Late 1940's RCA TV design- Adjacent Channel Rejection

[Penthode]

It is interesting to study the evolution of the design of various manufacturers televisions set from after WWII. RCA was a leader and as such introduced popular circuits as their synchroguide horizontal AFC in 1947/48. RCA was also one of the last manufacturers to stick with their split sound design until 1952 with audio take-off from the tuner and later after the second Video IF amplifier.

The earliest post WWII RCA televisions, the 630TS, 621TS and 721TS featured minimal adjacent channel rejection. The 630TS featured full 4MHz video bandwidth and a single adjacent channel video trap (19.75MHz) and adjacent channel sound trap (27.25MHz). The 621TS/721TS design had no adjacent channel rejection other than the roll-off response of the Video IF amplifier.

You may recall 1948 was a crucial year for the television broadcast industry as set sales were booming. But 1948 also yielded the peak of the 11 year sunspot cycle. Sunspot activity in 1948 interfered greatly with VHF television propagation causing co channel interference from distant broadcast television stations. The result was that the FCC issued a halt on the construction of new stations for the next four years which resulted in the opening of the UHF band (470MHz to 890MHz) band for television.

The 1948 series of RCA television sets beginning with the KCS28 chassis included not one but two pairs of adjacent channel video and adjacent audio traps. I always thought it excessive but during the days of analog television broadcast, it did make a brilliant DX'ing receiver!

It occurred to me that it was too much of a coincidence that this design was introduced coinciding with the sunspot cycle peak. You will find the dual traps pairs in almost all RCA designs until 1952, when the traps disappeared. By the mid '50s, only the most expensive RCA sets featured both traps but most often included only an adjacent channel sound trap or no adjacent channel traps at all.

Below I have included the shematic diagrams for the Video IF amplifiers of the 1946 630TS, 1946/7 621TS/721TS and 1948 8T241/3/4 using the KCS28 chassis. The adjacent channel traps have been boxed in red.

[ChrisW6ATV]

Some very interesting notes. Thank you for posting your comments and the diagrams, Penthode.

[Polaraligned]

Originally Posted by Penthode

You may recall 1948 was a crucial year for the television broadcast industry as set sales were booming. But 1948 also yielded the peak of the 11 year sunspot cycle. Sunspot activity in 1948 interfered greatly with VHF television propagation causing co channel interference from distant broadcast television stations. The result was that the FCC issued a halt on the construction of new stations for the next four years which resulted in the opening of the UHF band (470MHz to 890MHz) band for television.

Wow. I never heard of this. How could they issue a 4 year halt to new station construction when TV was just getting off the ground?

[nasadowsk]

Originally Posted by polaraligned

wow. I never heard of this. How could they issue a 4 year halt to new station construction when tv was just getting off the ground?

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[Electronic M]

Originally Posted by Polaraligned

Wow. I never heard of this. How could they issue a 4 year halt to new station construction when TV was just getting off the ground?

The TV VHF band was planned based on pre-WWII RF propagation data. Pre-War VHF was bleeding edge technology that was basically the domain of HAM radio, and sun spot tropospheric propagation boosting was at a minimum. Post WWII sun spot activity was at a high peak...TV stations that were only supposed to cover a 50 mile radius were regularly getting cleanly DX'ed 1000 miles away by John Q (I'm not trying to DX) public...
Because of this stations on the same channel that the FCC thought they placed far enough apart to avoid interference were interfering with each other like mad. Adding more stations when the band was behaving outside of predictions and interference problems were rampant would only throw fuel on the fire.
The FCC froze new licenses while researching the band and solutions...The solution landed on was UHF.

[Penthode]

So that was the problem facing 1948. Hence why I think over the next few years the extra adjacent channel traps.

The net result for me years later (before DTV) that this design was brilliant for use in DX'ing!

[Polaraligned]

I guess the traps solved the issues in the end because growing up in the suburbs of NYC, UHF was pretty dead. UHF had a few channels, but no major networks. Everything we watched in the 60's thru 80's was on VHF.

[nasadowsk]

CBS, NBC, FOX, ABC, WOR, and of course PIX. 2,4,5,7,9,11. And PBS on 13. Like anyone cared. I’m amazed PBS survived in major cities, I can’t imagine how it does outside of them…

Oddly NPR does well. Maybe because they have PRI as competition…

[Polaraligned]

I did enjoy some PBS shows like Nova and.....well, maybe just Nova

[Electronic M]

Their kids shows in the 90s were pretty good till about age 9 then too childish and too much Pre-K educational (okay PBS I'm a big kid now...I can count way beyond 100, STOP trying to teach me how to count to 3).
Granted the other channels that ran Looney Toons, Tom & Jerry, and Scooby Doo I liked better.
Some of their adult focused science and history shows back then weren't bad either, but then again I was that weird kid that intentionally rented about half documentary video tapes, half cartoons from the library...
When I was a teen and they ran Monty Python I liked that.
Once that was gone besides the occasional Swing Music feature I haven't watched them.

