CRT TV Picture Blooming, Bad Flyback? (Solved: Bad Tube)

[titan91]

I have an issue with a 20" RCA (TTE) CRT TV. The picture will bloom and size of the image will change when a bright screen appears, then when a dark screen follows the picture will be too dark before returning to normal brightness. The first thing I did was replace all high voltage electrolytic capacitors, including the main input side filter capacitor. At first I suspected B+ power supply but I measured a steady 109v with about 0.4v of change, which from what I read is normal at 50% brightness. I also replaced the main switching regulator IC but the issue persists.

I'm now thinking the flyback is the issue, maybe a failing resistor or potentiometer in the screen and/or focus voltage divider circuits. The main reason I think this is due to focus drift between screens and different days. A week prior I adjusted the focus from its factory position against a black background and white crosshatch screen. A week later I connect my Wii which has moderate brightness at the system menu. The focus was way off again, too soft. I adjusted a second time and noticed I had to turn the focus knob just about all the way to the right to get it looking alf decent. I'm aware bad focus can cause the image to shrink and grow. I'm also aware the spark gap plates in the neckboard socket could also be corroded. The set was exposed to smoke as well. Should I replace the HOT, if that could get weak over time and sag as brightness/beam current changes?

This set has many thousands of hours on it, operating about 10 hours a day 5 days a week in an office for about 15 straight years. I don't think the issue is the tube. So I looked at the flyback in the set. The original is a JZ branded, I think, JF0501-19535. I see various JF0501s and the closest I can find on eBay is a 19530. Assuming that's close enough:

https://www.ebay.com/itm/402936860415

Even if it's compatible I also want to be wary of poor Chinese made sources. I assume they will all be China but I've read about poor quality ones with double folded over legs and failures within a couple of years. The above eBay one looks decent from the photos with solid pins on the bottom. The seller also has an insanely good rating.

Any help would be appreciated. I did a component video input mod on this TV before realizing the issues it has, unfortunately. So hoping to get it fixed up.

[gray117]

Bet is ok for blooming, but would assume something on deflection circuit perhaps for size?

[titan91]

Bet is ok for blooming, but would assume something on deflection circuit perhaps for size?

I did at first, but both the horizontal and vertical size of the image change and I've learned that if the screen and focus voltages drift that could cause it as well. Unless however the deflection voltages are derived from the flyback and some fault with a winding is pulling the other output voltages down. The B+ voltage going into the flyback does not change when the picture does. I wish I could attach files here, so here's a couple links instead with videos demonstrating the problem.

https://www.mediafire.com/file/ucppzom5 ... o.zip/file

https://www.mediafire.com/file/enutee2e ... ).zip/file

Is it safe to test vertical deflection with a multimeter by just connecting the probes to the yoke? Or would voltage spikes between sweeps be an issue? I can also trace and/or test the VCC power rail going to the vertical IC.

And here's the factory flyback:

[Josh128]

Does the screen increase in size with bright pics or decrease in size? ( known as "breathing"). I have a Hitachi 27MM30B that the screen shrinks on bright images, like the power supply voltage is being pulled down. I played with different caps and tried to swap some components from a known working mobo, but was unsuccessful in pinpointing the issue.

I did not change the flyback in trying to resolve this issue as I didnt have a spare. I dont think the flyback would cause this though. Its a transformer, and transformers dont really "drift". They tend to either short or open. That said, Im not sure if some models have capacitors in them that could potentially drift and cause this issue. I tend to think its more likely a power supply or voltage regulator issue when it comes to blooming and breathing.

You can check vertical deflection with a DMM, but Im not sure what you should expect to see-- you really need a scope to see whats going on.

[titan91]

Does the screen increase in size with bright pics or decrease in size? ( known as "breathing"). I have a Hitachi 27MM30B that the screen shrinks on bright images, like the power supply voltage is being pulled down. I played with different caps and tried to swap some components from a known working mobo, but was unsuccessful in pinpointing the issue.

I did not change the flyback in trying to resolve this issue as I didnt have a spare. I dont think the flyback would cause this though. Its a transformer, and transformers dont really "drift". They tend to either short or open. That said, Im not sure if some models have capacitors in them that could potentially drift and cause this issue. I tend to think its more likely a power supply or voltage regulator issue when it comes to blooming and breathing.

You can check vertical deflection with a DMM, but Im not sure what you should expect to see-- you really need a scope to see whats going on.

Thanks for the detailed post. I know some amount of bloom is normal for a consumer set but this seems a bit excessive. The picture expands horizontally and vertically when there is a bright image, so I don't think anything is being pulled down. Once it expands and goes from a bright screen to a darker image, the picture shrinks and gets dim, then a split second later is comes back in brightness and size to a happy medium. The medium looks fine, it's when bright and dark scenes occur that the geometry and focus are noticably affected.

The B+ staying in a stable 108.8v/109.2v range with no correlation to the breathing/blooming proves it's not a regulation issue on the input side. Plus I've replaced the regular IC to be certain. Why would it slowly recover back up to normal size and brightness, and not recover instantly? That behavior sounds like a bad capacitor somewhere, but I've replaced all high voltage electrolytic capacitors on the chassis and the neck board. Unless there is a bad cap inside the flyback but I'm not aware of any cos built in to the flyback itself.

