Video Problem need help
 

I have noticed a video problem in the restoration of the Hallicrafter 820 I just completed

I do not know the technical name for this phenomonea so I will just have to describe it.

There is a horizontal band in the picture. It is about 1.5 to 2 inches tall and stretches across the entire width of the screen. The band is slightly darker than the rest of the picture. It causes a slight horizontal shift of the picture to the right. The band starts a the bottom of the screen and slowly drifts up the screen. When it reaches the top of the screen another band starts at the bottom just before the upper band disappears off the top of the screen. It takes approximately 6 seconds for the band to drift from the bottom to the top of the screen.

If you look at the photo of the Outer Limits still on the picture below, you can see the band across the forhead of the alien on the screen.

Does anyone know what this problem is called and what causes it? And even better how do you get rid of this anoying drifting band?

Thanks in advance for your help. I really enjoy being part of this group. I am learning a lot by reading posts here.

http://audiokarma.org/forums/attachm...5&d=1165966935

It kinda looks like a hum bar to me. Lots of things could cause it, but some of the most likely things would be bad filter capacitors in the power supply, or heater-to-cathode leakage in a tube.

Nice Hallicrafters :)

John

I was going to say electrical interference? Because a Philips-sagnavox colour TV in my grandma's room at her house does the same thing whenever the dryer is on...

The dryer syndrome sound like some voltage on the neutral side of the AC. Measure neutral to ground and see if anything is floating around. Perhaps a flipped grounding adaptor or an AC strip missing it's ground plug and flipped also. It's happened to me.

Dave A

ohohyodafarted? c'mon....!!

Thanks John for the tips. The set was totally recaped including rebuilding the cans, so I think it is not so likely the problem would be there, unless I got a bad cap from my supplier.

I will check tubes and see if I can find something.

Checking the tubes may not show heater-cathode leakage. I would check by substitution if possible.

In the process of starting to trouble shoot this issue, I inadvertantly discovered the problem.

I have a dvd player which feeds an RF modulator so I can feed a 75 ohm to 300 ohm adapter to feed the antenna on the set.

I had the set running and I was about to remove the back of the set. So I disconnected the output of the RF modulater and, miracle of miracles, the picture interference disappeared. All over the air signals look just fine, using only a rabit ears antenna. I have a 75 ohm antenna switch to switch from DVD to Rabbit ears, but when I have rabbit ears switched in (to watch over the air) I will get the interference bar only if the cable from the modulator is actually connected tot he switch. If I disconnect the cable connected from the antenna switch to the modulator, and leave only the rabbit ears connected, then no interference in the off air picture.

At least there is no problem with the tv set. Don't have any idea if the interference from the modulator can be eliminated. I wonder if the signal from Our local chanel 4 vhf station, and the modulator which is set to chanel 3 may be causing some sort of interference with each other.

I am very close to the Chanel 4 tower and the signal from that station is VERY strong. Possible signal overload from chanel 4 and the chanel 3 modulator???? I guess I may have to just live with because, other than disconnecting the modulator physically, I have no idea what to do. And then to watch a dvd (as shown in the photo of the Outer limits image) there is not much else because I have to connect the modulator to feed the set with a dvd signal.

I do not know about modulators but two channels next to each other on the dial, one very strong/one very weak can definitely cause issues on older TV sets. When I was young, we had two channels next to each other. One, channel 9 was 80 miles away and weak. The other, channel 10, was 30 miles away and very strong. There was interference from 10 onto 9 all the time on our TV and other TVs of the day (1960/1970s).

My mother still lives an the same area, but now has Dish Network and a modern TV, so no problems. Plus ,the strong channel has re-directed their signal away from her area, so channel 10 is much weaker now.

I described this exact condition in another post. Photo here:

http://audiokarma.org/forums/attachm...0&d=1162180974

With the lighter zone (from the girl's collarbone to her cheekbone) starting at the bottom and slowly walking up the screen, just as you describe. I attributed it to the Ramsey VHF broadcaster kit I was using, as all of the TVs in my house were doing the same thing, when tuned to the Ramsey's signal. I never solved it, but knew it wasn't a problem with the set I was working on, so I didn't worry about it.

Hopefully we'll get a good technical explanation...? I'd like to make the problem go away too.

Hum bar
 

That's a 60 cycle hum bar caused by a ground loop, in both cases. Yoda, if you're lucky, your balun (matching transformer) is shorted, and you should be able to verify this with an ohmmeter. It should measure infinite between the 75 - 300 ohm ends. If it's not shorted, then you have poor isolation between your DVD player and your modulator. Kbmuri, your ground loop is probably between the source and the Ramsey. One way to narrow it down is by plugging individual equipment into an isolation transformer. Best to take your ohmmeter with you to Radio Shack if you find you need a new balun.