Newbie's First Questions RCA TS 721 Restoration

[bandersen]

I've been thinking you have poor gain and the only reason the Leader works is that is has a greater output level.

I suggest you test the four 270pF mica coupling caps in the IF. I've found quite a few bad ones in early RCA sets.

[Penthode]

Originally Posted by bandersen

I suggest you test the four 270pF mica coupling caps in the IF. I've found quite a few bad ones in early RCA sets.

I would refrain from touching anything in the IF strip at the moment. It will upset a alignment and we not have even a gain reference point at the moment.

I would suggest first Chris disconnect anything from the antenna switch on the tv set yo channel 4 and advance the contrast control fully clockwise and see if he can see snow on the screen.

It with the Leader connected with the contrast control fully clockwise if the picture loses sync and goes negative.

The CRT can tell many things if only we had a few photographs

[Electronic M]

I wonder if he could have kept the set upright with the CRT in while adjusting the oscillator. And accomplish that by hanging the tuner corner of the chassis off the bench and crouching below the bench to access it from the bottom.

If alignment is off it's possible there may have been interesting changes if it had signal injected into it.

I agree with the assessment that the signal source may be weak and or the set may have gain issues.

[Penthode]

Originally Posted by Electronic M

I wonder if he could have kept the set upright with the CRT in while adjusting the oscillator. And accomplish that by hanging the tuner corner of the chassis off the bench and crouching below the bench to access it from the bottom.

If alignment is off it's possible there may have been interesting changes if it had signal injected into it.

I agree with the assessment that the signal source may be weak and or the set may have gain issues.

The tuner oscillator screws are all available from the front of the chassis. Chris says he has the oscillator at 93MHz although that still has to be fully ascertained.

What would show if the set is healthy is to see the snow on the screen and rush of noise from the speaker with the contrast and volume controls fully advanced.

Going back to the suggestion on the IF coupling capacitors, the sound take off is before the video IF.

Also this set has no agc so the contrast control sets the RF and Video IF gain. The snow and noise from the speaker on channel4 with no modulator will tell us if there is gain AND if the oscillator is close due to the rd stages tracking.

[Chris K]

Btw I did the composite hookup and it looks very good. All I’m doing to run it is pulling the tuner oscillator tube. It works fine with my generator too through the tuner when I put the oscillator back in. The screen shot is of the composite feed. Of course my camera is blanking part of it

[Penthode]

Originally Posted by Chris K

Btw I did the composite hookup and it looks very good. All I’m doing to run it is pulling the tuner oscillator tube. It works fine with my generator too through the tuner when I put the oscillator back in. The screen shot is of the composite feed. Of course my camera is blanking part of it

I went to the trouble of posting three images showing the fine tuning behavior. But you have still not answered my question.

First, the picture looks washed out. Did you advance the contrast control? Secondly, does the fine tuning react as depicted in the three photos I posted? This is very important if you want to find the sound. Can you please answer this?

[Chris K]

Thanks so much guys for this fantastic and informative discussion. I really appreciate all the interest and all of the knowledge that you guys have shared with me through this rather arduous journey. It’s quite obvious that I do not speak the language well enough to be able to communicate most of the issues and let you guys know exactly what’s going on. My plan is to sit down tomorrow in front of the computer and write it as a scenario complete with pictures and hopefully I can recap exactly what situation the set is in at the moment. Thank you again so much for everything you’ve done. It’s really very kind and generous. I’ll just make one comment about what I read here. I think Bob is probably correct. My first impression when I went over to an over the air signal through my digital converter box I just felt like the leader was putting a much stronger signal into the television. When I had hooked up the digital box, the picture was very weak. Some of these generator boxes are notorious for putting out very strong signals so I could’ve been part of the problem

[Penthode]

Originally Posted by Chris K

... I think Bob is probably correct. My first impression when I went over to an over the air signal through my digital converter box I just felt like the leader was putting a much stronger signal into the television. When I had hooked up the digital box, the picture was very weak. Some of these generator boxes are notorious for putting out very strong signals so I could’ve been part of the problem

Did you rotate the contrast control? The 721TS does not have an automatic gain control. If the signal is weaker from the digital box, did you rotate the contrast control clockwise to compensate?

The adjustment of the contrast control in the earliest RCA receivers may be a little tricky. You must adjust it for each rf signal fed to this set. You did not say if the contrast control was set to maximum.

Sure the Leader may be putting out a higher level signal but you left out mention of the contrast control.

[Chris K]

And of course the image is rotated

[Kevin Kuehn]

That composite video injection trick is a good thing for anyone working on old TV's to have in their toolbox. Once you get proficient at using an oscilloscope you can look at the video signal at 1st video input and compare it to the 2nd video output. According to RCA service data the in going signal should be about 1 volt peak to peak, and likewise about 30 volts peak to peak at the second video output to the CRT grid. The sync pulse needs to be negative going into the 1st video grid. Each amplification stage inverts the signal, which is why the polarity is important going in. Obviously it's working correctly the way you have it. As far as I know negative going sync is standard for most video component's, but it's not always the case where you're injecting into various TV circuitry. For instance when the video is feeding the cathode of the CRT in needs to be inverted compared to going to the grid.

