Hi, I have a VT-71, recapped, including some micas. All tubes tested good. Problem is the image is jittery. I have dialed in the tuning pot for Ch3 the best I can, but still have the issue. See attached image, and video linked below.

What do you think?

https://www.youtube.com/watch?v=uVxP8FtQZZw&feature=youtu.be

Jon-

That is absolutely, exactly the same problem I have with my VT-71 as well, which is also a TS-4D chassis. I started a discussion (with a similar title) on the Antique Radio Forums TV section a month or so ago, and got a number of helpful ideas and suggestions, but nothing has fixed mine yet. Since I have been busy with several other non-antique-TV projects and events, I have not got back into my set for a while, but my next step was to put an oscilloscope into the horizontal, video, and AGC circuits to see what I could find.

Starting the the TS-4E chassis, Motorola added a horizontal AFC circuit (and that version is what is in the Sams Photofact manual, more or less), so I suspect that this problem is common in the TS-4D and earlier chassis to one extent or another.

The one trick/clue I have found is that when I played with the horizontal hold control, I got the set into some mode where the picture was split horizontally but seemed to be in sync; the "horizontal phase" was way off, I think it can be called, and the vertical sync bar was visible in the picture. I forgot if it was in or near the center, or off to one side. The big kick, though, is that the (out of phase) horizontal sync seemed very steady! So, if anyone has ideas of what that means, I would like to know, and maybe it will help both of our sets. My intent was to connect my scope if/when I can get the set into that out-of-phase mode again, as well.

I wouldn't call that hor jitters, looks more like noise to me. Try looking for sources of stray HV, or adding additional shielding to the IF strip. Can also be caused be signal interference, so make sure you run a cable signal right into it using a balun.

The one trick/clue I have found is that when I played with the horizontal hold control, I got the set into some mode where the picture was split horizontally but seemed to be in sync; the "horizontal phase" was way off, I think it can be called, and the vertical sync bar was visible in the picture. I forgot if it was in or near the center, or off to one side. The big kick, though, is that the (out of phase) horizontal sync seemed very steady! So, if anyone has ideas of what that means, I would like to know, and maybe it will help both of our sets. My intent was to connect my scope if/when I can get the set into that out-of-phase mode again, as well.

My TV exhibits the EXACT same behavior! Does it have something to do with the horizontal oscillator? Unfortunately I don't have a scope or the skills to know how to operate one anyway.

I wouldn't call that hor jitters, looks more like noise to me. Try looking for sources of stray HV, or adding additional shielding to the IF strip. Can also be caused be signal interference, so make sure you run a cable signal right into it using a balun.


That is the direction that I would be heading also. It does appear to be noise to me. But I am NO expert, only a hobbyist.

Can you inject a composite signal to these sets? If so, where?

It does look like interference, but considering it cleans up when the horizontal hold is out of phase, not sure that is the issue.

Jon-

Yes, it should be possible to directly connect a composite-video signal, BUT: Of course, an isolation transformer MUST be used to power the set during such a test (and, in fact, during all service of these sets!). The logical place to connect video is to the control grid (pin 1) of the video amplifier tube, a 6AU6.

Knowing that your set can do that same "out of phase but jitter-free" trick is good to know; that means it is not a unique quirk to my set. One of these days, I need to get back into troubleshooting my TS-4D, and not just reading and posting. :)

I replaced all mica caps except for the ones in the IF chain and for the heck of it rebuilt the network for the audio, no change in the behavior of the set.

Next I will try to inject the composite signal to see if it goes away.

Jon-

...

Starting the the TS-4E chassis, Motorola added a horizontal AFC circuit (and that version is what is in the Sams Photofact manual, more or less), so I suspect that this problem is common in the TS-4D and earlier chassis to one extent or another...



Can you post or point to a schematic of the sync and horizontal section for the 4D chassis? Curious about what they used if they "added AFC later."

http://www.earlytelevision.org/images/motorola_VT71_TS4D.jpg

Jon-

Yes, it should be possible to directly connect a composite-video signal, BUT: Of course, an isolation transformer MUST be used to power the set during such a test (and, in fact, during all service of these sets!). The logical place to connect video is to the control grid (pin 1) of the video amplifier tube, a 6AU6.

Tried this but could not get it to work. Do I need to remove the over the air signal wire and replace with composite on pin 1? I just soldered composite signal to pin 1 and ground to ground

Hello Jon,
First off, I am not experienced or qualified enough to give anyone advise on fixing thier tv, but I do like to try to learn from these forums. So, for what its worth, my thought is could it be the crystal diode in the 2nd detector getting leaky?

http://www.earlytelevision.org/images/motorola_VT71_TS4D.jpg

Well, no components wasted there.

In general, you expect a direct triggered circuit to be much more noise sensitive compared to an AFC circuit. If L42/R38 through R39 and C44 is the source of sync, then L42 may be rolling off some high frequency noise depending on its size. Probably they experimented with the roll-off but couldn't get reliable triggering with it rolled off more.

Since there is more than one example in existence, I'd guess this is a design limitation that won't be fixed reliably without changing the circuit somehow.

