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 Post subject: Motorola VT-71 has diagonal lines moving through picture.
PostPosted: Apr Wed 07, 2021 2:42 am 
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Hi Everyone,

I have a couple of Motorola VT-71 TVs with the TS-4J (late) chassis in them. As needed, I replaced all of the paper and wax capacitors.
So the B+ and B++ are solid now as well as the coupling caps. I also replaced the selenium diodes and resistors that were more than 20 % out.
Also made diode style replacement ballasts for them.

Both chassis start up and run very well. I know the IF strips are not quite right and I am working towards that alignment, but before that, I wanted to address
this issue of diagonal lines traveling through the picture on both VT-71s. At first I figured it was RF noise in the tuner or IF strip but it may not be.
Because when I inject composite video into the grid of the video amp - the picture sharpness is amazing - but oddly, I still get these diagonal lines that travel
across the picture and fade in and out. So I'd say it's not in the tuner or IF area. All tubes have been tested and replaced / swapped in an attempt to isolate the
problem but that didn't effect the noise.

The 6000 volt caps in the HV circuit are a mix of .001uf ASC caps and .0047uf Sprague. So the Sprague's are N.O.S. but seem to be good (how to test 6k caps?). They don't seem to be leaky as I can see 600 volts on one side and only bias voltage on the other, so I think they are OK.

This may also simply be the best performance I can expect from these old gems... I don't know what to expect.

Does anyone have any thoughts on these VT-71 TVs?

Thanks,
Dave


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 07, 2021 3:54 am 
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These little Motorolas (and the 7" Admirals as well) can show amazingly good pictures when everything is right.

Can you please show us video of the set in operation so we can see first hand what you are describing?

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 07, 2021 4:07 am 
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What are you using as a video source? What you describe sounds like Macrovison and other protections schemes and garbage information that is in many DVDs. Not all DVDs have it but many do. Can you stream from the internet or try another DVD? There are ways of getting rid of it if that is the case.


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 07, 2021 6:00 am 
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There was a post from 2004 by David Kulka showing how to eliminate the retrace lines.

Let me attempt to provide a link:
http://www.antiqueradios.com/forums/vie ... ccc8d1f238[1/6/2011 12:08:07 PM]

Bob

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 07, 2021 12:27 pm 
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Location: Long Island, N.Y.
The Motorola VT-71 and similar models are good players when properly restored, yet are always finicky. Every one I have has it's own quirk of some kind. They don't like DVD's that have copy-guard like a lot of popular classic TV shows box sets. You'll get an intermittent buzz and scan lines. After collecting early post-war TV's for 30+ years, I've learned to be forgiving with them and not need to have them play absolute perfect.


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Thu 08, 2021 9:47 am 
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Have a look at this post. There is a picture in it of a TV with macrovision lines.
viewtopic.php?f=3&t=332446&p=2783185&hilit=Playing+DVDs+on+old+TVs+a+solution#p2783185
Is this what you have?

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Fri 09, 2021 2:22 am 
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davedrez wrote:
Hi Everyone,


Both chassis start up and run very well. I know the IF strips are not quite right and I am working towards that alignment, but before that, I wanted to address
this issue of diagonal lines traveling through the picture on both VT-71s. At first I figured it was RF noise in the tuner or IF strip but it may not be.
Because when I inject composite video into the grid of the video amp - the picture sharpness is amazing - but oddly, I still get these diagonal lines that travel
across the picture and fade in and out.

Thanks,
Dave



Dave, a video would help, it could be the macrovision lines as suggested.

I suspect it is possibly something else.

These sets have RF power supplies for the generation of the CRT's EHT and they run around the 100kHz region. Without the shield case around the supply area they produce markedly noticeable tilted fairly evenly spaced lines in the CRT image that move and drift. Even with the usual metal shield around the entire EHT cage, the radiation is not 100% shielded and even with the cage screws done up tightly, you can still sometimes see a trace of this type of interference coming & going in the image.


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sat 10, 2021 1:05 am 
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Thank you all for your thoughts!

I went back to study the issue again before I stated something that may not be true...

I have injected both RF into the antenna terminals and composite video through an isolation transformer into the grid of the video amp. The set itself is on an RCA isolation transformer as well. The test signal source is a Sencore VA48 so there wouldn't be Macrovision issues and I have done a retrace modification (the BANDERSENTV way) so it's not a retrace issue. So the source of these lines is "down stream" from the video amp.

I sent a blank video signal into the set so the screen is blank and I can switch over to a test pattern as needed. I let the set run with the blank screen for maybe a half hour in order to see if it changes as it heats up. The answer is - not really because these lines are always gently moving and reversing direction changing from a tight line structure into a wider mellow line structure, occasionally they just fade away completely then return. So my feeling is that it is noise - an oscillation that sums up and subtracts from the the sync driving the CRT.

