Newbie's First Questions RCA TS 721 Restoration

[Penthode]

Originally Posted by Chris K

For the crosshatch pattern I'm using a Leader LCG-396 that I have tested on a Zenith 12" Sidekick I restored. There's an audible tone that accompanies the crosshatch pattern. I'd have to look up the frequency...probably 1KHz. Last week, I injected an audio output tone from a Knight KG 650 RF generator (range 160 KHz to 115 MHz for the RF...I don't know what the audio tone frequency is) all the way back to the output of the discriminator 6AL5 and got a loud audio tone from the speaker that responded to volume pot changes so everything from the volume control forward is working. The problem has to be in the RF circuit correct? Now, this morning I ran the pattern generator into the set and I could hear a very low 1KHz tone coming through the speaker that responded to volume control inputs but when I say low output, I mean I needed to put my ear close to the speaker to hear it...but it was there and that's the first time I've heard it.

As for injecting carrier signals with information into the RF circuitry, that's about where my knowledge base stops. I made some idiotic attempts to get something out of a carrier signal input to the RF circuit but they are not worth mentioning for the purpose of troubleshooting analysis.

Another potential issue for the future...I did try to input a digital over the air broadcast signal through a digital to analog converter box into the TV. It's a commercial unit I've used before on my other vintage TVs and it works OK. On the 721, I wasn't getting a discernable image just some image jumps and flashes when I changed channels on the converter box. The pattern generator injects images fine but my guess is I'm going to need some sort of a signal boost if I'm going to watch anything other than a crosshatch pattern. That's for another day.

I thought you were attempting to inject a 21.25MHz IF signal to test the audio chain.

Do you understand how the RF IF system of this set works. It is a split sound design and if the tuner local oscillator is way off, you will get no sound.

The alignment of a split sound set is more critical than the later intercarrier sets as the sound IF/ discriminator has to be precisely aligned with the Video IF otherwise you get good sound and lousy picture or good picture with no sound. Also in the split sound receiver, you always use the tuner fine tuning control to adjust for best sound and not best picture.

I you are not familiar with split sound vs intercarrier sound systems, this forum will help enlight you.