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#1
07-13-2020, 12:05 AM
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What kind of interference is this?
Aside from the obvious horizontal pulling issue, is the apparently diagonal pattern of little dots shown in the below video what they call a 4.5 mc beat pattern? I'm not able to find much information or examples that match by googling.
I have the signal coming in from an off the shelf converter into the antenna terminals. TV is powered from an isolation transformer. Not sure where this is coming from. I checked the heater to cathode resistances on the tubes, other than the audio and tuner, and no shorts found. However on a couple of tubes there are 2 instances of resistances between heater and cathode being 7Ω and 10Ω. Could it be a leaky cap to ground along the filament string? The only suspect I can think of would be a ceramic along that line just before one of the suspicious low resistances. https://www.youtube.com/watch?v=bL0HkjeImIE 
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#2
07-13-2020, 12:45 AM
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4.5 MHz patterns dance in time with the audio. This looks more like some external interference from nearby equipment, perhaps a computer or something with a switch-mode power supply.
Check that you don't have a broken antenna wire / bad signal connection.
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#3
07-13-2020, 12:50 AM
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Looking again, maybe it is varying with the audio (hard to see)? In the youtube video, it seems to be spaced lines of dots. Is this right, or just the way it looks on youtube?
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#4
07-13-2020, 03:16 AM
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OLD TV NUT
Yes, it is evenly spaced lines of dots. Sometimes it's diagonal and sometimes straight across. A little hard to see in the video and depending on how much contrast there is. Of course much easier to see in person. Doesn't look to me like it moves with the sound. It may be the converter box. I'll have to test with my NTSC gen to rule that out. Here's another video you may be able to spot it more toward the middle to end... https://youtu.be/LmjJ_O4L034 Last edited by Jon1967us; 07-13-2020 at 08:13 AM.
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#5
07-13-2020, 10:39 AM
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1) Yes, try another source.
2) look for a device nearby or on the same power line (could be line-conducted interference) - switch things off one at a time. This includes fluorescent and LED lamps.
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#6
07-13-2020, 11:46 AM
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It looks like the converter box could be the culprit. When I run the NTSC box into it, not only is the picture more stable, but I don't see the dot pattern.
https://youtu.be/ujVDmaDpexY https://youtu.be/U_wqz0QPc0k Broadcast signal, once the converter box was moved away from the isolation transformer: https://www.youtube.com/watch?v=g5vzneETsGc https://www.youtube.com/watch?v=oIIx9Rzd29g Last edited by Jon1967us; 07-13-2020 at 01:49 PM.
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Linear Mode
