Large Black Horizontal Bar

[vts1134]

Howdy gents. I am currently repairing a GE HM-171 (restoration will come after repair). The set had been heavily worked on before, and came to me with some problems. I've been working through and correcting the issues one by one. Everything I've corrected so far I've either seen personally, or read about here on the forums. I've now run across one that I'm not familiar with. When I feed the set an RF signal I get picture, but there is a large black horizontal bar across the middle of the screen (approximately one third to one half of the raster's height). It's almost like some one is censoring out the middle of the picture. I believe the problem must be coming from upstream of the detector as I can get a picture free of the black bar by injecting video there. Has any one seen this before? Any ideas on what it could be?

[N2IXK]

Does the bar stay fixed in the middle of the picture, or does it roll very slowly up the screen?

A rolling black bar is caused by 60 Hz hum getting into the signal path somewhere. The roll is caused by the slight frequency difference between the 60Hz line frequency and the 59.94 Hz vertical scan rate. Look for a bad cap or H-K leakage in an IF or RF stage ahead of the video detector.

A bar that stays fixed across the screen while viewing an NTSC signal would indicate a vertical sweep frequency signal getting into the video somehow. Look for a bad filter or bypass cap on a supply that is common to both the vertical sweep and the RF/IF stages.

[vts1134]

I've seen a 60 cycle hum and this doesn't look anything like what I've seen. I like the vertical sweep theory, I'll sniff around in that direction.

[old_coot88]

Do you still get the black bar with a different RF source?

[vts1134]

What do you mean by different? Different modulator, different DVD, or different video source component? I can tell you that the modulator, DVD, and BluRay player that I am using is known good, or at least was known good 2 weeks ago when I was finishing up another set, but maybe you're asking for another reason .

[Username1]

I think N2IXK hit it! If it don't move its not 60hz, so its coming from vert. inside the set. Cap, leaky or shorted tube. N2IXK for president ! ! ! N2IXK for president ! ! ! N2IXK for president ! ! ! N2IXK for president ! ! ! N2IXK for president ! ! !

[old_coot88]

Originally Posted by Username1

I think N2IXK hit it! If it don't move its not 60hz, so its coming from vert. inside the set.

To prove this, turn the vert hold and see if the pic rolls relative to the black bar.

[zenith2134]

Any progress on the repair/restoration? The HM-171 is among the more interesting sets I have read about.

[vts1134]

Originally Posted by zenith2134

Any progress on the repair/restoration? The HM-171 is among the more interesting sets I have read about.

Sorry no progress. I have put the project on hold while I finish my basement. I need to have a permenent place to display all of my sets. Once I'm finished I'm sure I'll get back to the GE.