[DavGoodlin]

An example of this VHF interference problem was noted in the wikipedia for WGAL-TV, signed on as VHF 4 on March 18, 1949. It wasnt until 1952, that competition in the form of UHF came to this area from towns only 25 miles apart, bowties were everywhere .

"In 1952, WGAL increased its power from 1,000 to 7,200 watts. On December 31, 1952, the station moved to channel 8 as a requirement by the FCC in order to prevent interference with WRC-TV in Washington.....

On January 1, 1954, WGAL presented its first color television broadcast of the Tournament of Roses Parade. It has always been an NBC affiliate, but also carried some programs from CBS, DuMont and ABC until 1963"

Most TVs owned locally are RCAs prior to '52 and all sets were sold with UHF tuners if later than 52. Brand loyalty in an area where RCA employed many people at the New Holland Ave plant making vidicon and CRTs explained much of WGAL's early adoption of technology. Zenith really only became popular after mid-60s.

[old_tv_nut]

Oops - typo should say "...all sets were sold with UHF tuners if later than 62."

[DavGoodlin]

Originally Posted by old_tv_nut

Oops - typo should say "...all sets were sold with UHF tuners if later than 62."

I mean to say that TV sets sold in this county/area (had only one local VHF-8) so they were ordered with UHF option from factory as soon as that was available, prior to them being required as standard starting in 1964.

The most popular UHF antenna was a corner reflector bowtie or a flat bowtie (not 4 bay). There are many 65+ Y.O. UHF antennas still on rooftops as part of the original installation, not added later on mast below the VHF antenna. https://www.bing.com/images/search?v...t=0&ajaxserp=0

[Jeffhs]

Originally Posted by DavGoodlin

I mean to say that TV sets sold in this county/area (had only one local VHF-8) so they were ordered with UHF option from factory as soon as that was available, prior to them being required as standard starting in 1964.

The most popular UHF antenna was a corner reflector bowtie or a flat bowtie (not 4 bay). There are many 65+ Y.O. UHF antennas still on rooftops as part of the original installation, not added later on mast below the VHF antenna. https://www.bing.com/images/search?v...t=0&ajaxserp=0

My family's first new TV was a 1965 Silvertone all-channel set. We were some 30+ miles from the first and only (at that time) UHF station in Cleveland (PBS channel 25) and did not have a decent UHF antenna on the TV at the time (in fact, our first UHF antenna was a wire loop connected to the UHF antenna terminals). Channel 25 came in all right in our area, but not well; in fact, the station was just barely visible through a hail of snow on the TV screen. It was just as well, I suppose, since this station was an NET (National Educational Television) affiliate, the programming of which was meant mainly for the schools in northeastern Ohio; the station signed off at midnight every night during the week. The station did not go to 24-hour programming until quite a while (read a decade or more) later, after it became an affiliate of the Public Broadcasting Service (PBS) in the mid to late 1970s.

BTW, I did not realize, until I read your post, that your area only had one VHF TV station in the '60s-'70s. That part of the state of Pennsylvania must have been "in the middle of nowhere" as far as TV reception was concerned in those days, with little or no programming available on TV, even from Erie or whatever city was closest to your area at the time. Erie only had one VHF station, WICU-TV (NBC) channel 12, for years until channel 24 (WJET-TV, ABC) and channel 35 (WSEE-TV, CBS), not to mention WQLN-TV (PBS, then NET) channel 54, arrived in the city.

I can remember actually seeing WICU-TV's programming in northeastern Ohio during VHF band openings in the late '60s-early '70s, when I lived in a Cleveland suburb. The station, being an NBC affiliate, practically duplicated the programming of Cleveland's NBC station, WKYC-TV on channel 3; channels 24 and 35 in Erie likely duplicated the programming of our channels 5 and 8, ABC and (at the time) CBS (this was, of course, before that big TV network swap in Cleveland between channel 8 and channel 19, where 8 took Fox and 19 took CBS in the mid-1990s, IIRC).

[Electronic M]

I think you're both wrong...
All new sets sold were mandated to have UHF tuners in 1964.

UHF started in 1952 and was optional in new TVs from 1952 till the 1964 mandate.