5AHP7A Vintage SSTV The Deep image

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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Thu Feb 02, 2017 5:13 pm

I am not sure what you call them but i will call them magnetic beam position rings on the deflection yoke i have them on the yoke and i noticed how much effect both rings have on the beam as in that last test posting .

Klass if you see this i don't see them on your yoke ?

I think i will remove these when i have a chance .....even way back on the tube as in the picture it still effects the beam !
I would expect the scan to be much better with out them.

Just moving the second ring way back there.... back and forth the the scan line follows as touch so i think this is the problem.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Sat Feb 04, 2017 9:06 pm

Harry, the rings are there to place the picture in the centre of the screen. But they should not be at the botton of the tube, but almost against the deflection unit. Both rings are magnetized and it works as follows:

- if the rings are placed in a certain orientation both magnetic fields will cancel.
- then, if you rotate them simultaniously, nothing happens with the position of the scanning field.
- First try to find this mutual orientation of the two rings
- and mark both rings in some way (felt pen?).

- Now if you rotate both rings, one to the left, the other to the right, The resulting magnetic field comes on.
- This will move the scanned area in a certain direction.
- Then you can again rotate both rings together to get the direction of movement where you want it.
- The centre of your picture (scanning field) moves in a (small) circle over the screen.

So to recapitalise:
- always rotate both rings at the same time,
- in different directions to increase or decrease the displacement (size of the said circle),
- in the same direction to change the direction of displacement (positon on the circle).

The rings have no influence on the scanning geometry or scanning sizes. This is an adjustment that you do at the end, when you have a picture on your screen. In fact this is a correction on the magnetic field of the earth. If you replace the TV the TV-picture is no more centered on the screen. With the rings it can be recentered.

The same result can also be got by applying some DC current in the coils, but in TV sets there was never any DC in the deflection coils, they were AC coupled. For SSTV this is normally not the case and you can shift the picture over the screen by some trimming potentiometers. So the rings are not needed and if you place them in the "neutral" position they don't harm either. Only if you replace the monitor in the earth magnetic field, they can be used.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Sat Feb 04, 2017 9:34 pm

For larger TV picture tubes there are also magnets used, that you have to place close to the cathode around the neck of the tube. They are called "Ion Traps". At higher acceleration voltages it may happen that positive ions of the phosphor on the screen are kicked off and travel back to the for them heavily negative cathode and destroy it after some time. This effect is called cathode poisoning.

To prevent this the cathode and the control grid are placed under a small angle in respect to the larger part of the electron gun. It is no more than 10 to 15 degrees, but you can see it very well. The electrons are then "bend by 10 degrees" by a permanent magnet placed around the neck of the picture tube. This is adjusted for maximal brighness on the screen. Ions that travel back are far too heavy to be bend back by this relatively weak magnetic field and collide against the grid (a plate with a small hole in it, they mis the hole).

This ion current is very small, so it doesn't disturb the functioning of the picture tube, but the ion material is poisonous for the cathode material.

This trick is from the B&W television era, the radar picture tubes are older and they don't need this type of magnet. Besides that, I can't see the "bend" in the electron gun in your photographs, so you dont need it. It is also not for my 5FP7.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Sun Feb 05, 2017 12:14 am

HI Klass
Thanks for the reply on the position magnets ..i was not sure as they were really there for the past 50s large CRT not really made for this p7 tube.
i only had the magnets at the back of the tube to see how much they effected the scanning .
I have for now removed them as i needed to know whats causing the bend on one side of the scan...its a touch better without them but the bending is still happening...i have tried reversing the horizontal coil the bend is then just on the other side of the scan ..
I don't notice it or see it if i swap the horz for the vertical coil but the horizontal scan line is much smaller less wide so it could be this why i don't see it as well.
Can only think the bend now is due to the shape or misshape of the sawtooth .
Trail and error testing here one thing at a time
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Tue Feb 07, 2017 6:42 am

Harry, if possible try to rotate the deflection coil while it is working, or do it in small steps. Observe what happens on the screen. As the cathode ray tube is circle symetric it is expected that the shape of the scanning field is not affected by the rotation of the deflection yoke, so the distorted square keeps its distortion, in whatever position the yoke is. This prooves that the deflection error is in the yoke, or the sawtoothed deflection currents.

