Hello all,

This is my first post here!

To start: I (hope) I know what I am doing inside a CRT, that I need to discharge it and ground it before touching anything.

I've got a number of these Apple monitors. They're somehow brand new but likely DOA or used by service centres as there are RMA numbers on the boxes. I was trying to make one or two working out of a pile I found :)
I brought home a couple, one unfortunately had a broken neck.

The other one would make a fast clicking noise when powered up. The Power LED also flashes. I know this usually points to the flyback but it looks like the PSU section to me. I do not see arching and the clicking noise comes from the PSU (I've confirmed using a plastic hose!). I've monitored one of the test points mentioned by the service manual for voltage and it goes on and off with the clicking.

So I swapped the boards between the two monitors and... same clicking. Now, I cannot be sure these monitors ever worked. Chances are those boards have the same fault on them. Again, both the boards and the tubes are spotless.

I took a look around, I could not find anything shorted around the PSU area and my thermal camera does not show anything warming up.

Unfortunately I do not have schematics - only a service manual which is pretty generic.

The question I have for you is: can a faulty CRT cause this kind of issue? When I swapped the board, I swapped the neckboard with it so there is nothing left in the monitor!

I have a feeling this is a "simple" SMPS issue but it's not easy with no schematics. The board is also pretty much inaccessible - I guess I would need to take it out and power it up on a side to test something, but still pretty difficult without schematics.
The board is manufactured by LG and it's PN 6870T024A

Any pointers would be welcome! :) Thank you!

Service manual can be found here: https://1drv.ms/b/s!AukePwK7XyEPhOZLULQXflR93X5v0Q?e=64MaJY

Pics

To discharge short out the anode to the bare ground wire. It probably
is bleeding off anyhows. Wont kill you, I got bit once a week for 40 yrs !

Ticking usually from SMPS. Lets say there is a short on the chassis, most
likely the Hoz output transistor. The PS comes up then detects overcurrent.
It turns the PS off then repeats until the short is cleared.

Set looks like a Gold Star A.K.A. LG. At least the CRT is. To find manual go
to FCC.gov, enter the FCC ID number OR type acceptance number.
This will USUALLY cross to the OEM & OEM model ##. Look for a real
manual from the OEM.

BTW since the chassis has almost everything you may try unplugging the yoke.
Do it only as long as you need to see if it clears the short. Also there
may be a B+ jumper on the plug !

Good luck
73 Zeno:smoke:
LFOD !

ahah good to know it's not going to kill me - but I'll be careful anyways! I'll check the horizontal transistor, thanks (if I can find it!).

One of the tubes on another identical set is a Goldstar indeed - too bad it did not survive the shipping.

I'll try unplugging the Yoke - won't allow the CRT to burn, no worries! Can I check what you mean with "B+ jumper on the plug"

Thank you!

Jumper just kills the B+ to the hoz / hv so if you forget to plug it
in you wont fry the CRT. May have it, maybe not.

Zeno

ah I see. So a safeguard in case I don't plug it back to stop the CRT from being damaged by the lack of deflection.

Would the set be able to power up with that safeguard in place?

If the yoke is shorted & jumped it should. Maybe not full HV but
it will stop ticking. Yokes rarely go BTW

Update.

Horizontal transitor (C5129) showed a short between base and emitter (short in diode mode and 0.4Ohm in resistance mode). I took it out but the transistor is fine.

I inspected the PCB and I see the Base is going back to ground via a small transformer (labelled T701). This small transformer reads indeed 0.4 ohms on my meter.

BBut I believe this is normal?

Removing the Yoke plug did not improve things.

I took out the main switching transformer to understand how many voltages are generated. I counted 5. I can follow them on the back of the PCB, they all go through diodes and capacitors (obviously!) and then disappear around the board. I tested all I could think of but I did not find a short.

I've removed a connector which seems to take filament voltage and 12V to the neckboard but nothing.

The outputs sometimes go through transistors and voltage regulator but, again, I don't see a short there.

Any other ideas?

I might want to inject some voltage on the various rails to see if by any chance my thermal camera sees something?

I have a twin board which - surprisingly - is showing the same fault, I could use that one.

Thank you!

Well the thermal camera worked. I found a shorted small transistor A1275-Y. Datasheet says it's used in TV for vertical deflection or sound - I'd say it's for sound.

Reason I could not spot it is that it's connected to ground via 40Ohm resistor. I did notice one rail was showing this weird low-ish resistance but I thought it was ok.

Anyways, I'm looking for a replacement. Ebay sometimes is nasty as there are sellers selling ONE transistor at stupid price.

I've been looking for a replacement.

1275 datasheet: https://datasheetspdf.com/pdf-file/637525/KEC/A1275/1

I found this: http://www.unisonic.com.tw/datasheet/2SA1013.pdf

The original is Y so I need to make sure the replacement is P (does it matter if it's not as I see different manufacturers have different naming).

