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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Thu Oct 08, 2020 1:10 am 



Joined: 17 Aug 2020
Posts: 82
My Geiger counter arrived. It is the latest version of the GMC-300e Plus, which is available on Amazon for $75. It uses the M4011 Geiger-Muller tube or the J305 Geiger-Muller tube, which both use a thin glass casing, which makes it good for detecting both low energy and high energy x-rays. Revisions of the M4011 tube are known to be sensitive to harmless visible light radiation, but luckily the J305 tube does not have this issue. I opened the counter to check the tube model and luckily I won the tube lottery!
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Here is the reading that I get with the back plastic case of the GMC-300e Plus removed to fully expose the Geiger-Muller tube. Low energy x-rays have difficulty passing through solid materials, so removing the back plastic case helps increase the sensitivity of the device. Low energy x-rays also cannot travel very far in air. So I had it held up against the CRT screen in the center, then at the edges of the screen, the sides of the TV, and the back. It fluctuates around 0.08 to 0.1 micro-sieverts per hour when the TV is on at full white and the mod is enabled.
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Some context: if I walk around my house with this Geiger counter, I get the same range of readings is the same. In other words, the CRT is not emitting any x-ray radiation, even when the mod is active and the screen is full white (peak emissions). My granite counter top emits more radiation at around 0.13 micro-sieverts per hour. 0.03 micro-sieverts per hour up to 0.33uSv/h is considered normal, healthy background levels of radiation.

Modern CRTs, even with this mod, do not emit x-ray radiation. Significantly higher voltage (50 kilovolts) would be needed to cause a modern CRT to emit x-rays. The citations that I have linked explain why this is the case, and the measurements with a real Geiger-Muller tube that is sensitive enough to detect low energy x-rays confirms the theory is true. So fear not when moving forward with this mod. It is far below the voltage levels needed to cause your CRT to emit x-rays.


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Fri Oct 09, 2020 7:17 pm 



Joined: 17 Aug 2020
Posts: 82
Out of curiosity, I measured everything in my house using this Geiger counter. None of my CRTs emit any X, beta, or gamma radiation. However, the ceramic tiles in my bathroom do emit radiation. The tiles were made in 1940, and must have used a uranium oxide glaze, which was common during that time.
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CRT gamers should worry more about keeping a safe distance from their bathroom tiles and granite counter tops than their CRTs. Though apparently CRT TVs from the 1940s thru 1950s do trigger these Geiger counters because the screen glass is thin.


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Sat Oct 10, 2020 10:53 am 


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Joined: 24 Jul 2016
Posts: 307
Cheers for going the extra mile on this one. I appreciate the effort you've been putting in, and i'm sure others do too. I'd be interested to hear about subbing transistors and the other ideas you had too.

Hopefully soon i'll get around to trying out the mod for myself. Unless i can convince thumptech we need to get together and use one of his haul of old CRTs :)


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Sat Oct 10, 2020 12:47 pm 



Joined: 02 May 2017
Posts: 5
buttersoft wrote:
... Unless i can convince thumptech we need to get together and use one of his haul of old CRTs :)


I've been meaning to give it a go, I have a couple of trinitron sets I think this may work well with, but I can't decide if i want to wrap more turns on the flyback or make a SMPS to provide the extra bias. I would also need my mister back as that is my test pattern generator ;)

I could try it on the bench with the variac, but I would say there is enough evidence in this thread already to go with a permanent mod rather than prove the concept again.


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Mon Oct 12, 2020 4:00 pm 



Joined: 17 Aug 2020
Posts: 82
thumptech wrote:
buttersoft wrote:
... Unless i can convince thumptech we need to get together and use one of his haul of old CRTs :)


I've been meaning to give it a go, I have a couple of trinitron sets I think this may work well with, but I can't decide if i want to wrap more turns on the flyback or make a SMPS to provide the extra bias. I would also need my mister back as that is my test pattern generator ;)

I could try it on the bench with the variac, but I would say there is enough evidence in this thread already to go with a permanent mod rather than prove the concept again.


