Knox Studio 40.ND Character Generator Trouble

[uxwbill]

Originally Posted by Chip Chester

Found a couple of links you may have run across:

I'd run across one, but not the other. I also found some auctions for the earlier beige model with the built in keyboard. (The pictures were readable enough to figure out what the major function keys were by process of elimination.) The only info I had was the sticker on top of the unit itself, which isn't helpful if you don't have the specially labeled keyboard. (The Studio 40.ND uses a separate, electrically compatible with the IBM AT, keyboard.)

I thought sure that someone would have scanned the documentation or at least the schematic for this unit. Apart from people who are selling them, not much information ever came up. Makes me wonder how many of them Knox Video actually sold...

Of the sellers I asked about copying that information from their unit, none responded.

Originally Posted by Chip Chester

Also, there are two adjustment pots (with hex nuts) tucked up high on the back panel... you may have a go at those... after marking their current position fairly accurately. Best to do once you get video scope going.

Those are some kind of video centering/position adjuster.

Originally Posted by NJRoadfan

If an S-Video cable doesn't resolve the problem, I would start tapping the chroma signal throughout the unit. The first place to check would be at the input pin of the Harris chip.

I finally managed to find an S-video cable after much looking around. Funny thing is, I know I had some Apple Desktop Bus cables hanging around and now they seem to be gone.

I fed the unit video through its S-Video input and there was really no improvement at all. Colors were still wrong, though I didn't notice the "beating" effect nearly as much. I also found that it was possible to get a correct rendition of color bars by tweaking the trimmer in the 3.579 MHz oscillator circuit for the chroma processor. Yet this was very unstable and normal video gained two rainbow colored bands running through it. At no point could I get correct color from anything other than the color bars.

So then I turned my attention to picking up the chroma signal from the input pin to the CA3126E after removing it from its socket. I moved the S-Video cable to the main output, and cut into it so I could tap the chroma line while leaving luminance in place. Then I took the chroma line and touched it to the input pin on the IC socket.

Not having an S-Video capable monitor, I set a Sony DCR-TRV27 Handycam to accept video input. I hooked up another Handycam for video input to the Knox unit.

I didn't get a very strong rendition of color when trying to pick up the signal at the IC socket, but I figured that maybe the resistor in series with the chroma input was attenuating the signal too much for the Handycam to really pick it up. So I moved backwards in the circuit and picked the chroma signal up before the resistor and cap that are normally in series with the chroma input.

At that point I had good, if a little bright and washed out, color on the Handycam display. Test video here.



I think I'm finally comfortable with the conclusion that the IC has gone bad.