I've learned that flyback transformers also have resistors and potentiometers to form the voltage divider circuits for screen and focus. I don't know if increasing the screen voltage will affect the size of the picture, without affecting actual deflection circuits. I can test that by adjusting the screen pot. Like, I think the loss of focus is causing the lines to spread out and make the picture larger. Like a pin point of light in a magnifying glass going out of focus.

I was curious to see that although a meter would show an average of the vertical sawtooth, it should tell me if that average goes up or comes down. This video shows a flyback replacement brought this effect back down to normal levels, no longer excessive like mine:

https://www.youtube.com/watch?v=7z6T64Elm-s

[Syntax]

Have you checked the polymer caps on that circuit?
I know it sounds archaic, but if you grab some rubber hose and use it to listen to caps while the set is on you can hear the polymer caps that are on the way out.
They have a higher pitch whine.

[titan91]

Have you checked the polymer caps on that circuit?
I know it sounds archaic, but if you grab some rubber hose and use it to listen to caps while the set is on you can hear the polymer caps that are on the way out.
They have a higher pitch whine.

I have not actually, but I did consider those. I just didn't mention it in the last post since I didn't want to throw too much information out there at once. Your method is interesting though, I also have an audio amp and inductive coil tap used for desk phone handsets to record calls. Not sure if that would just pick up a lot of general noise from the drive circuit or allow me to zero in on noise coming from any solid state high voltage caps.

I'm not familiar with narrowing down which ones I should be looking for though. Should I check specifically in the horizontal drive circuit closest to the flyback, on the output side? When I tested B+ someone else told me to connect to the positive lead on a diode that goes directly to the input pin on the flyback, no other capacitors between that point and the transformer. Would solid state capacitors on any of the output windings of the flyback cause issues with focus, brightness, and deflection all at once?

I'm assuming you meant ceramic or film capacitors, not polymer. Polymer ones tend to be associated more with small surface mount capacitors. Maybe this type?

https://www.arcadepartsandrepair.com/st ... h-cap-kit/

[Syntax]

I like the hose because its fully insulated, and works stupidly well for honing in on a single part. Plus its cool just having a listen to everything
IDK why I said Polymer sorry, yeah the film caps in the lower right, give them a check.

[titan91]

Sounds good. I have a component/capacitor tester on order and some hose from a pond water pump. I'll listen then test those caps out of circuit. Hopefully that's all it is!

[titan91]

I was able to get to a PC and upload that test video to Odysee:

https://ody.sh/EEnKOzABqI

I tested C912, inline with the main switching MOSFET's output on the primary side of the main transformer, and do see that voltage fluctuating quite a bit depending on brightness. That being said, I had ground connected originally to the hot side ground but didn't get a reading. So I disconnected ground from my meter and left it floating. I don't expect the actual reading to be accurate because of that, but it does show the overall trend. Following a white screen the meter does show the MOSFET output dropping to below average then it slowly comes back up as brightness and picture size return to normal. I also tried the other cap coming off the MOSFET, C914. I didn't get anything but I think my probe was grabbing the insulation looking closely at the photo.




I don't know if the above result is even normal. The switching transformer and caps on the other side should condition the B+ power going to the flyback. But I'm not going to rule out film caps in the B+ section.

[titan91]

Well this just keeps getting weirder. I thought I had the focus turned all the way to the right (clockwise) but that's not the case. It's somewhere in the middle, where it should be. I listened for anything odd, and learned 20FT 1/2" PVC hose does not transmit high frequencies, lol. I didn't hear anything with the naked ear, until I loaded the Wii menu again. As predicted, focus went soft again but this time I did hear a fluctuating hissing sound coming from the main transformer area. I'm not sure if it's the transformer or C921 to the right of it. But that indicates a correlation with B+ and that just happens to be a cap in the regulation feedback circuit that DC isolates the hot and cold sides:



But what really surprised me is what happened when I adjusted the screen voltage. I decided to turn the screen pot down some (to the left, counterclockwise) to see if that had any impact on the blooming. Not only did it have no impact on blooming but it also wrecked the entire picture. Now the picture is darker but also has too much contrast, too much color, and horizontal streaks in many areas. With the TV in this state I also noticed the Homebrew Channel's blue background was flickering near the top of the screen. And when the blooming happens on brighter screens the color bleeding gets progressively worse.

I decided not to touch the screen adjustment again since something is more wrong than I was aware of. Throughout my testing today I never saw or heard anything strange from the flyback, no arcing or hissing. But, would these symptoms after barely turning down the screen pot still suggest a bad flyback? I know C921 is definitely suspect at least and I'm going to swap that one first.






Edit: Oops, I missed replacing electrolytic cap C928, right next to a smaller voltage regulator. Hmm...

Edit 2: Similar issue with screen pot turned down, possibly a cap in the screen circuit:

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

[titan91]

Something else of note, I had another TV once that had a flyback problem. Often the picture would be really washed out, would shrink vertically, and show this same type of smearing. If I poked the flyback when running it would often correct itself, then the screen would mess up again. I'm assuming the issue with that one was a failing screen pot.