If not, if the shape of the distorted square changes, there is something in the picture tube or its surroundings. May be that you used iron bolts in the front of the monitor, that are magnetized. Such a magnetic field may distort the square, but it you rotated the yoke by 180 degrees, so all acannings are reversed, the distortion is still on the same position of the screen. Then search for this disturbing magnetic fields, and if you found them, try to eliminate them.

If the pattern on the screen simply rotates with the rotation of the yoke, then the error has to be the yoke itself, or the deflection voltages and currents. For me this would be the next step.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Tue Feb 07, 2017 4:44 pm

Klaas Robers wrote:Harry, if possible try to rotate the deflection coil while it is working, or do it in small steps.


Yes it is Klaas i made so i have room to rotate it ,i noticed that shape raster i have does move around the tube rotating the deflection yoke.

Observe what happens on the screen. As the cathode ray tube is circle symetric it is expected that the shape of the scanning field is not affected by the rotation of the deflection yoke, so the distorted square keeps its distortion, in whatever position the yoke is. This prooves that the deflection error is in the yoke, or the sawtoothed deflection currents.


I sure hope its not the yoke not something easy to fix i think it would be a nightmare ...lets hope its the electronics !!! i did notice reversing the line deflection yoke connections the scan raster distortion is reversed ,,,i can't think if that shows up any thing about the yoke if it is a yoke coil problem .

Since i last posted i have put together another saw tooth circuit and almost finished the deflection part which will be done later in the week ...the sawtooth circuits are pretty much a SSTV version of Steves 3bp1 circuits with a standard lm741 power trany deflection driver circuit .i was thinking the only way i will know for sure is change the circuit ..the saw tooth in this circuit is a very good sawtooth waveform that can only help things.


If not, if the shape of the distorted square changes, there is something in the picture tube or its surroundings. May be that you used iron bolts in the front of the monitor, that are magnetized. Such a magnetic field may distort the square, but it you rotated the yoke by 180 degrees, so all acannings are reversed, the distortion is still on the same position of the screen. Then search for this disturbing magnetic fields, and if you found them, try to eliminate them.


I was thinking it might be that but didn't think of something in the monitor case ...the EHT cable is close to the tube and yoke i was thinking it might be that a while back but i moved it away ffrom the tube and it was the same .


If the pattern on the screen simply rotates with the rotation of the yoke, then the error has to be the yoke itself, or the deflection voltages and currents. For me this would be the next step.

[/quote]

If the distortion was in the same position or caused another shape when i reversed the yoke coil connections i would think it was some magnetic field but its just a reversed raster image so i would think the its just showing the reversed wave form driving the coil what i don't have yet is another deflection circuit to see if it improves things ....i would be head scratching if this new one gives the same results later this week ....

Thanks Klass the advice sure helps ...i will keep at it .
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Thu Feb 09, 2017 5:38 am

Very good Harry, this helps. If reversing the connections to the coils (for H and for V) gives the same scanned field, but virtually rotated by 180 degrees, It is unlikely that it is the deflection yoke, nor a parasitic magnetic field in the enclosure.

I have seen two circuit diagrams of the saw tooth generators and of the deflection amplifiers. Any way you should choose the deflection amplifiers that feed in a voltage into one of the ends of a deflection coil, while the other end goes to a resistor to ground. The voltage over that resistor is then fed back to the input of the deflection amplifier. This voltage represents the CURRENT through the coil. That current should have a perfect saw tooth form.

An other circuit that I have seen from you is a simple voltage output without any current sensing and feed back. That is a bad circuit, because the deflecting magnetic field is caused by the CURRENT through the coil and NOT by the VOLTAGE over the coil. The coil is not a simple resistor, but has also inductivity, which alters the current if you apply a certain voltage.

The feed back voltage coming from the current sensing resistor (some 5 ohms or so) should in some way go back to the minus input of the operational amplifier. Some times it is needed to solder a resistor of about 100 ohms in parallel to the deflection coil, to prevent oscillation. I see that I also placed a capacitor of 10 nF parallel to the 100 ohm resistor. If you like, I can give you the circuit diagram of my deflection amplifiers, which are without any opamp, but using discrete transistors. However they run on + 6 and - 6 volt, but can be simply redesigned for + and - 12 volt.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Thu Feb 09, 2017 6:50 am

Harry, this is the circuit diagram of my deflection amplifiers. I use two identical, one for H and one for V. For you the circuit is recalculated for + 12 and -12 volt.

The transistors are BC 549, BC 548 and BC558, but you may replace them by other good LF transistors.
The BD 135 - BD 136 can also be 137 - 138 or 139 - 140. They need a small heatsink.

