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Ekco T.U. 211, A possibly pointless restoration!

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Cathovisor
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Lloyd said
Well, those resistors are probably working right on the edge of their voltage rating! I've looked them up on Rapid's website, and they are rated max working voltage 250V, max overload 500V. It doesn't state AC or DC. Anyway, I'll not use the set until I've got replacements.

Use Vishay VR37 or HVR37 (3500V DC) - in both cases the RMS ratings are √2 less. There are also equivalents by Welwyn.

 
Posted : 22/05/2017 9:27 pm
Nuvistor
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At least you have it under control now. 

Frank

 
Posted : 22/05/2017 9:54 pm
Lloyd
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Cathovisor said Use Vishay VR37 or HVR37 (3500V DC)

Thanks for that, got some on order now thumb_gif

Regards,

Lloyd.

 
Posted : 23/05/2017 11:35 am
Forum 142
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Lloyd said
It was interesting to note the effect of A1 voltage on the picture, lower voltage made the picture more contrasted, but poorer focus, and higher voltage made the contrast more washed out, and flyback lines appeared. Now that's where things get unclear to me, as in the link above, it says that the A1 voltage needs to be +200-400V with regard to the cathode, so does that mean that I would be measuring between the cathode and A1? I was assuming it would be between chassis and A1. Also, the cathode in this set sits at about +160V.

By universal agreement in the industry all CRT electrode voltages are measured WRT (With Respect To) the Cathode.

To cut-off the beam the grid (also called Grid1) must be negative (WRT the cathode), by 40 - 80V typically.

For video display (i.e. TV and the like) there's two ways to drive the CRT, Cathode or Grid. The cathode is more common, as it requires less drive voltage swing compared to G1 drive, but requires inverted video (making the Cathode more negative brightens the picture)

An easy way to get G1 more negative than the cathode is to elevate the cathode by connecting it to HT, as in your case. Therefore all the other electrodes will need to be elevated by the same amount.

You've made some interesting progress, and landed a new CRT, that with a bit of wrangling will work well in this project.

 
Posted : 24/05/2017 5:28 am
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FordAnglia said

For video display (i.e. TV and the like) there's two ways to drive the CRT, Cathode or Grid. The cathode is more common, as it requires less drive voltage swing compared to G1 drive, but requires inverted video (making the Cathode more negative brightens the picture)

I would point out that the reason for cathode drive is that it being a lower impedance input compared to the grid, the frequency response is better due to the reduction in 1/2πCR frequency losses.

Also, I'm a little confused by your statement about ... The cathode [...] requires less drive voltage swing compared to G1 drive.

Surely, given that the beam current is proportional to the difference in potential between grid and cathode - the traditional method of measuring Ia/Vg characteristics -  it makes not one iota of difference whether the drive is to the grid or cathode in terms of absolute voltage levels but merely the polarity?

 
Posted : 24/05/2017 10:30 pm
Forum 142
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Cathovisor said

FordAnglia said
For video display (i.e. TV and the like) there's two ways to drive the CRT, Cathode or Grid. The cathode is more common, as it requires less drive voltage swing compared to G1 drive, but requires inverted video (making the Cathode more negative brightens the picture)

I would point out that the reason for cathode drive is that it being a lower impedance input compared to the grid, the frequency response is better due to the reduction in 1/2πCR frequency losses.

Agree with that. The cathode capacitance (to all other electrodes) is smaller, with less loading on the driver.

As high voltage PNP transistors are less common and more costly than NPN types, driving the cathode is practical.

In a colour CRT there are three channels, with G1 and G2 common to all three leaving cathode drive as the only choice.

Also, I'm a little confused by your statement about ... The cathode [...] requires less drive voltage swing compared to G1 drive.

Surely, given that the beam current is proportional to the difference in potential between grid and cathode - the traditional method of measuring Ia/Vg characteristics -  it makes not one iota of difference whether the drive is to the grid or cathode in terms of absolute voltage levels but merely the polarity?  

Logical but not always true. I don't have my copy of "How Television Works" (Holm) handy as I'm at work. My recollection is that as the G1 thimble surrounds the cathode it's job is to also focus the beam entering the G2 cylinder, giving rise to a non-linear relationship for Vg1 - Ik compared to Vk - Ik.

I hope that clarifies my earlier comment. 

 
Posted : 24/05/2017 11:52 pm
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