It really does seem like RF noise from the HV oscillator and somehow it gets in there. Moving the CRT leads around will change it for sure but I have not found a position where the noise goes away and stays away. If this is RF from the HV cage (all the screws are in place and tight), any thoughts on shielding?

I'm gonna try plugging in a different CRT that is not mounted so close to the cage... will see if that works differently.

Will report back!

Dave


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sat 10, 2021 1:11 am 
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You might be picking up interference from nearby equipment. I was going nuts trying to track down the source of random "sparkle" interference on several VT71s. Finally realized it was coming from the over the air ASTC converter box I was using!


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sat 10, 2021 6:16 am 
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Location: Belrose, NSW, Australia
It does sound like noise from the 100kHz EHT supplies.

Precise choice and placement of capacitors around the HT and heater feeds to these supplies can be critical to keeping the signal out of the rest of the set. It's one of the reasons the idea of RF EHT supplies went the way of the Dodo.

Switched mode power supplies in early SS color TVs were often genlocked to the hor scan so that any noise from them would be stationary on the screen.

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sat 10, 2021 8:07 pm 
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the RF HV supply is not just shielded by a cover. The base is also insulated from the chassis and decoupled with .1mf capacitor and a 470K resistor. Perhaps your HV base has been connected to the chassis with a screw or something.
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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sat 10, 2021 9:03 pm 
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Also there should be a funnel shaped shield on the CRT that should be grounded. Normally grounded by connecting it to the speaker frame which is grounded by having one of the voice coil terminals connected to the frame. You then have to have the correct speaker wire connected to that terminal.

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sat 10, 2021 11:27 pm 
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Thanks again for everyone's input!

I ruled out the interference from another device in the room. I learned that one the hard way the other day. It took at little while to realize that the sudden degradation in picture quality was related directly to a 110v to USB adapter I had plugged in to the same power strip! I couldn't believe something so little could effect the performance of the set so greatly. Yes, these are very sensitive!

I plugged in my test CRT into the back of the chassis which moved it away from the HV cage. The results were the same as before so it's not RF next to the tube neck.
I then pulled the chassis back out and gave it all a good look. It's a very clean chassis and the HV is not accidentally grounded so we can rule that out.

I even tried to see if moving the HV capacitors (as much as the leads would allow) would have any effect and it does not.

However the one thing I did notice is that as I rock the brightness control gently, the speed and direction of the lines changes directly as the control is moved. So somehow these two things are related. The brightness control does not have high voltage going through it, but I might assume that as the brightness changes, the HV needed to drive the CRT does change. So, that's where I'm at. That is the only thing I could see that directly effects the interference lines. I would expect that as the brightness of a scene changes, so would the lines. That explains why the lines don't look so bad with a passive blank image, but are very distracting when there is material in motion displayed. Thus, I haven't been able to get a good picture that demonstrates this effect.

As I mentioned, the HV caps have been replaced but now I'm wondering if one of them might be leaky? There is no good way to test for leaks at full voltage. I don't want to assume that just because the caps are new or NOS doesn't mean they are up to specification. I have some 6KV disc caps with the correct values, and maybe installing those is worth a try. Big project to get those in and out though...

Any thoughts about the HV caps? Is there a good way to test without just a replacement?

Thanks so much,
Dave


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sun 11, 2021 7:37 am 
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OK so from your last posts we now know the issue is noise getting into the video from the HV supply.
You can rule out leaky caps, the HV is working. Bypass caps on the B+ and heater lines are another matter. What did you replace these with? If you replaced some disk ceramic caps with polyester, for example, the RF bypassing may be inadequate.

Try shorting G1 of the video output tube to chassis with a very short clip lead. Does the noise go away or is it still there?
If it goes away it's getting in ahead of the video amp (vision IF stages). If not, it's probably coming in via the power supply or the heater wiring.

Also check that the shield covers on the HV system are clean and correctly fitted. The trouble with RF HV systems is the RF energy gets radiated through the air and gets into everything. Hence the importance of through shielding.

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Sun 11, 2021 3:20 pm 
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Disk ceramic HV caps work OK as HV filters on 7" sets, but there have been reports they don't always work well as the coupling caps, particularly in the vertical circuit. So if you do use any disk ceramics, I'd limit them to the HV filters and use new film type caps (ASC or similar) for the rest of them. If your old stock Sprague HV caps have red lettering on them they are probably OK, if they have yellow lettering they are too old to trust even if they were new.

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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Mon 12, 2021 3:29 am 
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Thanks very much for the tips...

I tried grounding the grid of the video amp to B- and the lines I have reported went away. I was surprised because I see the noise when I inject composite video into the same grid. But never the less, this tells something about the set.

