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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 14, 2021 4:36 pm 
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Joined: Apr Wed 07, 2021 1:40 am
Posts: 6
Thanks again for the information and perspective of these electrostatic sets.

I was wondering about the idea of shielding this RFI from getting into other parts of the circuit and/or the CRT. What might be recommended
as some additional shielding? What materials and where might it go?

I currently have two working VT-71 sets which are made up of 4 1/2 different sets... I have seen evidence that others have tried to
do some kind of shielding on the side of the HV cage, mostly using cardboard and tape. It seemed to me like that would have been ineffective.
I also noticed that on two of the chassis that had original Motorola caps in the HV section, that two of them (horizontal HV caps) were always taped
together. Could this have been done to minimized interference? I am just guessing. In my theater sound system days, it was common to use Mu metal
shields around the magnetic pickup heads in a projector. Is that a material that would help in any way in shielding an electrostatic set?

Thank you!

Dave


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 14, 2021 7:44 pm 
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Joined: May Fri 29, 2009 4:35 am
Posts: 2274
Location: Chicago, IL USA
Here's a shot of the way they were shielded out of the factory.

Make sure the metal wrapped around the neck is making good contact with the bell shield and that it is grounded to the speaker frame.

Image


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Wed 14, 2021 10:21 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1629
For this type of problem, mainly it is the magnetic component of the radiated field interfering with the CRT's beam.

Many scopes and early TV's used mu-metal shields as they are particularly good at screening out low frequency alternating magnetic fields, like those radiated from power transformers, and to a degree direct constant magnetic field from permanent magnet sources. I don't know if they are quite as good with high frequency fields, but I'm not certain about that.

The fundamental principle of a shield, take a copper flux band for example wrapped around a power transformer, is the alternating field induces a current in the shield material, which produces a magnetic field equal and opposite to the one inducing the current. But for a constant magnetic field, a copper band would be useless.

In the Admiral 19A11S set, it used a power transformer. If you look at the design of the set, the transformer is at what looks like a peculiar angle on the chassis, they rotated that for minimum beam interference, radiated from the transformer, at the line power frequency and that was required even with the very good mu-metal shield they used. Of course transformer-less sets escaped this problem and some sets with the 7JP4. Pre-war sets with the 5AP4 and 5BP4 didn't bother with a mu-metal shield because the power transformer was spaced a fair way away from the CRT.

With a high frequency power supply the box or enclosure needs very tight fitting seams and a relatively thick wall. There is a good section on magnetic shielding in Terman's book. For a very effective shield ideally it would be two concentric boxes, but if course that would raise the cost, so its hardly every done.

With these sorts of interference issues, in the end in most cases, it requires experimentation to cure. You could try some copper foil , or aluminium foil around the crt neck and cone, or perhaps around the EHT cage. Copper is easier to solder together.


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Thu 15, 2021 3:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Crystal Bay, NV
While the discussion of possible interference is interesting, I do not think it is an explanation of the problem first reported.

The original description of the "interference" was "diagonal lines traveling through the picture" The key word here is diagonal.
Retrace lines could be described as diagonal. We never had a picture.

When Dave eliminated he video input (grounded the grid), the lines went away. Thus, we can eliminate signals reaching the CRT beam from unshielded sources.

Motrorola made over 100K of the 7" sets and I doubt that they would have been successful if many suffered from the problem described here,
Also this problem exists in both sets, if I read the first post correctly.

The change in the brightness had an effect on the direction of the lines, but then this could not be retrace lines.
The swap of the 1B3 made a major improvement, so that takes us back to the HV supply.

Very strange.

=====
Ron


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 Post subject: Re: Motorola VT-71 has diagonal lines moving through picture
PostPosted: Apr Thu 15, 2021 10:51 pm 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1629
TahoeTV wrote:
While the discussion of possible interference is interesting, I do not think it is an explanation of the problem first reported.

The original description of the "interference" was "diagonal lines traveling through the picture" The key word here is diagonal.
Retrace lines could be described as diagonal. We never had a picture.

When Dave eliminated he video input (grounded the grid), the lines went away. Thus, we can eliminate signals reaching the CRT beam from unshielded sources.

Very strange.

=====
Ron


Yes, if the lines do indeed go away with the electrical signal to the CRT's grid-cathode circuit grounded to AC voltages, it cannot likely be radiation to the CRT from the HV supply.

The lines I was referring to , from radiation, look a few mm or more wide, closely spaced often, slightly diagonal (tilted) and the wander around in position and width of the lines(number of them) exactly like carrier wave interference. With the shields properly fitted on the EHT box they are just visible with the the brightness set to medium, if you know what they look like. Without the shield, they are grossly obvious. So if anyone wants to see them easily, just remove the shield on the EHT box.


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