Mod a CRT to increase its TVL

[titan91]

Yes, that JVC is using a simple zener regulator to produce a positive voltage.

I've got a Sony 32" TV that has a simular positive (~30V) G1 bias using a similar approach (with resistors)



However I amazed about the quality of this TV including scanlines all around.

I plan to measure the EHT and I suspect you get a similar effect by having high EHT (thinner beam increasing EHT), however increasing EHT comes with many issues so controlling G1 is much easier.

Thanks for the reply. I don't intend to use a potentiometer for the G1 mod as I don't have one, and don't want to adjust it once it's dialed in. I also don't want the risk of it getting bumped and being set too high (voltage too low). Though someone got away with over -200v without causing arcing in the electron gun evidently.

I'm more inclined to somehow get a negative voltage out of the video B+ circuit. Video B+ should be around +200v on my TV, I will have to verify that. Ideally I would need to find where that is being generated and see if I can tap the negative side of the rectifier and just use a smoothing cap and resistor to lower that voltage to around -100v/-150v?

Edit: Doesn't seem that simple:



Can this simplified circuit posted earlier be modified to invert the +199v DC video B+ rail that's already rectified and filtered? Or am I better off just using a pin on the flyback with a negative pulse?

[vol.2]

Question about the G1 mod. Mike Moffitt did this on his cab screen, and he said it made the image dimmer and he had to crank the G2 to compensate: https://mikejmoffitt.com/pages/ms9-hax/index.html

Wouldn't cranking the G2 wear down the cathode faster?

[titan91]

I don't believe so, the cathode is just a piece of metal that repels and accelerates electrons past the control grid (G1). I believe what actually wears are the emissive barium layers, and the phosphor on the screen if the brightness/beam current are too high. Operating G1 with a negative voltage evidently used to be standard practice in CRT design. I'm sure most if not all VGA computer monitors do that.

[geckotdf]

I have a Trinitron kv21se40 but no luck with the mod.
Can isolate the G1 pin, generate -120v but the weird thing it's doesn't change. If I put -1v or -120v in another CRTs it's quite noticeable because the picture get dimmer, but here its like nothing.
Probably it's not modeable or need more negative voltage - Can't find any point in the flyback that give me more than -120v

[titan91]

Has anyone done this with a Trinitron KV-20FS12 or BA-5 chassis? Sharpness on mine is pretty bad (may just need a simple focus adjustment) but being a flat CRT it also has worse focus in the corners and along the edges. Doing this mod I have read will improve depth of field for more even sharpness.

[KPackratt2k]

Has anyone done this with a Trinitron KV-20FS12 or BA-5 chassis? Sharpness on mine is pretty bad (may just need a simple focus adjustment) but being a flat CRT it also has worse focus in the corners and along the edges. Doing this mod I have read will improve depth of field for more even sharpness.

Have you tried disabling velocity modulation? From what I remember the last time I worked on that chassis, there should be a setting in the user menu to toggle velocity modulation (should be the setting labeled "VM"), disabling it should reduce the sharpness artifacts significantly.

[titan91]

Has anyone done this with a Trinitron KV-20FS12 or BA-5 chassis? Sharpness on mine is pretty bad (may just need a simple focus adjustment) but being a flat CRT it also has worse focus in the corners and along the edges. Doing this mod I have read will improve depth of field for more even sharpness.

Have you tried disabling velocity modulation? From what I remember the last time I worked on that chassis, there should be a setting in the user menu to toggle velocity modulation (should be the setting labeled "VM"), disabling it should reduce the sharpness artifacts significantly.

Thanks, I haven't. I was just looking at the service manual for this series of TVs with the BA-5 chassis and see there's a line coming from the microcontroller to the VM board. The VM board itself appears to drive the horizontal yoke. So I assume disabling VM in the user menu just turns off the signal going to the VM board.

I also discovered there are not only unused E-YUV inputs on the jungle IC, but also RGB1. RGB2 is used for the OSD. Thus, I assume RGB2, when switched, has precedence over RGB1 for maintaining the OSD. So in theory, both an RGB and component mod would be dead simple. After tying in the input jacks add 2 switches: one for VCC power going to a second rocker switch that sends that either to YUVSW for component blanking or YS1 for RGB1 blanking. That is, assuming you have the correct video input selected for sync.

[titan91]

Just saw this posted on Reddit. Phenomenal improvement for sure.

https://www.reddit.com/r/crtgaming/comm ... d_kv32s26/

[Konsolkongen]

Not really a good comparison. That's composite vs RGB. RGB would always look much better

[KPackratt2k]

Agreed, it certainly would've helped if he provided "before and after" photos of the G1 voltage mod on the same input.

