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 Post subject: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Thu 20, 2019 2:48 am 
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For old timers like me, this is elementary. But as we drift away from the analog age, the process of fine tuning a pre-digital age TV is worthy to discuss.

Analog TV required a number of analog compression tricks in order to optimse picture resolution despite the limitations on transmission channel bandwidth to successfully deliver it to the home. Analog broadcast television used vestigial sideband am modulation. Essentially almost a complete sideband apart from a vestige was cut in transmission. To compensate at the receiver, the IF response placed the video carrier at about 50% up the pass band slope.

When fine tuning early TVs using intercarrier sound, you therefore tune for best picture detail immediately prior to the sound wormy waves appearing in the picture. For sets employing split sound, you tune for best sound and when if the set is properly aligned, the picture carrier will be at 50% for optimum picture coinciding with best sound.

Of course, RF and more particularly IF alignment must be correct. Split sound sets are more critical in this regard and the alignment is therefore important.

I mention this because I see snapshots of very smeary and soft pictures off screens with little or no picture detail.


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 Post subject: Re: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Thu 20, 2019 4:03 pm 
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Location: Pewaukee, WI
Many modulators don't conform to broadcast standards.

Also not all sets are in alignment, or owned by people who can/will correct the issue.


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 Post subject: Re: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Thu 20, 2019 7:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Lafayette, CO
I use an antenna with a converter..... or ......RCA's old selectavision CED's. Life's too short to figure the details of better and better. Many of these early sets offer a surprisingly good picture. All have a fine tuning control, except some Philcos. Craig


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 Post subject: Re: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Sun 30, 2019 6:02 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Phoenix, Arizona
Normally you can adjust for best sound and that centers the video as well. With the crappy modulators used in set top boxes and various converters you are lucky to get sound.

Some of the early Philco TV's used the audio discriminator voltage to AFC the tuner worked like a charm.

Earlier TV sets were designed for 20% aural power from the TV transmitter. I find some starved for aural signal. My RCA 621TS comes to mind.

Jim


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 Post subject: Re: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Sun 30, 2019 9:34 pm 
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All of the cheap consumer modulators are double sideband AM video. This means that lower video sideband will bleed into the lower adjacent channel. No problem as it will be the only channel (3 or 4).

What is important : an intercarrier set fine tuning (which is the majority since split sound sets were extinct by 1953) must be adjusted for best picture detail. This occurs just before the sound "wormy lines" appear in the video. The reason being that the video carrier must be at 50% on the side slope of the IF response. The sound should be optimum at this point.

Look at the set instructions. They generally tell you to adjust the fine tuning until you see wormy lines in the picture and back off where they just disappear.


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 Post subject: Re: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Sun 30, 2019 10:01 pm 
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I've owned many working early post-war TV's and they typically are very finicky. Sets got better in the early '50's. Every one I have is somewhat of a mystery to how it will perform the next time I turn one on. You often have to switch the tuner back and forth and adjust all controls. Philco's are the worst. At this point the only Philco I've kept was a Predicta. RCA's can be touchy with audio. They often have a precise "sweet spot" on the fine tuner. Admirals are the best. GE's, Sentinel's/Airline's, Motorola's, and Emerson's are usually good too. I've learned to be happy with a watchable picture and decent sound with these sets. TV's came a long way since that time. Style-wise no, but in reliabilty, worlds apart.


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 Post subject: Re: How to Adjust Fine Tuning on a Vintage TV
PostPosted: Jun Sun 30, 2019 11:34 pm 
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The tuning sweet spot on the early sets with split sound design are finickity by design! It is important however to recognize and understand the difference between split sound design sets and later intercarrier sound sets.

Split sound sets sound IF carrier is 4.5MHz away from the video IF carrier e.g. video IF 25.75MHz sound IF 21.25MHz whereas an intercarrier set video IF of 25.75MHz will have a sound IF of 4.5MHz.

In an intercarrier set, the video carrier itself becomes the local oscillator to the sound such that the beat between the two is always 4.5MHz irrespective of the fine tuning control. It s only important to keep the ratio of sound and video IF carriers are kept within a specific ratio.

A split sound set which includes all RCA televisions until about 1952, means that the fine tuning control must be set for proper fm demodulation. The finickity bit is getting the audio tuned between the two FM discriminator nulls. The way the tuning is performed is as you approach the correct tune, the audio will be a bit distoted but will get louder then dip to a null and the audio will be the loudest and clearest. Further turning the fine tuning will reach a second null followed by distorted diminishing audio.The benefit of split sound is no intercarrier buzz because the video and audio channels are essentially fully independent.

Hence one must remember that an intercarrier set is tuned for best picture and sound is always present and a split sound set must be tuned for best sound.


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