Quote:
Originally Posted by 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.
|
I so appreciate the wealth of information you have provided here! I did read through almost all of your 721 restoration thread and it was very informative (...those old, reformed electrolytics still hanging in there?...). I do know the carrier signal splits into 2 paths for the audio and video and I have not messed with the converter transformer T3...in fact, the adjustment screw is soldered so you know you shouldn't move it! From the tracing I've done, the issue is in the 6BA6-6AU6-6AL5 IF chain. Video is fine and an audio signal applied to the grid of the 6AT6 1st audio is loud, clear through the speaker and is adjustable via the volume control. All these tubes are NOS and lit...filament voltage is spot on at 6.3V AC for all tubes. I did screw around with both the top and bottom of sound IF transformer (T107) and the sound discriminator transformer (T108) a little bit. No change in getting any sound and I believe I returned the adjustments to approximately their original positions. When I read the service data on how to precisely adjust the oscillation of these transformers as a function of adjusting both the top and bottom precisely, I thought I screwed it up for good. I have not checked the pin voltages on all of these tubes yet but I do remember the 6AL5 having some really low pin voltages specked out...-0.5V...stuff like that that I did not get when I did some random voltage checks. I will do a more comprehensive voltage check and report back. Thanks again so much for taking the time to reply!