Before you switch on for the first time place the 2k2 varable resistor in the zero ohm position. This sets the idle current of the power transistors to zero, so no large current is drawn from the supply voltages.

The input voltage should be a saw tooth of - 1volt to + 1 volt, symmetrical around zero. An input voltage of 1 volt will cause a current of 300 mA through the deflection coil. If that current is too large or too small for maximum deflection, replace the current sensing resistor of 5 ohm. Make it smaller for a larger deflection, or larger for a smaller deflection. For small corrections adjust the input voltage.

You will see on the screen that in the middle of the scan the spot is somewhat brighter. That is the cross-over distortion of the amplifier. This can be made smaller, almost invisible, by adjusting the resistor of 2k2. However, don't adjust it further than needed, that will cause unneeded current drawn by the power transistors, they will heat up. Later when the monitor is running with video this is better visible, so you may delay this adjustment to later and keep the resistor on zero. This is safer for the time being. In the mean time you see a thin horizontal and a thin vertical line in the middles of the vertical and the horizontal scan. This also indicates the centre of your picture. In my monitor this is still slightly visible, if you know that it is there.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Thu Feb 09, 2017 10:48 pm

Thanks for the circuit there Klass the one i hope to be using is similar in a way in the driver side to yours ,i would of made yours as is if i remembered that you may have posted it in one of the sstv posts ,i should of had a look before i started

I will use it at some stage it might be best to change the original driver circuit to yours and give it a go .

The one here is almost finished just have to see i have made no mistakes .i will post up results
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Fri Feb 10, 2017 6:56 am

Harry, do these deflection amps use a low ohmic resistor from the bottom of the deflection coil to ground? And feed back from that point to the input of the opamp?

Those are the better circuits.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Sat Feb 11, 2017 5:04 pm

Klaas Robers wrote:Harry, do these deflection amps use a low ohmic resistor from the bottom of the deflection coil to ground? And feed back from that point to the input of the opamp?

Those are the better circuits.


IT did Klass but testing the circuit today it was no where as good as the last one to the point where the odd resistor was over heating have not had that happen in a while ...well i was not pleased with the result of this one so taken the deflection circuit off that board left the sawtooth circuits and i will now try yours with it .
I Still have the first circuit just put aside for now ..while trying these .
So see how yours go's ,i don't mind the building of them i enjoy wiring soldering ...you gave such a good circuit description least i can do is try it ! :wink:
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Sun Feb 12, 2017 7:40 pm

I put together Klass your Deflection circuit i am having trouble with the vertical side but the horizontal seems to work .

I had to put a capacitor between the input resistor to the deflection circuit as a direct connection cuts out the sawtooth oscillator other wise ,funny enough it does not seem to be the case for the vertical side as i can direct connect that with out it effecting the saw tooth oscillator .

The size of the deflection line is what is shown as i am adjusting the 2k trimmer since i am using a different yoke with higher resistance to the coil than yours i may have to adjust those 100 ohm resistors across the yoke coils .

i heard a crack when i switched it on i think one of the vertical side transistors might of blown good reason nothing is outputting to the yoke there ...easy to make a mistake never know i will double check /

The scope is on 2 volts per div on the second gif and thats whats outputting at the deflection coil ...
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Mon Feb 13, 2017 4:05 am

Harry, the deflection amplifiers expect at their inputs a sawtooth that goes from -1 volt to + 1 volt. For the vertical deflection this is a kind of DC voltage that very slowly rises or drops. If you soldered a capacitor in that signal path, the slowly changing voltage is converted to a DC voltage of about 0 volt. So that will not work.

Use you oscilloscope to check the input signals to the deflection amplifiers. They should be linear sawtooth voltages going from -1 volt to + 1 volt and/or reverse. Then look at the voltage across the resistor at the bottom of the coil. This should have almost the same shape, although somewhat larger or smaller. The VOLTAGE going to the deflection coil may have a strange shape. But it is not the VOLTAGE that does the deflection of the beam, it is the CURRENT. This current goes to ground through the resistor of 5 ohm and makes again a voltage of it. In this way you can monitor (see) the current.

If the horizontal deflection works, you may test the vertical deflection by connecting the wires of the vertical coil to the output of the amplifier. Of course after disconnecting the wires of the horizontal coil.

