Newbie's First Questions RCA TS 721 Restoration

[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.

[Chris K]

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.

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!

[Chris K]

I've gotten it about as good as I can. I do have a decent image from an HDMI to RF Coax feed from my computer. Unfortunately, my new cell phone blanks out the image every time I take a photo. I haven't done well with the Android to iPhone transition so far!
Just for the information because it was mentioned here before by Kevin...how would I install the means to inject composite video and sound into the unit???

[Penthode]

Did you follow my instructions to set the local oscillator? What did you find?

To facilitate injecting video directly you will need to build a 75 ohm interface. It could be connected to the grid of the first video input stage. Then you could inject audio to the volume control. It will take a bit of fiddling as you have to ensure the video polarity is correct.

It appears you were very close connecting by the antenna. What happened?

[Chris K]

Originally Posted by Penthode

Did you follow my instructions to set the local oscillator? What did you find?

To facilitate injecting video directly you will need to build a 75 ohm interface. It could be connected to the grid of the first video input stage. Then you could inject audio to the volume control. It will take a bit of fiddling as you have to ensure the video polarity is correct.

It appears you were very close connecting by the antenna. What happened?

I need to go on ebay and get a radio with a signal strength meter. I thought I had one but I didn't. Couldn't work on the oscillator just yet. I did get an RF modulator that injects excellent, clear video in my Zenith B&W TV into the antenna from my computer's HDMI output and I fed that into the antenna on the RCA and I got a decent picture...a little snowy but decent...but still no sound.

Could I go into the 12AU7 grid with a composite video signal? The modulator I got has RCA video and sound outputs and I can mount 2 RCA female connectors on the back of the 721 chassis and run the signals to the volume pot and the video amp grid. But if this TV has some funky video system, this might not work.

[Penthode]

You don't need a fancy radio. Don't you have an FM radio kicking about? What kind of FM radio do you have?

[Chris K]

Originally Posted by Penthode

You don't need a fancy radio. Don't you have an FM radio kicking about? What kind of FM radio do you have?

I have an old channel master super fringe FM radio

[Penthode]

Originally Posted by Chris K

I have an old channel master super fringe FM radio

Tube model? That should work in a pinch. What does it look like? Model number?

It doesn't have to be fancy. You may hear the silent carrier as you tune to it.

Right now we a just fishing for the local oscillator. Use as your reference the radio stations I indicated and the oscillator should be right next to the 92.9 KHz low power station in Philly. It is probably well off from that. Just connect the TV antenna terminal with the set on channel 4 directly to the radio antenna and tune until you hear what would be like a silent radio station.

[Chris K]

OK will do. Unfortunately, I’ve got a 430 wake up call tomorrow! If it’s possible, I’ll pick this up around 8 PM Eastern daylight time tomorrow night. Thank you so much for sticking with me through all of this. I’ll get everything set up and be ready to go. See you tomorrow!

[Penthode]

That's okay. You have gone this far and it would be a shame not to figure this out.

I guess the most important aspect of this hobby I have learnt through the years is patience and perseverence.

[Chris K]

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.

I have the Channel Master tuned between WMMR 93.3 and WXTU 92.5. What's the next step? I have a working, good Tektronix oscilloscope too if that'll help with this.

[Chris K]

Actually, the scope won't help...it's a 60MHz scope

[Penthode]

Connect one TV antenna terminal to the radio antenna. Turn on the TV, tune to channel 4 and center the fine tuning control.

We have identified where the TV local oscillator is supposed to be at exactly 93 Mix. Try start tuning the radio between 92 and 94MHz until you pick up the local oscillator. The oscillator is inoculated and once you think you have located it, you can change the fine tuning slightly to confirm you have indeed found it.

[Chris K]

Originally Posted by Penthode

Connect one TV antenna terminal to the radio antenna. Turn on the TV, tune to channel 4 and center the fine tuning control.

We have identified where the TV local oscillator is supposed to be at exactly 93 Mix. Try start tuning the radio between 92 and 94MHz until you pick up the local oscillator. The oscillator is inoculated and once you think you have located it, you can change the fine tuning slightly to confirm you have indeed found it.

So what should I be hearing on the radio when I have this hooked up? I hear a buzz and a "wee-oh-weee-oh-weee-oh" but the wave of the wee-oh isn't very fast. It's about half a MHz below the 92.5 tuning of WXTU where I get this

[Chris K]

When I flip the channel to 3 or 5, the radio blasts in. I think the oscillator for channel 4 is tuned to about 91.5 or 92 MHz