TK-31's peculiar spot!
 

This inquiry is directed to those readers who were active video engineers at television studios in the '50s and '60s and who repaired or ran video on RCA-TK31A cameras.

I've run into a peculiar problem on my '31A which I've not seen before on other similar cameras or on TK-30s. If you look at the attached pictures of off-the-screen shots from my picture and waveform monitors, you'll see a conical spot, dead-center in the picture which is not being produced with either the target or beam voltages "up." (Both controls are at zero position.) This is not a dynode spot. It does not move during alignment of the camera either for geometry or true beam alignment. The only control which seems to have an effect on reducing it is the Multiplier (Multi) Focus control which when turned counterclockwise to minimum, blends the spot into the background (out-of-focus across the screen.)

Some considerations: 1. The camera is equipped with an orbiter coil on the orth section (factory installed) which is not active. 2. The spot has been visible on all 5820s, 5820As, 4401s and 6474s I've tested (a total of more than 20 tubes.) 3. The spot was NOT visible on a Bi-Alkali IO I've tested.
Anyone seen this before and if so, what was the solution?

My first thought was that the tube was just a bit gassy, but as you have tested so many (20?) tubes it is unlikely to be a tube problem. Now you say the beam is at zero volts, so no beam current, therefore it is not associated with the front part of the tube. I would check the voltages in the dynode multiplier chain and all the other tube voltages are within the normal range.

The conical shape of the spot is interesting, as it is a picture ( with dimensions) some scanning must be happening of whatever is arriving at the dynode section, I assume the amplitude changes as you adjust the dynode gain control.

If you turn the beam up do you get any sort of a picture?

Best though so far is that one of the electrodes is way off correct voltage and electrons are being dragged of it through the scanning on its way to the dynodes.

does any of this help? or even make sense??

Best

Brian

Yes, the camera puts out an excellent picture except for the bright spot in the middle of the pix. It easily makes it to 600+ lines except for the bialkali version which tops out at 400 lines. I've tried adjustment to g5 and g6 and at any setting setting, except for being out of focus etc., there is no improvement.

Just a wild thought - did this occur suddenly, or was it like this from the first? If from the first, is it possible some coil connection is reversed and producing the wrong polarity field?

Edit: I was thinking alignment coils, but in review I see the adjustments go both negative and positive anyway. So, now thinking the multiplier voltages, especially the multiplier focus, which is apparently at one end of the range (?) Warning: I have never operated an image orthicon, so take these free comments for what they're worth!

This is very odd.

The tube(s) and camera produce a good picture apart from the spot, so all tube voltages, scans, etc must be OK.

The newer Bi-Alkali IO works ok, no spot, resolution a bit down but o/wise OK.

The older soda glass target tubes have the spot which brings me back to my first thought that is is a tube fault, even thought several different tubes have been tried.

next step might be to try a known good tube tested in different camera, and to try your tubes in the other camera.

As an aside:-

I am told that IO tubes, and for that matter other high vacuum devices, have a failure mode were natural helium in the air "seeing the vacuum in the tube" goes though the glass as the helium is so small. The helium then accumulates in the tube.

If anyone can confirm or deny this I would be interested to hear what is said.

I wouldn't be so sure. I would think the multiplier focus should have an optimum value, and get worse on either side. The description says the spot goes "out of focus" (blends across the image) at one end of the adjustment - it seems to me this is the desired condition and should happen near the center of the control range (unless, of course, the picture is degraded).

Question: when showing a picture, does the picture itself (not the spot) get better or worse (dimmer or less contrast) when the spot is prominent or when it's spread out, or does it stay the same?

Certainly possible, assuming you ran the tube in a helium-rich atmosphere. :)

This is the principle behind the helium leak detectors used on high vacuum systems. Helium atoms being able to diffuse through tiny cracks is certainly true. It often happened with early He/Ne laser tubes, but in the other direction, where the He slowly diffused out of the tube (through the epoxy seals), depleting the mixture of Helium until the laser stopped lasing.

A coworker once was able to rejuvenate one of those tubes by placing it in a chamber of pressurized helium for a while.

Some high speed counter (dekatron) tubes that included H2 or He in the gas mix were prone to issues as well.

This is interesting. When we got our first high-def Saticon cameras, we were warned to fire them up for a while at least once every few months to get rid of accumulated gas. I never heard an explanation of where the gas comes from. Also, I'm guessing the cathodes in camera tubes were more susceptible to degradation due to being so small.

Wayne, why does high-def Saticon sound like an oxymoron? Now back to our regular thread.

Frankly, it's been quite a while since I got this camera working and I don't remember whether or not the spot was there from the beginning, but it sure has been there for several months now. I might suggest, that if any of you are in the Los Angeles area, you'd be welcome to come over and take a look. Just let me know and I'll send you directions, etc. Write to: ralphsargent@earthlink.net

(PS: I have noticed that a number of these tubes improve their quality of picture when they have been run daily (for several hours) over a period of days or weeks since their long hibernation!)

:D

Actually, special models with extra-fine spot size. As a result, the expected life was considerably shorter than the standard-def ones. We ran our first set for much longer than the expected life and replaced them eventually just out of caution, but kept the original set as emergency spares. The replacement set cost $85,000. (The camera originally cost $800k, which represented Zenith fully funding the engineering charges. AT&T bought a second one.) They were straining to get sufficient bandwidth without excessive noise. BTS limited the bandwidth in their cameras to about 22 MHz, where something more like 33 MHz was needed for full resolution. We had to wide-band ours for the proposed 1280x720 progressive format, and then we had a huge shielding problem because the BTS cameras used a 27 MHz internal clock for the control system. Sony supplied the Saticons to BTS, but Sony had proprietary low-noise FETs for their cameras and insisted BTS remove them and use BTS preamps. The preamps were soldered directly to the front of the Saticon, so BTS had a critical desoldering and soldering job to avoid damage.

And now back to the original thread again! I still wonder if the multiplier focus voltage isn't off towards one end.

Dave: You had the same idea I did. Yesterday I went through the complete dynode divider chain and here are the results I found:

Typical voltages suggested by the datasheet:

Grid #1 and 1st dynode 300 vdc
2nd dynode 600 vdc
3rd dynode 800 vdc
4th dynode 1000 vdc
5th dynode 1200 vdc
Anode Voltage 1250 vdc

Voltages measured after changing a few off value resistors due to aging:

Grid #1 and 1st dynode 68 vdc
2nd dynode 490 vdc
3rd dynode 613 vdc
4th dynode 987 vdc
5th dynode 1160 vdc
Anode Voltage 1187 vdc

Not bad, but not terrible either. Any other guesses or opinions? The white spot is still there!

Those voltages are terrible. Photomultipliers need to have the voltage ratios
be as speced.

As to helium gas, yes, it really does diffuse through glass.
But its important to note that the effect is hugely different for different
glasses. Its least in plain soda-line glass, worse in Pyrex, and absolutely
terrible in fused quartz.

Correction: Grid 1 and Dynode 1 voltage is 287v. Dynode 3 is "Orth Gain" which is set by a 1 meg pot and is therefore adjustable. Dynode 2's voltage is divided off of Dynode 3 and therefore is variable depending on the setting of Dynode 3.

Also remember: This is NOT a photo multiplier as might be used in a scintilation counter. It simply a low noise variable amplifier with adjustable gain!