Author Topic: CRT TV Repair Brightness / Contrast Won't Adjust Down (Read 3761 times)

Mikek400 · « **on:** October 03, 2021, 05:56:32 am »

Hi I know many might ask why bother with a CRT TV.. Simple Answer is it fits well in the space I have and I like it. It is also used for playing old video games which don't work well on newer flat panel technologies. Chalk it up to nostalgia if you like. Now that is out of the way on to the question.

The issue is that the screen suddenly became too bright. As if the brightness control was turned all the way up. You can adjust the brightness and it seems to make it a bit dimmer, but the contrast doesn't seem to work at all. I need to verify this to be sure.

Does anyone have any idea where to start looking around? After looking at the schematic, I didn't see anything about brightness or contrast and am not sure where to start.

I have a 19" CELERA model CL2002 Chasis CN-12C from 2002. I have found a schematic that appears to be close and it is for an APEX model AT1302 Chasis CN-12C.
The board layout is a bit different but the components seem to be the same for the few I checked and in similar locations.

The service manual is attached.

EDIT: Added second Schematic I found

james_s · « **Reply #1 on:** October 03, 2021, 06:01:50 am »

Lots of people still have CRT TVs, they are collectible now.

The main thing that determines the brightness of the picture is the voltage on the G2 electrode (also called a screen grid). This is usually controlled by a pot on or near the flyback transformer labeled "screen". You might try gently tweaking that and see if it makes any difference. I've seen this same symptom before although it's been years, seems like I've seen it caused by a resistor going open more than once. Have you looked for a schematic for the chassis?

Edit: Just noticed you attached a service manual, I'll take a look.

james_s · « **Reply #2 on:** October 03, 2021, 06:10:34 am »

That schematic is hard to read but at least it's something. Looks like a fairly standard setup where the Screen pot is in the flyback and there's a wire that goes from that to the neck socket. IIRC screen is usually around 300-600V depending on the setting and Focus (the other wire besides the HV anode wire) is around 1kv so be careful probing that one. Both often go directly into the CRT socket instead of connecting to the neck board.

Mikek400 · « **Reply #3 on:** October 03, 2021, 06:27:08 am »

Hi James,

Thanks for the reply and nice to see others appreciate CRTs as well.

I had assumed that adjusting the pot on the flyback would dim it down, but this happened suddenly and I'm sure there is a failure of some sort. I'd rather fix it knowing that there is a problem. A burned out resistor is at least a starting point. I guess I will start at the flyback and work my way back through the circuit.

andy3055 · « **Reply #4 on:** October 03, 2021, 06:37:06 am »

Look for high value resistors in the high voltage areas. I have also seen this typical fault but many moons ago!

Kerlin · « **Reply #5 on:** October 03, 2021, 11:25:13 am »

Does it have faint diagonal lines on the screen? These are retrace lines.

Mikek400 · « **Reply #6 on:** October 03, 2021, 08:18:52 pm »

I didn't notice any retrace lines. At the moment I have the chassis pulled from the set and can't easily double check but pretty sure there weren't any.

james_s · « **Reply #7 on:** October 03, 2021, 09:06:57 pm »

I don't mean just adjust the pot and call it good, the goal is to see if the Screen control adjusts the brightness, if it doesn't work at all then you know where to start looking. I do suspect something has failed, hopefully it is not part of the HV divider built into the flyback.

Mikek400 · « **Reply #8 on:** October 03, 2021, 09:40:38 pm »

Found another schematic that is more clear but yet again a slightly different model.

Mikek400 · « **Reply #9 on:** October 03, 2021, 10:45:42 pm »

Got the Chassis re-installed in the TV and the Screen Pot on the flyback does indeed adjust the brightness down. At first I thought it might be sufficient, but the red channel is over saturated when you turn it down. It only takes 1/16th of a turn or so to get it where the black levels are good.

So I think that rules out the flyback as being the problem.

Oh and there are not any scan lines. The picture seems to be perfect aside from black levels.

I have attached a picture that shows the black levels. The bars in the picture are only there because of my camera. This picture accurately shows the black level and it should about the darkness of the bars running across the picture.

Mikek400 · « **Reply #10 on:** October 11, 2021, 07:17:55 am »

I have probed around and it seems the flyback may be the issue.

