Argus Antics

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Argus Antics

Postby Panrock » Sun Feb 21, 2016 4:29 am

I've been waking up my Argus again, with an eye to the forthcoming Convention.

First off, I've made a magnifier from my old 7-inch bench lens... and a flowerpot !

The signal source is the Aurora World Converter. Note the too-slow frame flyback. For some reason this is only a problem on some of the standards. Constructional details are http://www.radiocraft.co.uk/agus.htm. I have however improved the frame linearity by messing around with the little capacitors between the anodes and grids.

This photo shows it running on the 'German' 180-line standard. By twiddling the line hold, I can view anything from 120 to 240 lines. However, when the frame rate of the standard in question is 24Hz rather than 25Hz, I can see rippling on the picture due to hum. This hum is not due to the power supply, it seems to be magnetic interference from the mains transformer into the tube, not completely soaked up by the mu-metal screen.

I am wondering whether I should further modify the timebase and its sync arrangements, so I can explore the 'mechanical' standards below 120-lines.

Steve O
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Re: Argus Antics

Postby Klaas Robers » Sun Feb 21, 2016 8:17 am

Steve, I don't know how "open" your mains transformer is. It helps if you construct a winding around the whole transformer, including the iron yoke. I did this in my pseudo Heathkit SSTV monitor and the hum is not completely gone, but it became much less. I have no mu metal shield for the picture tube.

Look to the photo's in the thread "Off Topic", "Retro SSTV anyone?" and then page 14.

The winding is of a single thicker copper wire and it is shorted (visible) from the end to the beginning. I have also seen somewhere a single shorted winding made of copper foil, not too thin and soldered end to beginning. Electricity wire is easier. Eddy current will compensate for the the stray field of the transformer.
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Re: Argus Antics

Postby Harry Dalek » Sun Feb 21, 2016 3:08 pm

Great monitor there Steve !
On the mu metal Klaas was mentioning i came across this on another forum .
You have probably heard about something called mu-metal that does provide magnetic shielding. It does exist, but it is fantastically expensive; it's only really practical around things like CRTs in many-thousand dollar oscilloscopes.
Cold-rolled steel is an inexpensive substitute for mu metal. I learned about from a consultant, an engineer familiar with physics, who was helping us shield a monitor lab from the main electrical boxes on an outside wall. If you have room for several layers its a practical solution. Steel plate is also used to shield adjacent areas from magnetic fields generated in MRI imaging.

I noticed you have a shielding around your Argus CRT Steve did you make it your self ?

Reading and noticing now what i didn't before the transformers in some old tvs had shielding around the mains transformer which i suppose is doing this instead of the crt and what i think klaas has done in a different way on hes SSTV monitor .
Wonder as in this you tube video you could do the test with a transformer .
https://www.youtube.com/watch?v=OHEzD7uP6hI
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Re: Argus Antics

Postby Panrock » Sun Feb 21, 2016 10:07 pm

Klaas Robers wrote:Steve, I don't know how "open" your mains transformer is. It helps if you construct a winding around the whole transformer, including the iron yoke. I did this in my pseudo Heathkit SSTV monitor and the hum is not completely gone, but it became much less. I have no mu metal shield for the picture tube.

Look to the photo's in the thread "Off Topic", "Retro SSTV anyone?" and then page 14.

The winding is of a single thicker copper wire and it is shorted (visible) from the end to the beginning. I have also seen somewhere a single shorted winding made of copper foil, not too thin and soldered end to beginning. Electricity wire is easier. Eddy current will compensate for the the stray field of the transformer.

Hi Klaas,
Thanks for this idea. It is most interesting and yes - I will now give it a try on my transformer, though it won't look as neat as on yours! Presumably this winding must run perpendicular to the active transformer windings (as indeed it appears to in your case) or it would act like loosely coupled shorted turns and increase the loading..?

Harry Dalek wrote:Great monitor there Steve !
On the mu metal Klaas was mentioning i came across this on another forum .
You have probably heard about something called mu-metal that does provide magnetic shielding. It does exist, but it is fantastically expensive; it's only really practical around things like CRTs in many-thousand dollar oscilloscopes.
Cold-rolled steel is an inexpensive substitute for mu metal. I learned about from a consultant, an engineer familiar with physics, who was helping us shield a monitor lab from the main electrical boxes on an outside wall. If you have room for several layers its a practical solution. Steel plate is also used to shield adjacent areas from magnetic fields generated in MRI imaging.

I noticed you have a shielding around your Argus CRT Steve did you make it your self ?

Hi Harry,
This is a mu-metal shield originally manufactured for VCR97-style tubes. However, at one time I painted it with Hammerite, which later I removed. Getting the Hammerite off was difficult, involving some serious scraping, and I later learned that mechanical shock can reduce mu-metal's permeable properties... as is also stated in your reference material.

I built the Argus back in 1990, but construction was interrupted by a back-packing trip round Asia, including a stint up the east coast of your lovely country from Sydney to Cairns then across to Darwin. It was a great trip but what was I really dreaming about all that time...? You guessed it - the Argus! :oops: I couldn't wait to get back and finish it.

