Quote:
Originally Posted by dieseljeep
The low end of the AM loop antenna goes to a feed-thru capacitor on the back of the chassis. The schematic is Beitmans 1960, available at the build-a- radio website.
The 731 chassis has a two gang tuning condenser and the slug arrangement, similar to the 845 series.
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Thanks much for the information.
What is the URL for the "build-a-radio" website? I never heard of it.
I noticed the terminal for the feedthrough cap on the chassis of my C845, though I mistakenly thought it was a ground terminal. I'll have to go into the set tonight and reconnect the loop. I don't listen to AM that much, but it will be nice to have the radio working on that band anyway. Surprisingly, the radio still picks up the local station (1kW day/0.5kW night) without the loop, although since that station's transmitter is only about two miles away or so I'm probably getting a tremendous signal from it, so I shouldn't be surprised I am hearing it with no antenna.
BTW, I did not realize the K731 has a similar type of slug-tuned FM tuner to the ones in the C845s. The '731 must have been a high-end radio in its day, as most AM-FM radios of the '50s-'60s do not have such a tuning arrangement for FM; most sets I've seen use the tuning capacitor for both AM and FM, with the RF amplifier stage used only for FM. I read, however (I think it was in this forum some time ago), that there isn't much point in having an RF amplifier for AM in the '845. The tube serves some other function on that band; I'm not sure what, though.
There wasn't much need for RF amplifiers in AM radios by about the late 1950s or '60s, anyway (the external AM antenna connector on the back covers of many AC-DC radios of the period disappeared about the same time), since by then most areas of the US were within range of at least one AM radio station. Many small towns, suburbs of major cities, and even rural areas had at least one low-power local station as well; many of the big-city stations were operating with 50kW clear-channel signals too, so they could be heard almost anywhere at night. Station WLW in Cincinnati has quite a story to tell about its 500-kW signal in the 1930s, which lit up light bulbs not even in sockets (!) and which probably could be heard practically world-wide after sundown. WLW was forced to reduce its power output to 50kW later on in the decade.
This changed, of course, when the FCC abolished clear channels in the mid-1980s; instead, under new rules adopted in that decade, former daytime-only or limited-time stations could now operate full-time, often with decreased power output and directional signals at night and during critical hours. Some stations in very small towns, however, still sign off at sunset for financial or other reasons; one station in northeastern Ohio (WBTC-1540 in Uhrichsville, Ohio, near Canton and 70-some miles from Cleveland) does this, operating with a measly 5 watts at night and signing off at nine p.m.; the night sign-off is to protect at least one full-time station (KXEL-1540 in Waterloo, Iowa, IIRC) from skywave interference. Station WJIB-AM 740 in Boston also operates with just five watts nights (250 watts days); the station cannot be heard in much of Boston outside the city itself at night, but unfortunately this was the only way the station could operate past local sunset.
I noticed the terminal for the feedthrough cap on the chassis of my C845, though I mistakenly thought it was a ground terminal. I'll have to go into the set tonight and reconnect the loop. I don't listen to AM that much, but it will be nice to have the radio working on that band anyway. Surprisingly, the radio still picks up the local station (1kW day/0.5kW night) without the loop, although since that station's transmitter is only about two miles away or so, I'm probably getting a tremendous signal from it so I shouldn't be surprised I am hearing it without the antenna connected.