Meanwhile, my guess is to try to eliminate possible sources of noise, as others have suggested.

Tried this but could not get it to work. Do I need to remove the over the air signal wire and replace with composite on pin 1? I just soldered composite signal to pin 1 and ground to ground
Jon-

There may be more to it than I made it sound like, as I just looked at the schematic diagram again. Yes, disconnecting the signal from the IF/detector circuit is likely needed, especially to eliminate those circuits (and the earlier tuner/IF stages) as a possible source of noise. However, I am not an engineer enough to know for sure if it will be that simple. At the least, I would recommend using an oscilloscope to look at the video signal that is there from the tuner/IF now; it could be more than the one volt that is standard from DVD players and so on, and it could even be inverted polarity. I will do such tests when I try that type of connection, and also, I have a couple of video test-signal devices (Sencore VA62, B&K 1077) that can output variable-voltage video signals as needed.

Oh, and thanks for quickly posting the TS-4D schematic link for old_tv_nut.

I have a question for you, as well: When you replaced the mica capacitors, does that include the two 900pF ones (c137 and C138 on that schematic, I think) that connect between the horizontal sweep signal outputs and ground? I think they are rated at 1000 or 2000 volts, large "domino"-type capacitors in my chassis. That is on my list of suggested possible fixes, but if you already tried them without success, they would likely not fix my set either.

When you replaced the mica capacitors, does that include the two 900pF ones (c137 and C138 on that schematic, I think) that connect between the horizontal sweep signal outputs and ground? I think they are rated at 1000 or 2000 volts, large "domino"-type capacitors in my chassis. That is on my list of suggested possible fixes, but if you already tried them without success, they would likely not fix my set either.

I did replace both of these. However I did not have the mica's on hand, I used 1000pf instead of 900pf, I don't know if this would have an affect.

Thanks

OK, I doubt that the 10% difference in value would affect the jitter problem, and if your set is working the same (no better, but also no worse), then they may be fine. Thanks for the update.

Hello Jon,
First off, I am not experienced or qualified enough to give anyone advise on fixing thier tv, but I do like to try to learn from these forums. So, for what its worth, my thought is could it be the crystal diode in the 2nd detector getting leaky?

Hi, I am not sure. I don't have any 1N34's on hand only 1N4007. They are cheap enough however might be worth a try.

I have also attached an image of the chassis as it is now.

I have still not got back into my TS-4D VT-71, but Decojoe made this comment in another discussion of VT-71s, so I have to wonder how widespread this issue is:
Nice work! I've only had one of these Motorola's that had a real crisp picture on it. My current set, and others I've had, have that "sizzling" or "pie-crusting". Not bad from a distance, but up close rather low definition.

Here is a close-up picture showing the problem as it appears on my set. It is being fed from a digital-TV converter on channel 3 or 4, with direct RG-6 coax into a matching transformer attached to the antenna terminals. (The picture shows up rotated 90 degrees, and I don't know how to fix that; I even made a copy and re-uploaded it.)

that is a verticle problem , i have 3 of these sets i restored and i would suggest looking close at the verticle section including micas in the verticle there are several for that section. i had the same lines you have there and i changed the micas in the verticle after i recapped the set thinking they were alright. looks like the frequency is off alittle seemingly consistant with one of those micas. i listened to everyone telling me that micas rarely go bad, well they do and they went bad on all 3 sets i got. there are 2 dog bones off the 6sl7 which can be replaced with ceramic discs, and the tie strip to the right of the 6sl7 there is another dog bone. the vert hold pot , there is a tie strap there in that area which has 1 mica and 1 dogbone, they should be changed as well. the chassis in 1 set i got which gave me the worst headache was the ts4d. and if you go back and change those micas and dog bones you should also go back and tweek the channel 3 coils, IIRC there is another coil there that will have to be tweeked as well there is not only 1. i wouldnt use a 1n4007 in place of the 1n34. whats up with those blue caps in the hv section there, there should be all those white caps.

I'd first suspect an HV arc/corona issue....I've seen that type of issue on electromagnetic deflection sets when the HV is arcing.

Micas most often go bad in the deflection section (where they take the most abuse), so they should be on the list under the "after everything else tests good or has been replaced in the ____sweep circuits if problems remain change the ____" heading. Never change RF/IF stage micas without first PROVING they are bad.

Rf & if I agree but take a look at the pic of the chassis, different caps for the vert and horiz, not a good match......

I'd first suspect an HV arc/corona issue....I've seen that type of issue on electromagnetic deflection sets when the HV is arcing.
The one catch with that is how, if the set is adjusted for a "horizontal split picture" as mentioned earlier in the discussion, the jitter goes away (and Jon was able to get the same effect on his set). Very odd. (That should reasonably rule out a weak/noisy signal too, it seems to me, but maybe not.)

Rf & if I agree but take a look at the pic of the chassis, different caps for the vert and horiz, not a good match......
That picture is of Jon's chassis; I used ASC tubular capacitors for all of my HV replacements and have the same problem.