Quote:
Originally Posted by Chris K
...and this is where my foundations get a bit thin. What are these "traps" for and why are they called traps? I think, from what I've done and read, they are basically filters that remove unwanted noise from the signal, shunting noise to ground? The ten different megacycle signals from the signal generator fed into the converter grid where the voltage across the resistor is adjusted to a minimum...are those the frequencies we want to pass through the traps with the noise filtered out? Is that the purpose of this set of adjustments? I'm certain my description is devoid of correct terms and electronics specific language but, best I can do!!!
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The traps are to remove or suppress undesired signals. They essentially suck out or remove certain frequencies.There are three pairs of traps: two lower adjacent video carrier traps, two lower adjacent sound carrier traps and two in band sound carrier traps. The adjacent channel traps were to supress interference from adjacent channels eg if you wish to watch channel 9, the traps will suppress channel 10 video and channel 8 sound if present.
The adjacent channel traps are less important today.But it is important that the traps are correctly set as they will affect the overall video and audio performance of the set. Note the heavy suppression of the 21.25MHz sound carrier allows the video bandwidth to reach the full 4 MHz wide response. Remember most sets of the 1950's bandwdth extended only to 2.5 MHz to around 3 MHz, so this set can produce a much better resolved picture than most other sets.
History; When this set first appeared in 1948, it was when the FCC imposed the channel construction freeze. In 1948 was the first cyclical sun spot peak since television began. The effect was to cause interference between TV stations between cities due to the FCC based upon only line of sight planning. The sun spot peak showed that the signals could travel considerably further than the horizon. Hence in this set design to help mitigate interfence, thorough adjacent channel trapping or suppression was added.
Between 1948-1952, TV stations construction was halted while the industry explored a solution which ultimately resulted in UHF channels 14 to 83 being approved by the FCC in 1952.
A sidenote: I have a model 8T243 (same chassis) and used it for DXing (fishing for distant stations) during the sunspot peak in 1972. I was then able to receive from Toronto Canada, KCMO TV channel 5 Kansas City MO and WPTV channel 5 West Palm Beach Florida. Channel 5 was unallocated in the Southern Ontario Canada/ Western NY region but a strong adjacent channel 6 (CBLT Toronto) prevented all other sets from seeing the channel 5. (After 1952, the adjacent channel suppression in the design was a lot less because of the opening of the UHF band). The set was brilliant for DXing (long distance stations). I last used the same set for DXing in 2009 to view the 'night light' few remaining analog TV stations across the US. I was living in southern Illinois at the time and in 2009 on this set via a 50 foot outside antenna received stations in Phoenix AZ, Albuquerque NM, Denver CO, Regina Saskatchewan, Fredericton New Brunswick, Boston Mass, West Palm Beach FL and Monterrey Mexico. All from the same antenna! So this set was terrific for DXing because of those the traps!
I your case, the most inportant traps to be spot on are the 27.25MHz (lower adjacent sound) and the 21.25MHz sound trap as they especially affect the overall video response. The 19.75MHz trap adjust is only important to ensure it does not tune into the video and audio pass bands as they could distort the response.
09-13-2024, 01:06 AM
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