I expect the results generated by this mod to vary quite significantly from CRT to CRT. The electron gun design will determine what the spot size vs G1 voltage curve looks like, and the flyback design may limit the peak G2 voltage that can be adjusted (without more significant modification). So just getting very convincing results from a consumer Trinitron, even if that is done with a bench power supply, will be very useful for the community to see.


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Wed Oct 21, 2020 12:32 am 



Joined: 17 Aug 2020
Posts: 82
I am going to prototype an alternative version of this mod that upgrades the RGB drive amps to ZTX658 amps that can tolerate a higher voltage. It is only $2.67 for all 3 amps. Then I will use a simple voltage divider to tap into approximately 400 volts off the screen voltage and use that as the voltage source for the RGB drive amps collector pin.

The advantage of this variation of the mod is it will be easier to implement since charging the G1 requires adding another winding to the flyback, which is labor intensive. Modifying the RGB drive amps directly is also a good opportunity to increase their bandwidth through an upgrade to a more modern amp. The disadvantage to this approach is it requires even more headroom in the screen voltage potentiometer because whatever increase in voltage of the cathodes relative to G1 needs to be matched with an even larger increase in screen voltage relative to G1. Charging the G1 anode directly avoids this.


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Mon Oct 26, 2020 8:05 pm 


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Joined: 08 Jan 2016
Posts: 610
Location: Mountain View, CA
I have found a monitor that applies -68V on the G1 pin from the factory. I will follow up with more information and the results of doing so on a similar monitor that does not.
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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Mon Oct 26, 2020 8:35 pm 



Joined: 17 Aug 2020
Posts: 82
mikejmoffitt wrote:
I have found a monitor that applies -68V on the G1 pin from the factory. I will follow up with more information and the results of doing so on a similar monitor that does not.


It will be very exciting to see others experiment with this mod, especially because results will vary considerably based on CRT model. The magnitude negative voltage you will need to apply will vary greatly from CRT to CRT. It will largely depend on the physical distance between the cathodes, G1 anode, and the G2 anode, especially the size of the aperture in the G1 anode. Consumer CRTs use a very large aperture in the G1 anode, which causes them to have a very thick cathode ray, and therefore they need a very large magnitude negative voltage than what you'd see in a professional CRT, which tend to use a very small G1 aperture.

The Diamondtron based Viewsonic P225f applies -180v to G1 in its factory design. It is also one of the highest TVL CRTs ever made, being able to both address and resolve 1920 TVL! It outputs a cathode ray that is incredibly thin!

Applying a negative charge to G1 shifts the burden of outputting a very high cathode voltage relative to G1, from the RGB drive output amplifiers to a separate power rail for charging G1. The Viewsonic P225f uses RGB drive output amps that operate at a much lower voltage gain, which allows them to have a very high bandwidth as a result, since the cathode to G1 voltage differential needed for cutoff is accomplished through separate negative charging of G1. This is why extremely high TVL CRTs charge G1 to a very negative charge.

This page is a great overview of the physics of electromagnetic equipotentials that explains why increasing K-G1 and G2-G1 voltages decreases spot size. Basically, the cathodes and anodes are not point voltages in a circuit. They are three dimensional structures that give off electromagnetic fields. The "Grid" aka "G1" is a metal tube that surrounds the cathodes and has a tiny aperture at the end of it. The fields create a sort of path of greatest attraction that the cathode ray is pulled through by the G2 anode. The size of the surface area of the cathode where the voltage equipotential between G2 and cathodes is absolute zero shrinks as K-G1 and G2-G1 voltages increase.
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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Fri Nov 13, 2020 9:59 pm 


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Joined: 14 May 2015
Posts: 116
Location: Australia
This is one me of the coolest CRT modding ideas - simple to implement and effective at achieving its goals. Will there be any updates or progress reports anytime soon?


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 Post subject: Re: Mod a CRT to increase its TVL
PostPosted: Sat Nov 14, 2020 11:22 am 



Joined: 07 Aug 2018
Posts: 31
I'm still planning to do that mod on a Sony consumer CRT. They are in the basement though and I don't have enough space upstairs at the moment. I need to get rid of.some CRTs first.


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