I have a Trinitron kv21se40 but no luck with the mod.
Can isolate the G1 pin, generate -120v but the weird thing it's doesn't change. If I put -1v or -120v in another CRTs it's quite noticeable because the picture get dimmer, but here its like nothing.
Probably it's not modeable or need more negative voltage - Can't find any point in the flyback that give me more than -120v

I performed this mod on a KV-20S41 using the same circuit that ElBartoME posted a while back and although there was no change in the screen voltage, the image was definitely sharper to my eyes. It went from having a slight "bloom" on bright colors when contrast is in the middle to having more pronounced scanlines on the same contrast setting (although not as thick as the scanlines you'd see on a PVM, ofc). I've measured -50V on the G1 pin once the mod was installed. On the older sets I've done G1 mods to, I had to adjust the screen control on the flyback in order to get an image to display. Perhaps the flyback works differently on this set compared to the older designs?

[titan91]

I also put together a "simple" version of this design, without the pot and ripple filtering. The filtering is overkill, and the pot is unnecessary if you are going to stick with a single setting. As you can see, this simple version is very easy to build, and it does not require adding a new winding to the flyback:

Here's a mock-up of the simplified circuit I roughed in to test this on a 19" Philips Magnavox using a new winding on the flyback. Winding is white wire, neckboard ground is black, and G1 is red. I'm tying in the other side of the winding to the neckboard ground, I believe this is correct. I have a 220Ω 1/2 watt (I think) resistor that should be rated for high voltage, a couple of 400v fast recovery rectifier diodes, and a couple of 250v ceramic capacitors that are both less than 5nf. Not quite as much capacitance as the schematic specifies (the first one is just for isolation) but I expect these will be fine.

I wish I had larger project PCBs on hand but I found these tiny ones in a spare parts lot. It's large enough to fit all the components and connections. Before I solder everything in and clip the excess component leads, let me know if you see something wrong. I probably won't be running G1 at more than 120v.

Spoiler

[titan91]

Here's what I found just testing the circuit:

1 turn: - 15v
4 turns: - 35v
7 turns: - 58v

Here is the TV with the G1 pin floating with nothing connected:

Spoiler

Here is my mod board connected:

Spoiler

And here is the result at -58v, no change. I did have to increase G2 a small amount as expected. I started out with no picture.

Spoiler

I upped it to 12 turns, bringing me to - 90v:

Spoiler

The result on this tube is spot size increases and the convergence gets worse, especially in the corners. Not a good choice for the G1 mod. This tube was operated 24/7 in a laundromat for many years and while it still has a bright picture with good emissions, its geometry doesn't respond well to increasing G2. Even with the picture barely visible, the spot size did not get any smaller.

[titan91]

Last update from me for a while, I attempted this again on my heavily worn 27" JVC tube. Unfortunately a worse result than the high hour Samsung tube, I started out at 10 turns which gave me -75v. No improvement. I upped it to 15 turns, -108v. This time I didn't even have enough emissions left to bring G2 high enough without streaking the picture, overdriving the CRT.

I doubt my lack of success is a Samsung/JVC thing. LukeEvansSimon and I talked about worn tubes on Reddit and we assumed this mod would improve things, but it only makes them worse. Oh well.

If you have blooming or a dim picture and bringing up G2 doesn't help things, don't bother with the G1 mod. Your tube is too far gone.

[SuperSpongo]

Thank you for your tests! While it did not bring any positive results, it made for an interesting read
Maybe you could try a rejuvenator, if you have access to one?

[titan91]

I don't unfortunately. If I can find one dirt cheap on eBay with the needed CR-23 socket adapter I may consider it, but at this time it's not going to be cost effective for me unless I come across more worn CRTs.

That being said, I do plan on trying with a couple of low hour Funai tubes and a 20" Trinitron flat tube. Those TVs are on the back burner at the moment while I'm doing a cap kit on the JVC.

[titan91]

OK it seems I can pick up a B&K 467 rejuvenator, calibrate it, and modify it with a modern harness for not an insane amount of money, if one ever comes up on eBay. If I find the emissions on this 27" JVC tube are poor I'll try a clean & balance on it, nothing too aggressive.

[titan91]

Just came across this video, seems to be the only one in English:

https://www.youtube.com/watch?v=USRP-l8v0b4

[fromkok]

Has anyone tried the mod with 27 inch Sony BA-4?

I have a kv-27v42 and I purchased the mod board from rgbmods.com and I think I need some help here.
(Here is the website: https://www.rgbmods.com/products/g1-mod-kit)

Here are the steps:
1. 200v from Flyback's pin 4.
2. Ground from Flyback's pin 5.
3. Isolated G1 by cutting the trace around it.
4. Also used the ground near G1. (No difference when removing the ground from Flyback's pin 5.)

After installing it, the output voltage is -78.6v with the full rotation of the knob to the right.
I had to increase the G2 voltage since the screen became black.
I didn't see an improvement at all.

Based on the description, I think I am supposed to get -200v when the knob is fully rotated to the right, but -78.6v is the max. What can I do for further testing?