Late 1940's RCA TV design- Adjacent Channel Rejection
 

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.

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

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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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.

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!

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.

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…

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

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.

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.

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

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.

UHF stations had a hard time in the 1950s. For example, Pittsburgh's channel 53 was on and dark twice, from two different locations, before staying on for good in the late 1970s. Cable TV was a big help; the UHF tuners in some of the old sets were not too sensitive.

You are correct. the all-channel act was passed in 1961 and FCC regs based on it required all sets to have UHF starting in 1964.