1. Home
  2. Electronics Repair
  3. Know Studio 40.ND Television Character Generator chroma issues

Know Studio 40.ND Television Character Generator chroma issues

I am trying to troubleshoot chroma issues with a Knox Video Technologies Studio 40.ND television character generator. This device is capable of accepting input from up to two video sources (composite or S-video can be selected by a switch). It appears that all genlocking and synchronization functionality is built into the unit--there is no obvious way to input a sync reference and the unit will function as a text generator only when no video sources are attached.

All of the character generation functions work correctly and appear to be stable. Color of the text and backgrounds appears to work correctly.

Input from either camera connector (tested with S-video and composite input) demonstrates a serious chroma stability problem. The color will shift at random times and at different places in one "frame" of video. The chroma signal from the cameras goes to a Harris CA3126E chroma processor IC. I have removed this IC and tapped the chroma input at its socket, and the signal looks good up to that point. I have also tried another IC and careful adjustment of the chroma oscillator circuit (after marking the position of the trimmer in that circuit). No improvement has resulted.

Tracing the circuit has not been successful as the board is dark and appears to have at least two layers. I called Knox Video and was told the product is "ancient", that they threw away all of the documentation and the person who worked with these is no longer employed by the company. They cautioned against attempting any adjustments on the video board, stating that the controls interact.

I am hoping that someone might have repaired a similar fault or that someone may have a user's manual or schematic for this unit. I'm just about to give up on it.

William

Reply to
William R. Walsh
Loading thread data ...

Do you have ny video test equipment? I.E. Video test generator & Vectorscope? You need a known video signal with a reference output, and the ability to measure the subcarrier phase. Without these, do the usual of checking ESR on the electrolytics, and try a new crystal.

Reply to
Michael A. Terrell

Responses have been inlined.

I have a working color bar generator that I've fed into the unit. (I've found that it's just barely possible to get stable and correct color with the bars by tweaking the trimmer in the color oscillator circuit of the character generator.)

As far as a vectorscope goes, no, I don't have one. It was brought to my attention elsewhere that I could use a VirtualDub plugin and video capture card to provide a vectorscope in software. I'm not sure if I'd understand how to use it. Nor do I know if it would be accurate (enough).

I may try a new crystal and make the effort to replace electrolyic capacitors at some point in the future. Right now I've set it aside as I'm tired of fiddling with it and have no pressing need for it to work.

William

Reply to
William R. Walsh

'Color bar generator' is a vague description. Some don't produce a good subcarrier, since they are used for TV repair. Old TV shop grade video gear is almost worthless. There is a lot of working, broadcast equipment on Ebay for pennies on the dollar. I recently picked up waveform monitor & vectorscope for $20 (Tektronix 1730 & 1720) and a video test generator for for $25 (Tektronix TSG-170A) I bought an Ikegami 9" color studio monitor for $20 a few months ago. All of it in better shape than what I used at the last TV station I worked at as an engineer. Now I need a bunch of quality, new 75 Ohm BNC video cables.

I really don't see how that would help. You have no way to test or calibrate the video board without the proper equipment.

Reply to
Michael A. Terrell

Software based vector scopes are used for correcting digitized video they are unlikely to be helpful in calibrating equipment.

Reply to
nick

I just saw this TSG170A NTSC video test generator for $35.99 plus shipping:

TEKTRONIX-TSG-170A-NTSC-TELEVISION-ANALOG-SIGNAL-GENERATOR-RACKMOUNT-1U-

formatting link

Reply to
Michael A. Terrell

Hi!!

This is a 3M Mincom unit made sometime around 1974. I've found virutally no information about it, but the build quality is outstanding. I'd dare say it was professional/broadcast grade equipment when it was new. (It came with several other pieces, including a video distribution amplifier and a subcarrier distribution amplifier.)

It has a number of options that appear not to be present on this model, including a switch to go between color bars and a black burst.

For a one-off job such as this, I'm not sure that it's worth the acquisition costs, even at pennies on the dollar. I was really hoping that someone might have the owner's manual or a schematic buried in a forgotten file cabinet. I get the feeling after obtaining this unit, that they weren't common or popular. If I better understood what all the circuitry in the unit was trying to accomplish, I really feel that I'd have a good shot at repairing it.

I agree with that observation. This was suggested by someone else.

William

Reply to
William R. Walsh

William R. Walsh forklarede den 14/04/2013:

Would it be possible to use a Raspberry PI, which have both HDMI and composite video output, to generate whatever video signals one would need?

--
Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske  
beslutning at undlade det.
Reply to
Leif Neland

A vectorscope consists of an oscilloscope and a special graticule, nothing more.

Just draw a circle on a piece of clear plastic and mark it with the markings from a protractor or something.

Also, I doubt this will help solve the problem. This is almost always a PLL problem, probably a cap somewhere, but not for sure.

Question, it has two of these chips and they're both doing it ?

Reply to
jurb6006

more.

On what planet? Look at the online manual for the Tektronix 1720 and you'll see that it's nothing like the low grade TV shop crap. A real Vectorscope is synched to the signal via a reference input.

You can get a free PDF of the manual here, if you don't have an account with Tektronix:

Page 71 has a block digram

Here is a photo of the rear panel of that unit:

from a protractor or something.

problem, probably a cap somewhere, but not for sure.

Reply to
Michael A. Terrell

3M had a broadcast & industrial video division into at least the mid '80s.

They sold several base models, configured to the customer's needs. That's what makes finding any documentation so difficult.

It would have to be some TV station with a small engineering budget. Larger stations & production companies tended to use Grass Valley or Tektronix.

Reply to
Michael A. Terrell

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.