Predicta Holiday - First power up attempt

[madlabs]

Been doing some more tinkering. If I crank the video modulation of the BT up to the point where I start hearing over-modulation noise it gets much better. So maybe something in the tuner or IF? Moving the channel selector switch doesn't do it.

So I think I'll go through the tuner and IF tubes again. Last time I went through I wasn't testing looking to see if I could lower the video modulation as I went, I was looking for best picture.

EDIT: Playing with it more, one thing I should have included is that the BT modulator vdeo is well below where the red LED starts to flash but I still get over modulation hum. Right now I have it set to where the hum is bearable and the picture is pretty stable, no rolling and a small amount of hum.

[Electronic M]

The vertical bounce in your video is sync separator, vertical integrator, or vertical stage related (might also be 60hz line freq leaking into said stages).

Originally Posted by madlabs

Been doing some more tinkering. If I crank the video modulation of the BT up to the point where I start hearing over-modulation noise it gets much better. So maybe something in the tuner or IF? Moving the channel selector switch doesn't do it.

So "it" is the vertical bounce? and the hum/over modulation noise is in the audio, right? How do other working sets do with the same signal?

Originally Posted by madlabs

EDIT: Playing with it more, one thing I should have included is that the BT modulator vdeo is well below where the red LED starts to flash but I still get over modulation hum. Right now I have it set to where the hum is bearable and the picture is pretty stable, no rolling and a small amount of hum.

Some BTs have an 'aural level' control for the sound carrier level that also should be tweaked for best sound (and can cause hum if badly set).

[WISCOJIM]

Originally Posted by madlabs

Hmmm. The link works for me. This is the first time I've ever uploaded a video to youtube so I may be doing something wrong.

Just checked now, link works fine. Earlier it acted like a private video.

.

[madlabs]

Tom, I don't have another TV to try it with. I should look for a cheapie at the thrift store.

Yes, "it" is the vertical jiggle and the over modulation buzz is in the audio. The red over mod LED is not coming on. The instructions say to crank it until it is flashing red then back off until it only occasionally blinks. If I do that I get way to much buzz to deal with. So I am setting it where the jiggle isn't too bad and the buzz is tolerable.

[madlabs]

Duh! I remembered that the LCD screen I use for my computer is actually a TV and to my surprise is does indeed have a cable input. Hooked up the BT modulator and it looks fine and I don't get the over modulation buzz. Didn't have time to really fiddle with it and see how low I could set the modulation and still get a decent picture on the LCD.

So, the question becomes: Why do I get over mod buzz at lower mod levels on the Predicta? I'll puzzle on that while I'm at work for 48 hours.

[bandersen]

Have you tried adjusting the buzz control in the Predicta?

[madlabs]

Yes. When I had the PCB board off to replace the audio adjustment coil A11/12 I even removed the buzz pot from the board, cleaned and tested it. It makes no difference to the over mod hum. In fact it doesn't seem to do all that much except that there is a sweet spot where the audio volume is the loudest.