Steve O
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Re: Argus Antics

Postby Panrock » Mon Feb 22, 2016 1:24 am

Enthused by Klaas's suggestion, I have wrapped the mains transformer in three rings of foil, each insulated on the back. See pic.

Since there is another mains transformer below decks - the EHT transformer - I also tried reversing the mains input to the first transformer, in case their fields were tending to add rather than cancel.

To be honest, not a lot of difference has been made. I attach a photo of the 120-line 24Hz picture. It's cropped at the sides because there is a 6:5 aspect ratio on this standard.

Steve O
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Re: Argus Antics

Postby Klaas Robers » Mon Feb 22, 2016 2:38 am

No Steve,

the short circuit coil or foil should be parallel to the windings of the transformer and it should include both outer legs of the transformer core. In your case it should go over the top and under the bottom of the transformer, such that it hides the paper or plastic insulation layer wrapped around the windings. As far as I can see it, it should be at the left and the right of the transformer, as it is mounted in your monitor. You can look through it from front to back.

The trick is that it forces the magnetic field, emitted by the central leg of the iron core, to go back through the side legs and not through the air. That last is the stray field of the transformer.

Do not try to place it through the narrow split between the windings and outer legs of the core, because then you produce a short circuit winding of the transformer itself.

Very good that you made a photo of it, now I could see how you made it.

What I asked myself when looking at the circuit diagram: Is this TV receiver tuned to VHF channel 1, 2 or 3 only? I missed an oscillator and a mixer. Or is it only an IF-receiver?
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Re: Argus Antics

Postby Panrock » Mon Feb 22, 2016 3:54 am

Klaas Robers wrote:No Steve,

the short circuit coil or foil should be parallel to the windings of the transformer and it should include both outer legs of the transformer core. In your case it should go over the top and under the bottom of the transformer, such that it hides the paper or plastic insulation layer wrapped around the windings. As far as I can see it, it should be at the left and the right of the transformer, as it is mounted in your monitor. You can look through it from front to back.

The trick is that it forces the magnetic field, emitted by the central leg of the iron core, to go back through the side legs and not through the air. That last is the stray field of the transformer.

Do not try to place it through the narrow split between the windings and outer legs of the core, because then you produce a short circuit winding of the transformer itself.

Very good that you made a photo of it, now I could see how you made it.


I'll study what you have said and come back to you on this when I next get a mo'.

Klaas Robers wrote:What I asked myself when looking at the circuit diagram: Is this TV receiver tuned to VHF channel 1, 2 or 3 only? I missed an oscillator and a mixer. Or is it only an IF-receiver?


It's a single channel TRF tuned to Channel 1. So yes, it's a bit like an IF receiver with a vision IF of 45MHz...

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Re: Argus Antics

Postby Panrock » Mon Feb 22, 2016 5:08 am

Right Klaas, I'm back.

I attach a photo showing the transformer in your lovely unit, and another picture of mine - but this time with two of the three foil rings removed so you can see how my transformer sits.

My transformer's windings are on their side compared to yours, so aren't my foil rings actually placed correctly?

Steve O
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Re: Argus Antics

Postby Klaas Robers » Tue Feb 23, 2016 8:44 am

No Steve, they should run over the yellow paper band and over the top and the bottom of the transformer, as it is mounted now. When they are in place, the only thing you can still see completely is the pertinax plate withe the connections and the voltages impressed and printed. And may be there is something comparable on the now invisible back side.

And be sure that it is firmly short circuited. It is not the material itself that shields the magnetic stray field, it is the induced current in the ring. So the ring should be firmly closed, e.g. by soldering.
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Re: Argus Antics

Postby AncientBrit » Tue Feb 23, 2016 8:18 pm

Steve,

Very neat use of the flower pot for a tapered viewing hood.
(Doubtless the programme material will include an episode of the 'Flowerpot Men' !)
I must admit I have spent time in Poundland on a similar search for a non magnifying hood for a small scope tube.
Also plumbing supplies have also proved fruitful.

Cheers,

Graham
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Re: Argus Antics

Postby Panrock » Tue Feb 23, 2016 9:17 pm

Thanks for the nice comments as ever, Graham. I bought three such flowerpots at B & Q; they were very cheap. I cut out the outer rim from the second one then pushed it inside the end of the main one, + some glue, to act as a lens retainer. I was very lucky to find a housing that fitted my lens so accurately.

Unfortunately it doesn't look like it will be practical to install the winding in the way you suggest, Klaas. I also tried 'hum-bucking' yesterday by placing a varying number of spare 4v windings on the transformer in series with the timebase HT feed. Obviously tried 'em both ways round. Still no luck. To tell the truth, the results are okay on the 25Hz standards already. I watched a programme on it (not the Flowerpot Men!) yesterday on 240-lines/25Hz and it looked almost HD! The only difference being the greater flicker from this non-interlaced standard.

I am now thinking I will bring my 1960 Philips 17TG100-U to the convention instead, and demonstrate my optical link on 405-lines. Jeremy has already approved the plan to show 405, and the display will be so much bigger and brighter.

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