I went ahead and ordered that replacement flyback as well, as it seems I have issues on the B+ and the horizontal side. Seller offered me 10% off so why not. I hope it's compatible, only the last digit in the part number is off. Even if it doesn't end up being the flyback I'll have a spare at least.

I did some rough math and this TV has about 30,000 hours on it. So who knows what's going on. I'll post updates as I progress with these cap replacements.

[tongshadow]

You'll need a proper oscilloscope to test it properly and draw proper conclusions. I recommendo the DPOX180H, very affordable and more capable than more expensive options.

[titan91]

I pulled C928 and tested its ESR. It was pretty bad at 9.3 ohms. I checked some new ones I had and replaced it with one below 2 ohms. I realize at 1 microfarad ESR can be a little more lax but 9.3 ohms can't be right. I powered it back up, not expecting that to fix the issue. It didn't, same dark streaking screen and blooming. I then checked that ceramic disc cap in the B+ regulation circuit I thought was noisy, and the one for the screen circuit on the neck board. The B+ cap is slightly low but within its 20% tolerance rating. The screen filter cap is the same way, only off by 5 picofarad which is pretty impressive for its 5% tolerance. I know the component tester is accurate because I tested it against brand new Nichicon caps and all of those show slightly above their rated capacitance. And the new Chinese caps I have read low more often than not. Still giving that flyback transformer the stink eye until I can eliminate anything else. Next I will try testing some film caps, from what I learned they can be a bit more troublesome than ceramic ones if they are not made correctly. And yeah, I'm aware putting these things in circuit under load can cause failures that don't show up in a low voltage test.

[titan91]

Before I dive deeper into this I wanted to get some better knowledge. I began watching Randy Fromm's CRT Workshop series from 2020. In the Workshop 3 video they discuss where focus and screen voltage drift and seem erratic when adjusting the pots on the flyback. I can't recall exactly where they talk about that. But one of the guys was poking the pots with a screwdriver and the picture would pop in and out of focus and brightness as he did it. The pots on mine are recessed, so not sure how easy that will be to try.

https://www.youtube.com/watch?v=I52_WTDWHVQ

Also in Workshop 6 Randy just flat out confirms film capacitors always short when they fail. I trust what he says so that made it easy easy to test those in circuit. 26 minute mark:

https://www.youtube.com/watch?v=aNyYoaV2InI#t=26m

I found no shorts. But, now that I know what a step-down horizontal drive transformer is, I located a few more electrolytics around that area I have not changed yet. Fingers crossed.

[titan91]

Well lookie here:







C403 filters whatever is coming off of R402. According to Randy Fromm in CRT Workshop 10, in newer monitors the input of the horizontal drive transformer can be about 24 volts. I assume R402 is a pull-up resistor. With no filtering on that rail, I wonder if that caused the primary winding to sag, which would cause the secondary to sag, which would cause a drop in voltage on the base of the horizontal drive transistor. Could that in turn could cause the 0.7v bias voltage for the HOT to drop, and ultimately compromise its output power driving the flyback weaker than it should be? Might be far fetched, but seems logical.

There's no ESR reading on that cap. I wonder if it's leaky as well. C957 and C918 in that area are "fair" in ESR according to charts on meters, I was able to replace C958 and will have to leave the other 2 since I don't have 50v variants of those. Also have not looked at C824 yet.

[tongshadow]

I'd swap the tube to rule out the circuit. This approach will go nowhere if the issue is on the tube.

[titan91]

I'd swap the tube to rule out the circuit. This approach will go nowhere if the issue is on the tube.

I considered that possibility as well. Hopefully it's not the tube. I don't have another TV of this type of test with, or a CRT tester. I will declare it a tube issue if I have ruled out everything on the chassis board.

If it ends up being the tube, how could that fail without affecting any one color gun? Contaminated grids? I've heard of the phosphors getting old.

I do have two other brands of TV of the same size I can try swapping the tube on if the yoke impedance/inductance matches close enough but that's pretty unlikely. I'll have to consult some tube swap guides, I'm not going to attempt a yoke swap.

[titan91]

Yes, the issue is the CRT. Swapping a TruFlat tube with a curved one is not possible. While the vertical yoke resistance is correct at 9.5 ohms, the horizontal resistance is 2.2 ohms and needs to be 3.1 to match the original yoke. Also, the bolt spacing is different.

The poor picture and blooming persisted with the screen turned down a bit. I then decided to turn down the brightness and contrast. Then when slowly adjusting the screen pot back up, I heard a faint noise from the electron gun and the TV turned off. I pulled the neck board and found the blue cathode is now shorted to ground.

It has been a fun experience learning more about this fascinating technology. This is now a parts TV.

[Josh128]

lol, at least you had fun trying!

[titan91]

lol, at least you had fun trying!

Haha yep, I basically eliminated every other possible cause and had a reason to watch Randy's CRT Workshop series. I highly recommend it if you want to dive deeper into monitor theory. I'm going to shoot him an email with my thanks for all his time and effort.

https://www.youtube.com/@randyfromm