Quote:
Originally Posted by Penthode
I just had an idea to set accurately the channel 4 local oscillator setting in your TV. Further you will be able to determine where it is before you start! I tried it out on my 721TS and worked a treat.
The local oscillator beats with the channel 4 video and audio carriers to create the difference frequencies used for the IF: eg 93.0 - 67.25 = 25.75MHz and 93.0 - 71.75 = 21.25MHz. Below, I have drawn a little pictorial diagram depicting the IF amplifier frequencies, the Channel 4 frequencies and lastly the local oscillator frequency generated in the tuner. The tuner always radiates via the antenna terminals a small portion of the oscillator signal. In the old days it was a problem and was difficult to suppress and would cause interference on radios and other TVs. We can take advantage of this. (Note: The IF frequency move from the 20 MHz range to the 40MHz range in the 1950's was to minimize the problem).
Fortunately for us, the frequency of the local oscillator for channel 4 falls smack dab in the middle of the FM band at 93MHz! So all you have to do is to connect one of the television antenna terminals to an FM radio antenna of which you can trust the dial calibration. The frequency of the oscillator must be exactly 93.0MHz so that is where you tune the radio. The radio needs a signal indicator as you wont hear anything as the oscillator is supposed to be unmodulated.
For tuning reference, I see in Philly there is a bunch of low power 92.9MHz stations and a high power WMMR a Rock station at 93.3. So the local oscillator will be found between these two stations.
I used my Sony portable radio with a digital display and a signal strength light for my test. I switched on my 721TS with the antenna connections made and the power light was brightest at 93MHz.
Because your oscillator is off frequency, you will be able by this method to find where the oscillator is currently tuned. If it is low, you turn the channel 4 screw clockwise and if high counterclockwise until you reach exactly 93 MHz.
Remember the signal is a pure unmodulated sinusoid so you won't hear anything on the radio.
Once you find 93 MHz, I will bet you a 6BG6-G (my Avatar) you will then find a decent picture and audio! I suspect the discriminator will be off tune slightly but once you have established exactly the 93MHz local oscillator and feeding a channel 4 TV signal to the set, you can touch up the discriminator to get best sound.
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Wow...thank you! Amazing information. This weekend got away from me but...I'm off tomorrow and after I watch the SLS/Artemis launch in the morning at 8:30am EDT, I'm going to try all of these generous ideas and techniques. You guys are incredibly kind for taking the time you have spent on trying to help this all thumbs newbie get this beautiful antique working and bringing pleasure to myself and my family. The former owners have been in contact with me asking how it's been going and they will be thrilled if WE succeed! They owned the TV since Daddy first brought it home in 1948 so I owe it to them and all of you to reboot myself and do this right. Fingers crossed!