The 190V supply is only at 130V. It appears to be generated in the flyback from a center tap transformer inside it. Maybe I'm looking at the schematic wrong but I can't see where else it would be coming from.

Any thoughts or ideas what else to check? I have another possibly good possibly bad flyback around here somewhere for this tv, but not sure if I want to chance putting it in since the tv it was in was struck by lightning.

james_s · « **Reply #11 on:** October 11, 2021, 05:43:31 pm »

If the voltage is low it could also be a bad capacitor, electrolytic capacitor failure is fairly common in TVs. Depending on the sort of damage it received from a lightning strike it's entirely possible that the flyback in that set survived, having a second flyback to try certainly makes it easier to test.

NSA · « **Reply #12 on:** October 11, 2021, 08:53:51 pm »

Check out 12VOLTVIDS on YouTube he fixes a lot of Crt’s and old school equipment.

Mikek400 · « **Reply #13 on:** October 13, 2021, 11:47:13 pm »

Finally found the issue. There is a 22μF 200V electrolytic in the 190V circuit from the flyback, on the neck board that had no measurable capacitance.

Any advice on getting the flyback set properly so it doesn't reduce the life of the CRT? I have it so that it looks good but not sure if that's good enough or if there is some voltage I can measure and set.

BrokenYugo · « **Reply #14 on:** October 14, 2021, 01:50:04 am »

Usually the service manual gives a detailed procedure to adjust the screen pot, but yours doesn't seem to. If you haven't adjusted any of the service menu controls and neglected to mark the factory position, adjusting for a good picture is probably close enough. The factory procedure in the manual I have handy, for a Sharp 27U-S100 is more about setting up the red/green/blue gun drives (which involves turning down the screen pot) and the last step just tells you to adjust the screen pot afterward "to obtain normal brightness range".

Focus pot you just display snow or a video game and adjust for best compromise between razor sharp corners and a razor sharp center.

It is wise to use insulated tools here, and a mirror if the flyback pots aren't adjustable with the back installed.

Mikek400 · « **Reply #15 on:** October 14, 2021, 02:05:32 am »

As part of the troubleshooting I replaced the flyback with another I had. It was still glued in its original position but was set for the set it came out of and was too high for this tv and had retrace lines.

Ok, well I will call it good. I have already set it so it looks correct.

Thanks everyone for the help!

andy3055 · « **Reply #16 on:** October 14, 2021, 04:41:36 am »

If you get the retrace lines, try adjusting the pot on the flyback. Some have them built on to the flyback. If I remember right, it does adjust the screen voltage.

james_s · « **Reply #17 on:** October 14, 2021, 07:31:53 pm »

In the absence of a proper adjustment procedure I usually set the user brightness control to maximum then turn up the screen pot until I get retrace lines then back it off a bit past where they go away.

Kerlin · « **Reply #18 on:** October 17, 2021, 09:24:03 pm »

So after following the usual "It could be this or could be that" and the "I had that once" type diagnosis.
Surprised the usual "check the ESR of all the capacitors" has not come up.
And as usual the thread just moved down the queue, so the next thing to try is some proper diagnostic analysis.

Turning the screen voltage down is only masking the problem and wont provide a correct luminescence set up. Check and adjust the colour balance and grey scale to see what I mean by that.
Hint - you are getting retrace lines which means the blanking is not working, which means........... so check for..........and then suspect.........anyone?

james_s · « **Reply #19 on:** October 18, 2021, 06:04:42 am »

I suggested checking the capacitors several posts ago. The retrace lines can be a fault, but it can also be caused by the screen control being turned up too high. He already twiddled with that control so it is not surprising that it needs to be tweaked.

Kerlin · « **Reply #20 on:** October 18, 2021, 09:57:37 am »

Think you totally missed and skipped over the points in my post, then continued on with "can be this and can be that" again.

I DID NOT say the problem is that he didn't tweak that adjustment, 0 out of 100 for reading comprehension.
Reading homework - read it again.

I said the problem is he did adjust it.

Do adjustments, checks and measurements as suggested, not guesses.

My post provides a complete guide on how to fix it.
I'm out of here, bye.

james_s · « **Reply #21 on:** October 18, 2021, 06:01:55 pm »

Your post doesn't really provide anything other than bad attitude, and you didn't show up here until it was already fixed.


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Author Topic: CRT TV Repair Brightness / Contrast Won't Adjust Down (Read 3761 times)

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