally, and the black level drifts up and down along with average picture level, there is no DC clamp.  Well, there's one transitor in the middle of a chain of AC coupled amplifier circuits designated as being a clamp, where it really can't work as a DC voltage clamp (either it's just retrace blanking, or some kind of limiter).

Do not treat any ground pin as a common ground for any signal, only the horizontal and vertical sync signals share a common ground pin.

Two 24 volt tally lamps are directly wired across pins 6 & 10, the top and rear tally lamps, in parallel.  The vision mixer these were commonly used with sent out a 24 AC voltage when the camera was selected.  Because they're directly wired to those pins, it is possible to change them for different voltages, or replace them with LEDs.

The intercom line is wired directly to the tip and ring of the ¼″ TRS jack, with a 3 volt (if I recall correctly) baristor from tip to sleeve.  The intercom is totally isolated from the rest of the camera, ground, and the chassis (the sleeve is mounted in an insulator).  The intercom headset has a 35Ω carbon mike between tip and sleeve, and a 160Ω earpiece between ring and sleeve, and is powered from the remote end (typically a vision mixer with in-built intercom).  Inside the remote end, is a low voltage DC power source going through an inductor, or transformer, with the headset mike and earpiece in series with it.  The various cameras could be coupled together by the transformers being linked together, or a resistive matrix if it's just an inductor.  It's an old-school telephone type of system.

The camera is directly mains powered via a mains lead going into the bottom of the camera.