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 Post subject: RCA 8TS30 "Picture" control operation
PostPosted: May Fri 22, 2020 5:00 am 
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Joined: Aug Sun 11, 2019 5:45 am
Posts: 5
Hi there. I have returned to an RCA 8TS30 that has been languishing in my shop for some time. When I turn the "picture" control too far CCW i lose both video and audio. I had been chasing a significant change in various voltages associated with the video and audio IF until I found a manual on this very site! THANKS to the uploader! I now see that the voltage variations I am seeing are to be expected. Most everything stays within 10 percent of what the service manual states. Can anyone tell me whether this is "normal" behavior? I would be surprised if RCA produced a TV that had no pix/no sound if a front panel knob was not tweaked properly. The control in question adjusts DC levels to half the doggone TV set! It feeds the tuner, all the video IF, most of the audio. Very interesting design.
All suggestions much appreciated!


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 Post subject: Re: RCA 8TS30 "Picture" control operation
PostPosted: May Fri 22, 2020 1:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2135
Location: Lafayette, CO
Yup.....that's the design. I have a Motorola VK101 with that design but it's labled contrast. It has twelve user adjustable controls not counting the tuner or centering. Early sets are interesting.... Craig


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 Post subject: Re: RCA 8TS30 "Picture" control operation
PostPosted: May Fri 22, 2020 7:22 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 1222
Location: Pewaukee, WI
The early sets like the 8TS30, 630.and sets of other makes predate the invention of AGC. The picture/contrast controls varied the contrast by controlling the gain of the IF circuits...they had to have a broad range since these sets were deployed everywhere from homes a block from the transmitter to DEEP fringe locations where you needed a 50' antenna tower to get a snowy picture with an antenna amp and the picture control cranked.

AGC is basically the TV equivalent of AVC in radio....the video signal at the detector (or after it but before the contrast pot) would be averaged and the IF gain adjusted to maintain the average... later on keyed AGC took over...Keyed AGC basically would connect a keying tube (amp biased hard into cutoff) to video and the horizontal sweep such that during retrace/sync pulse period the amp would suddenly be biased into conduction and pass only the sync pulse. the circuits would then adjust the gain to hold the sync pulses at a fixed level...this worked better than averaging the video signal since sync amplitude should be constant but video varies.
The 630 chassis that is also in the 8TS30 was built under license till 1960 and later versions had keyed AGC and modification to drive tubes as big as 27".


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 Post subject: Re: RCA 8TS30 "Picture" control operation
PostPosted: May Fri 22, 2020 7:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 28747
Location: Detroit, MI USA
That's definitely how it's supposed to work. You can't reasonably expect early sets to work exactly like later ones did. They can be very quirky compared to TV's from several years later, and there is some learning curve for each brand/model to be able to tune in the best picture and sound.

This set is actually the best of the best from that era, and works really well with a good signal and correct adjustment of the controls. You won't find another 1948 set capable of getting a better picture than the RCA, provided everything is working right.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: RCA 8TS30 "Picture" control operation
PostPosted: May Sat 23, 2020 4:35 am 
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Joined: Aug Sun 11, 2019 5:45 am
Posts: 5
To all that replied:
WELL! I have been chasing my tail on this for weeks! Thanks big-time for setting me straight. Once I got hold of the actual RCA schematic compliments of this forum, I saw that the 165 volt bus branched off everywhere. I learned that it could vary as much as 20 volts depending on the picture control position. I shall turn my attention to addressing other minor issues with the TV. GEEZ I wish this thing had a picture tube strap to hold the CRT in while probing the underside of the chassis! I will continue to muddle on and will report back one day soon. The cabiniet is in great shape on this, and I have the original stand for it.

Thanks folks


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 Post subject: Re: RCA 8TS30 "Picture" control operation
PostPosted: May Sat 23, 2020 2:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 28747
Location: Detroit, MI USA
Back in the day, technicians did not usually use the set's CRT while operating a TV chassis on it's side on the bench. There were test CRT's available, an example would be the 5AXP4, which were smaller and lighter in weight than any of the normal CRT's.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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