Regarding the capacitors: I re-stuffed the the multi-section caps with new components, mostly Nichicon and replaced the selenium rectifiers with diodes. That restored the B+ and B++ to where it should be. So, in short, all of the electrolytics are new. This chassis was very clean with most of the original paper and wax caps in place.
I replaced all of the coupling caps with the yellow (film) axial lead style made by Illinois Capacitor. I did not replace any mica or disc caps. I did replace some resistors that seemed to be too far out of specification. Then working my way down to the High Voltage section, I replaced the 6KV caps with the NOS Sprague (.0047uf) and ASC (.001uf). A few tubes were bad as well. That's about the extent of the work. The chassis fired up right away and it looked great- except for these lines that move, likely from the RF of the HV supply. The IF strip is in OK alignment but I thought I might cautiously look into tuning it up. Maybe the lines are coming in through the IF strip,
but there isn't anything about it that isn't the way it was. All the tube shields are in place as well.

As for the High Voltage cage, all the original screws were still in place! I did not try swapping the HV rectifier (1B3GT) maybe I should, but either it works or not, right?

I'm kind of out of ideas, other than the IF tuning and a tube swap. Maybe, this is simply the way they work, I don't know. In any case, it's been a fun project!

Dave


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Tue 13, 2021 3:32 am 
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One last follow-up on the moving lines...

As mentioned previously, I had not swapped out the 1B3GT tube which is the HV rectifier. I did have a similar 1B3GT in a box, however it was the shorter
version (newer or older version?) I removed all the screws from the HV cage and remembered that I had looked inside there when I first started the project
but was going to wait until I had it running before I did anything, such as test the tube. It was very clean in the cage and this was the original tube because this version of the set had the spring around the tube glued in place. This chassis came out of a VT-73 which was the "portable" version.

So I swapped the tube, put the cover back on and fired it up. Yup, the lines were almost completely gone! Great picture! I am calling this project finished...

I am surprised that the rectifier tube, as simple as it is, worked well but allowed these oscillations or ripple to get through. I figured that either it worked or it didn't but
this is what I found - there are subtleties to the performance of even a rectifier tube.

Thanks for all the help - try some tube swapping, maybe you'll get lucky.

Dave


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Tue 13, 2021 12:44 pm 
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davedrez wrote:
O

I am surprised that the rectifier tube, as simple as it is, worked well but allowed these oscillations or ripple to get through. I figured that either it worked or it didn't but
this is what I found - there are subtleties to the performance of even a rectifier tube.

Thanks for all the help - try some tube swapping, maybe you'll get lucky.

Dave


Generally, in these RF power supplies, the feedback is sustained by a metal clip around the rectifier glass envelope. Its ideal position is such that is sustains oscillations, but the feedback is not too heavy. If it is too heavy, the effective grid drive to the the amplifier tube is too high and this encourages additional radiation. So even changing to a different rectifier can alter the dynamics here and result in higher order harmonics as the output tube is pushed into saturation, or less, simply due to the internal architecture of the rectifier tube and the feedback capacitance from the spring clip resulting in higher feedback. None of this is mysterious, just basic Physics.

As I mentioned though, this method of EHT generation is never 100% interference free, it works as long as the balance of factors is ideal, and the shielding reasonable.

As a side note, exactly the same issue, but manifest in a different way, occurs in the Tek 464/466 series oscilloscopes that use a sine wave oscillator to generate EHT. The designers figured out (quite rightly) that a square wave smps could never do the job as the RFI would be objectionable. Still, its not "perfect" and the shielding not 100%, so if you look closely at the beam it is possible to see the radiation from the EHT supply manifest as a small sine wave modulation. To fix this in my 464 and 466 scope I added a shield plated over the existing vertical amplifier sections.


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 14, 2021 1:23 am 
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I have about a dozen 7" sets with RF HV supplies, and I have not seen any interference like described here. So, no, I do not think this is a problem common to electrostatic sets. The HV problems are often caused by corona, but I have not seen a 1B3 create interference.

I would not bet my experience or knowledge against some notable members of this forum (you know who your are), but I do not think this problem is totally solved. But, if you are happy, Dave, that is great.
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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 14, 2021 12:41 pm 
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TahoeTV wrote:
I have about a dozen 7" sets with RF HV supplies, and I have not seen any interference like described here. So, no, I do not think this is a problem common to electrostatic sets. The HV problems are often caused by corona, but I have not seen a 1B3 create interference.

I would not bet my experience or knowledge against some notable members of this forum (you know who your are), but I do not think this problem is totally solved. But, if you are happy, Dave, that is great.
====
Ron


When there is direct electromagnetic radiation to the CRT beam, a lot of factors come into it relating to the orientation of the RF power supply coil, the position of the RF power supply box to the CRT, the intensity of the EM radiation, the CRT shield effectivity and the shielding around the EHT box & coil. All these things are specific to any particular model.

It is not related to interference on the wiring leading in and out of the EHT box, bypass capacitors and their quality cannot help the problem. And it is not directly related to the 1B3 itself.

So you may find a number of sets (that typically use the 7JP4 & an RF power supply), depending on the exact design, where the beam interference is so negligible it cannot be detected, but in other cases it is detectable.

Both the Motorola VT-71 and the Admiral 19A11S suffered from this problem to varying extents.


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