[Penthode]

Originally Posted by Kevin Kuehn

That composite video injection trick is a good thing for anyone working on old TV's to have in their toolbox. Once you get proficient at using an oscilloscope you can look at the video signal at 1st video input and compare it to the 2nd video output. According to RCA service data the in going signal should be about 1 volt peak to peak, and likewise about 30 volts peak to peak at the second video output to the CRT grid. The sync pulse needs to be negative going into the 1st video grid. Each amplification stage inverts the signal, which is why the polarity is important going in. Obviously it's working correctly the way you have it. As far as I know negative going sync is standard for most video component's, but it's not always the case where you're injecting into various TV circuitry. For instance when the video is feeding the cathode of the CRT in needs to be inverted compared to going to the grid.

One should add a 75 ohm termination should be added to prevent reflections if the cable is more than a few feet long. The video level needs to be a couple of volts peak to peak in the 721TS to provide proper contrast. There is no gain control in the video amplifier to control contrast. Hence the the picture will lack contrast unless further additional amplification is added which includes an adjustment for contrast.

[peter scott]

If you want to take photos try turning the screen brightness down and select a longer shutter speed on your camera, something longer than a 30th of a second. If the camera doesn't give you access to shutter speed then try a lower sensitivity number (lower ASA in old film terms)

Peter

[Kevin Kuehn]

Originally Posted by Penthode

One should add a 75 ohm termination should be added to prevent reflections if the cable is more than a few feet long. The video level needs to be a couple of volts peak to peak in the 721TS to provide proper contrast. There is no gain control in the video amplifier to control contrast. Hence the the picture will lack contrast unless further additional amplification is added which includes an adjustment for contrast.

From my own experimenting with my Magnavox converter box I'm seeing closer to 1.5, possibly 2 volts peak to peak. Without a contrast control the contrast is way too much in a low light setting. Of course one could add video signal attenuation, but for my testing purposes there was no need.
Agree cable length is critical.

[Chris K]

OK...I think I'd like to make a statement here and hopefully exit all of this gracefully and with a degree of satisfaction I hope for all involved.

As I stated in the title of this 23 page, 340 comment thread, I am a novice at TV restoration. Radios...there I think I have developed a level of competence but TVs are a brand new adventure for me. With the help of everyone here, this project has come along so far, much further than I ever thought it would in my hands. I owe so much to the experts here for their continuing guidance and input. I've tried, to the best of my limited experience and skill set to employ all of the suggestions here. Unfortunately, my lack of a good electronic vocabulary and experience has frustrated some of my efforts and now is apparently beginning to frustrate some who have reached out to help me.

The discussion has taken a turn that is starting to make me feel uncomfortable. I thought I was doing an adequate job of being grateful and humble for the generous input of everyone here but I guess, for some, I haven't done that. Let me state I have never sought to take advantage of anyone's time or knowledge. If I have given the impression I'm here to steal some advice and disappear, I apologize for any discomfort I have caused. This is a hobby for me. I have 2 jobs, a wife, a new house and an ageing and challenged 93 year old father to care for. I don't have hours to sit at the bench and work on this and if my lack of responding, or being able to remember to do everything that has been suggested has angered anyone, I hope my explanation of my circumstances will give some perspective. Some of you have been so generous and have invested a lot of your time trying to help and if my responses have made you feel unappreciated or taken advantage of, again, I'm very sorry.

I think it's in the best interests of everyone we bring this thread to a close. I hope to be back with other projects and perhaps give back some of what I have learned to other newcomers. I am awed by the level of expertise and the generous way everyone has contributed. I didn't in any way want to make anyone feel uncomfortable.

Again, thanks so much to all of you. All the best.

Chris

[init4fun]

Originally Posted by Chris K

OK...I think I'd like to make a statement here and hopefully exit all of this gracefully and with a degree of satisfaction I hope for all involved.

As I stated in the title of this 23 page, 340 comment thread, I am a novice at TV restoration. Radios...there I think I have developed a level of competence but TVs are a brand new adventure for me. With the help of everyone here, this project has come along so far, much further than I ever thought it would in my hands. I owe so much to the experts here for their continuing guidance and input. I've tried, to the best of my limited experience and skill set to employ all of the suggestions here. Unfortunately, my lack of a good electronic vocabulary and experience has frustrated some of my efforts and now is apparently beginning to frustrate some who have reached out to help me.

The discussion has taken a turn that is starting to make me feel uncomfortable. I thought I was doing an adequate job of being grateful and humble for the generous input of everyone here but I guess, for some, I haven't done that. Let me state I have never sought to take advantage of anyone's time or knowledge. If I have given the impression I'm here to steal some advice and disappear, I apologize for any discomfort I have caused. This is a hobby for me. I have 2 jobs, a wife, a new house and an ageing and challenged 93 year old father to care for. I don't have hours to sit at the bench and work on this and if my lack of responding, or being able to remember to do everything that has been suggested has angered anyone, I hope my explanation of my circumstances will give some perspective. Some of you have been so generous and have invested a lot of your time trying to help and if my responses have made you feel unappreciated or taken advantage of, again, I'm very sorry.

I think it's in the best interests of everyone we bring this thread to a close. I hope to be back with other projects and perhaps give back some of what I have learned to other newcomers. I am awed by the level of expertise and the generous way everyone has contributed. I didn't in any way want to make anyone feel uncomfortable.

Again, thanks so much to all of you. All the best.

Chris

Personally, I think you've done fine so far, there IS a learning curve with any hobby, especially technological ones, and you have displayed patience above what could be expected from a novice facing a tough problem. I agree with your plan to put this set aside for a while, sharpen your skills on some sets of lesser desirability as your time allows, and then some day in the future come back to this set armed with the newfound wisdom of what your experiences with the other TVs will teach you......