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 Post subject: Philco Predicta Princess Horizontal White Lines
PostPosted: Mar Mon 30, 2020 1:15 am 
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I am restoring a Predicta Princess (10L43). I have a good picture, with an anomaly. Separated white lines appear in the upper left quadrant of the picture. They are accompanied by a 60Hz hum, and last about 10 seconds. Then they disappear. They re-appear after about 5 seconds. Please provide suggestions for a solution.


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 Post subject: Re: Philco Predicta Princess Horizontal White Lines
PostPosted: Mar Mon 30, 2020 2:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: San Jose, CA USA
Those are vertical retrace interval lines. Basically when the scan gets to the bottom of the image, it snaps back to the top, but during that time, there are several horizontal scan lines.

In the old days, the video signal was simply blanked (transmitted black) for the entire interval, but more recently, additional "stuff" is being inserted into this time interval, and old TVs don't do a good job of automatically blanking it out. New TVs new how to ignore this stuff.

The fact that you have a 60 Hz buzz with it is because the video signal is fully saturated with black-to-white transitions. Probably you get the same thing when you bring up the channel number or some menu on your set. That is caused by the audio demodulator not being able to fully suppress saturated video, and is really a separate problem.

To get rid of the retrace lines on the screen, either a circuit modification is needed, or a special box to modify the video stream to look like old fashioned video. The video source also matters; certain videotape and DVD systems are particularly prone to having this kind of information encoded into the vertical retrace interval. If you try an off-the-air converter or cable box, you might find the problem is different or even gone.

As far as circuit modifications go, I've done two types that work well:

1. Feed a little signal from the vertical deflection amplifier to grid #1 of the CRT. This works fine if your set happens to have a signal in the vertical system with the right sign and duration. Most sets do, but once in a while this doesn't work very well, no matter how much you experiment.

2. Use a one-shot pulser to apply a nice square black level pulse to the grid of the CRT during the entire vertical retrace interval. I have a Predicta where I implemented that with a tube-based one-shot circuit, and it works perfectly. It would be simpler to build a solid state version.

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 Post subject: Re: Philco Predicta Princess Horizontal White Lines
PostPosted: Mar Mon 30, 2020 3:25 am 
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Location: Pewaukee, WI
I'm guessing you are using VHS or DVD/Blu-ray as your signal source...most tapes and discs have macrovision copy protection on them that prevents VCR from making copies of the original recording...It also drives old TVs bonkers.

Macrovision is easy to distinguish from retrace lines because macrovision will create regularly spaced dashes identical to those shown but retrace lines typically sweep from one side to the other without and breaks in the line.

Get a macrovision scrubber and insert it between your signal source and RF modulator... That should fix it.


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 Post subject: Re: Philco Predicta Princess Horizontal White Lines
PostPosted: Mar Mon 30, 2020 4:23 am 
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Location: Belrose, NSW, Australia
Here is my earlier post on this subject with mine and others solutions:

viewtopic.php?f=3&t=332446&p=2783185&hilit=playing+DVDs+on+vintage+TVs#p2783185

Macrovision also messes with the sound when you use a modulator. That's the buzz you are probably getting.

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 Post subject: Re: Philco Predicta Princess Horizontal White Lines
PostPosted: Mar Mon 30, 2020 9:25 pm 
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Thank you all for your excellent, and timely, advice. I am using a DVD player as my signal source. I am going to try to clean up the modulator signal. I may also try the modifications suggested by Mr. Albrecht. Really appreciate it.


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 Post subject: Re: Philco Predicta Princess Horizontal White Lines
PostPosted: Mar Tue 31, 2020 12:04 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 840
Location: Belrose, NSW, Australia
Removing the macrovision without molesting the rest of the signal (particularly the syncs in the vertical blanking interval, causing loss of interlace and jitter) is non-trivial. Have a look at the circuit in the PDF in my post. I don't believe it could be done properly any simpler than that.

Prior to designing this thing I designed a mono using transistors and fitted it to one of my old TVs. It worked well, but then I discovered that the lines aren't the only problem.

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