The resistor of 100 ohm is indeed dependant of the resistance of the coil. It has a value 10 to 20 times the resistance of the coil that he is switched in parallel. But if one of the coils has a resistance much higher than say 10 ohms, then see if you can modify the coil. Those coils are always made up of two coils, that are switched in series or parallel. I remember that you wrote about a coil of 40 ohm. That will be two half coils in series. If you circuit them in parallel, the resistance drops to 10 ohm.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Mon Feb 13, 2017 9:03 pm

Klaas Robers wrote:Harry, the deflection amplifiers expect at their inputs a sawtooth that goes from -1 volt to + 1 volt. For the vertical deflection this is a kind of DC voltage that very slowly rises or drops. If you soldered a capacitor in that signal path, the slowly changing voltage is converted to a DC voltage of about 0 volt. So that will not work.


Hi Klass i have uploaded here Steves NBTV sawtooth generators i have changed R704 fixed resistor to a trimmer and also R725 the horizontal i have kept the cap the same as the frequency is near on the same the vertical i have slowed down with a changed cap marked with the trimer i can adjust from a 1 sec to a touch over 8 secs...
I connected the horizontal sawtooth from this circuit to a 10k trimmer which i replaced your 6k input resistor ...i was thinking adjusting the size of the saw tooth may be like size control and you wanted it reduced to 1 volt but testing even though it does adjust the sawtooth it has little effect on the deflection line .
Just connecting this horizontal sawtooth to your directly the steves circuit sawtooth stops perhaps a higher resistor may be the answer if you think the capacitor a 10uf i placed between this circuit and yours is not a good idea ..any case as is the deflection is sort of working but i will work on it later in the week.


Use you oscilloscope to check the input signals to the deflection amplifiers.


i have done this after the first transistor 549 there is a vertical sawtooth on the output on the vertical side but funny enough none of the horizontal which has me confused as its the one working and the other not... :roll:

They should be linear sawtooth voltages going from -1 volt to + 1 volt and/or reverse. Then look at the voltage across the resistor at the bottom of the coil. This should have almost the same shape, although somewhat larger or smaller. The VOLTAGE going to the deflection coil may have a strange shape. But it is not the VOLTAGE that does the deflection of the beam, it is the CURRENT. This current goes to ground through the resistor of 5 ohm and makes again a voltage of it. In this way you can monitor (see) the current.


Reason i want to try Steves circuit for this as its pretty clean sawtooth ...Yes the shape of the waveform on the coil does look bit off but i was thinking the yoke coil will do that .,,the result a touch smaller line than i was expecting so far but its a start and that did look ok .

If the horizontal deflection works, you may test the vertical deflection by connecting the wires of the vertical coil to the output of the amplifier. Of course after disconnecting the wires of the horizontal coil.


I will try that and see.

The resistor of 100 ohm is indeed dependant of the resistance of the coil. It has a value 10 to 20 times the resistance of the coil that he is switched in parallel. But if one of the coils has a resistance much higher than say 10 ohms, then see if you can modify the coil. Those coils are always made up of two coils, that are switched in series or parallel. I remember that you wrote about a coil of 40 ohm. That will be two half coils in series. If you circuit them in parallel, the resistance drops to 10 ohm.
[/quote]

I would expect adjusting this to help the deflection line size , but yes also i need to look into your procedure on the yoke coil i will look into this it has a cover i think i have to remove the yoke from the crt to get at the connections i do see your yoke coil readings are much lower than mine it would be good if i could get closer to yours.

I will look into it all later in the week when i have more time .
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Steve Anderson » Mon Feb 13, 2017 11:49 pm

Sorry to say Harry but the mixture of the NBTV sawtooth generators and Klaas's deflection amps are not going to work at all as they stand.

First off as Klaas has said more than once, the deflection amps need a +/-1V signal. There is no way my ramp generators can provide this, they are unipolar, positive going only. They were designed for primarily electrostatic tubes but the deflection amps could be modified to cope, but it would probably be messy.

Second, the output of the ramp generators need to feed a very high input impedance deflection amp, Klaas's with good reason are certainly not.

This is where cut-and-paste falls flat on its face. You have to start from left-to-right ensuring every required condition for each stage is met. Here I'm afraid that is certainly not the case.

Where to go from here? Stick with Klaas's deflection amps which are proven and re-design the ramp generators or use Klaas's versions.

Not wishing to sound too harsh, in future ask if these two arrangements (or whatever) will work together before switching on the soldering iron and wasting time and potentially components. Even modifying timing components as you've done here often isn't as simple as it may seem.

Steve A.
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