Newbie's First Questions RCA TS 721 Restoration

[Chris K]

Fail. Did it exactly the way we schemed out. I tuned the L60 adjustment screw for channel 4 until the radio quieted completely at 93MHZ. Then I reassembled the rig and fed my RF modulated, computer video into the set. Same meh picture and no sound. This thing has silver mica disease in the RF or something else way outside my ability to fix. I think it's time for some Ben and Gerry's and then bed. Thanks my friend for trying. I really don't know where we go from here. I think doing the composite setup and pulling some tubes is the way this is going to go at this stage in my journey with this TV. Maybe at some point I'll have the insight and skills to get this going from the rooter to the tooter but I don't think it's that time just yet. Right now, a decent picture and sound no matter how it's obtained is how I'm going to go with this. It'll sit in the corner in the living room playing YouTube recordings of The Honeymooners and I Love Lucy and I don't think anyone will care if I'm injecting video and sound right in it's heart.

Or maybe I'll feel different tomorrow! Who knows.

[Penthode]

Sorry to hear it did not work. There are a few checks you can actually make to ensure the local oscillator is correctly on frequency.

Remember the three screenshots I posted last week? When you adjust the fine tuning control across its range, do you get the bright smeary image with the control fully clockwise, the slightly less bright but sharper image in the middle setting and at the full counter clockwise the weak image? This is very important for the proper vestigial sideband tuning I outlined and it would confirm the oscillator tuning is correct. This is how all properly operating 621TS and 721TS sets should work.

[Penthode]

You are very close to getting it to work and we only have to methodically work it through.

Bypassing the RF and IF and feeding the video directly to the video amplifier I do not think is a viable option for you. There is no conventional contrast or gain control and you may have to construct a transistorized gain contolled amplfier to invert the video and ensure the video level is correct. It won't be a matter of directly connecting the video to the grid of the tube. If you cannot diagnose this simple fault do you think you are up to some electronic design and construction work?

I have gone through great pains to outline tests which you have not responded to. Do you understand what I have said about the tuning?

[Kevin Kuehn]

Originally Posted by Penthode

Sorry to hear it did not work. There are a few checks you can actually make to ensure the local oscillator is correctly on frequency.

Remember the three screenshots I posted last week? When you adjust the fine tuning control across its range, do you get the bright smeary image with the control fully clockwise, the slightly less bright but sharper image in the middle setting and at the full counter clockwise the weak image? This is very important for the proper vestigial sideband tuning I outlined and it would confirm the oscillator tuning is correct. This is how all properly operating 621TS and 721TS sets should work.

While I'm sure your 721 has been aligned to perfection, I think you're assuming the video IF in Chris's set is also in perfect alignment. I don't recall any mention that the image has ever respond to the fine tuning control, but Chris can confirm. It certainly would be reasonable that the Video IF on a nearly 75 year old TV could be compromised to the extent that the fine tuning control will not show such subtle fine tuning indications?

[Penthode]

There a number of variables at play here! Yes the alignment may be out. But the oscillator may not be at 93MHz. Maybe Chris tuned to an image or harmonic? How certain are we the oscillator is correct?

Certainly the fine tuning test will show if we are in the ballpark. If the video carrier is not on the slope at 50%, the picture will be smearyand awful anyway.

I see impatience here. Too often I see the knowledge we have gathered taken for granted.

If the alternative is to feed the video amplifier, fine. But will no contrast control in this set to set level and assuming the output composite analog video polarity of say a DVD player is negative sync you may not need to invert video. Is Chris up to devising a way to interface the video and manage signal level?

It seems simpler to me to resolve the relative simple problem the set has rather than introduce another problem. Maybe the best way is to let somebody else work on it.

[Kevin Kuehn]

Originally Posted by Penthode

There a number of variables at play here! Yes the alignment may be out. But the oscillator may not be at 93MHz. Maybe Chris tuned to an image or harmonic? How certain are we the oscillator is correct?

Certainly the fine tuning test will show if we are in the ballpark. If the video carrier is not on the slope at 50%, the picture will be smearyand awful anyway.

I see impatience here. Too often I see the knowledge we have gathered taken for granted.

If the alternative is to feed the video amplifier, fine. But will no contrast control in this set to set level and assuming the output composite analog video polarity of say a DVD player is negative sync you may not need to invert video. Is Chris up to devising a way to interface the video and manage signal level?

It seems simpler to me to resolve the relative simple problem the set has rather than introduce another problem. Maybe the best way is to let somebody else work on it.

I went back and reread all the posts. What strikes me odd is the Leader test pattern gets a signal through to the output of the second video amp, enough to drive the CRT well with a high contrast test pattern, however Chris has said his over the air box gets little to no legible picture. We have not been shown what the signal from the over the air box looks like, but he hints it has a poor picture on other TV's as well. I would assume the RF output has something wrong with it or it's signal is much weaker than the Leader. Is the over the air box outputting on the same rf channel as the Leader, or are we comparing reception on two different channels on the 721 tuner? Given the digital signal source, a properly working converter box should not produce a weak or snowy rf signal - they more less lock in or not. So without a decent rf modulated video source it's very difficult to observe the fine tuning detail you're looking for. I would prefer to see a signal from a known good DVD player feeding an rf modulator outputting on Ch 3 or 4.

[Penthode]

That is a good point.
So many variables.

More screen shots and photographs of the chassis and the setup would help.