Newbie's First Questions RCA TS 721 Restoration

[Penthode]

It would be good to review the rf/if design of the split sound televisions manufactured by RCA Victor in the late 1940s. The first post WW2 design separated the sound from the video at the tuner output. You will find that the video if carrier is 25.75MHz and the audio if is 21.25MHz. The 21.25 MHz sound IF signal goes through 2 or 3 stages and it is fed to a Foster Seeley FM discriminatory. The alignment procedure is very standard but you at least need a crystal calibrated accurate 21.25 MHz signal.

In a split sound design where the audio is branched off prior to the video detector, achieving the 21.25MHz sound from the tuner requires the tuner local oscillator to be tuned very precisely. The range of the fine tuning control is only about +/-250kHz and simply changing the 6J6 oscillator tube can affect tuning as much as +/- 2000kHz which means so my changing the oscillator tube or even removing the tube shield will be sufficient to lose audio! The later KRK 5 tuner has an adjustment to offset the effects of the minor capacitance changes when replacing the oscillator tube. I believe the earlier KRK2 tuner used in your set has the same. If so it would only be a,matter of making minor adjustments to it to bring the local oscillator in line. Since the tuner uses three 6J6 tubes, you may try swapping them to find one that brings in the sound.

If you are getting a picture through the tuner and can hear,audio if applied to the volume control, that leaves only the discriminatory and two sound if stages. Providing you nor anyone else has disturbed the adjustments, the likely only case for the path to fail is open filaments or lack of power to the tube elements. I would first check if the anode and screen voltages are correct. If so and the if and discrimator tubes are lit, I think your only problem is tuning the local oscillator in the tuner.

Lastly, a word about intercarrier sound. Most people are used to on later sets adjusting the fine tuning for best picture with the sound always present. This is because in the intercarrier design, the am video carrier beats with the FM audio carrier to produce a 4.5MHz signal, or the difference between the video and sound carriers. That means that if you adjust the fine tuning control, the difference between sound and audio if carriers remain the same and you will always get the 4.5MHz diiference. RCA was one of the last manufacturers to adopt the intercarrier design until 1952 I suspect because they wanted to hold out paying a royalty fee and because of customer dissatisfaction forcing their hand.

[Chris K]

Nice clean tone injecting an audio signal into the volume control.

[Chris K]

As I said before, I can hear a very faint 1KHz tone that responds to volume changes when injecting video and audio through the antenna from the Leader pattern generator but it is very, very faint. I have to put my ear up to the speaker to hear it but it is there.

[Kevin Kuehn]

Originally Posted by Chris K

As I said before, I can hear a very faint 1KHz tone that responds to volume changes when injecting video and audio through the antenna from the Leader pattern generator but it is very, very faint. I have to put my ear up to the speaker to hear it but it is there.

At any point throughout fine tuning does the audio get louder even though the picture may worsen, or does the best picture seem to track with the loudest sound?

[Chris K]

Originally Posted by Kevin Kuehn

At any point throughout fine tuning does the audio get louder even though the picture may worsen, or does the best picture seem to track with the loudest sound?

I get no change in the very faint sound no matter what I do to the fine tuning. I think I might need to get a list together of the "special" equipment Penthode mentioned I would need to get the transformers I messed with back to where they need to be. Nothing I've done so far has affected the picture. It's still rock solid.

[Chris K]

Probably won't be able to use the suggestions here until tomorrow night. Busy weekend. I'll post with my progress.

[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]

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]

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

[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!