Would that work?

Thanks!

I sniffed around a little & didnt find much. Odds are the ones on E-bay are
Chi-Com bootlegs. Almost every STK amp IC is. They fake EVERYTHING. Did you try the one from the other chassis ??

Zeno

Hey thanks for that.
I did find some from Ebay UK which were delivered today so I should be able to test soon.

How do I know if the ones I have are fake or not? I used this seller before and don't remember having issues - that doesn't mean I got some fake stuff and they just won't perform as per specs!

Update.

I swapped the transistor and checked things around.

The monitor powered up and stayed in standby happily (Yay!). When I applied VGA signal, the monitor fully powered up, I had HV, a weird image on screen and then The Magic Smoke! :)

The same transistor shorted (Base with Collector) and a resistor connected to it went on fire.

I've looked around and here are my notes:

- The transistor seems to be generating +70V for the neckboard. On the Neckboard there is a CVA2422TL CRT driver. I've noticed that I've got 90Ohm between GND and the 70V line on the neckboard - my other neckboard reads some Kiloohms and climbing.

CVA2422TL datasheet: https://pdf1.alldatasheet.com/datasheet-pdf/download/57164/CALOGIC/CVA2422TL.html

Shall I move my attention to the neckboard? Could it be the reason why the 70V rail fails every time?

Could a bad CRT cause this issue on the neckboard? Please note I read 90 Ohm with the neckboard disconnected from everything (but the Flyback)

I don't want to give up know. The area which went up in smoke seems pretty simple - I'm sure some of you can recognise the topology, I suspect it's some voltage regulation?

When the screen came up it was pretty weird, white with blotches. Maybe someone can recognise the symptom of something? See video below.

Also, I cannot assume that the trimmers around the PCBs haven't been touched. Still, I wouldn't expect components to go up in flame if something is misadjusted?

Here is the video of the accident (I have a small youtube channel where I publish my repairs):

https://www.youtube.com/watch?v=YqpgZc0HX2M

Attached some pictures - the main PCB is single layer so the circuit can be easily followed there!

Thanks so far!

Latest diagnostic: I've injected 7V into the 70V line and the CRT driver IC lit up! :)

You can actually see the short inside the IC!

First most new hoz outs have a damper & base resistor built in so
readings may be off. In 40yrs I had only one Hoz out open, All others
dead shorted. Hundreds of them.
Here are typical CRT voltages.

Cathodes R,G, &B 130 v
G-2 400v
So the lower the cathode V is the brighter the pix.
Go too low & you get over current then HVSD.
If G-2 gets too high same thing.
MOST the time the outputs run on a 200 V source from a flyback winding.
On TV's the filter cap for that is a super common thing on all brands.
Loose the 200 V & the K's go low then HVSD.
BTW I have no idea what that 70 V does !

Zeno

it powers the CVA2422TL which I believe powers the RGB signals? To be fair the CVA2422TL says 80V, but the PCB says 70V...

Good news!

I've replaced the resistors and the transistor. Swapped the neckboard. The monitor powered up... and stayed up! :)

The picture is all messed up but I believe this is normal when you swap a neckboard between tubes - all the adjustments are in there. Colours are off, brightness is too high and I can't turn up contrast without causing major smearing on the picture.

Only two potential issues
- Those blotches are still there. I am not sure the de-gaussing circuit works? I remember hearing this loud "boing" (sorry!) when turning on a TV and you'd see the picture wobbling for a few seconds while the coil was discharged. I don't hear/see anything here
- The 70V line. I was monitoring it and my meter only reads 31V. I'm wondering whether the picture is dim because the CRT driver is getting half of the expected voltage? Unless the voltage rises when adjusting the drive on the neckboard? Do you think that second transistor could get some feedback from the neckboard and drive the rail harder if necessary?

Anyways, the service manual has the complete procedure to re-adjust it. I'll put a mirror in front of it and try tomorrow!

For now: Yay! I found the fault! My first CRT! :)

Thanks so much for your help so far.

You're talking about those wrong-colored rings in three corners, right? Ouch!
Are they static or changing?

They may be evidence of a damaged shadow mask. I would suggest getting a manual degaussing coil to see if that will improve it. Such localized problems suggest to me that someone was playing with a strong magnet and bent the mask, or that maybe the monitor was dropped. Degaussing with a strong coil could improve it or prove that it can't be fixed.

Also, look for small magnets (like refrigerator magnets) glued to the bell of the CRT. Some CRTs used them, placed as needed at the factory to fix small regions of impurity. If one falls off, it could create a limited area of impurity.

I did try moving a small magnet close to the screen (without touching it) those circled spots were changing but then going back to the same thing - while the blotches were changing colour.

I'll have a look at the magnets on the tube, I think they are safely taped in position.

Where would I get a de-gaussing coil these days? I saw one on aliexpress which runs on mains and I got shivers. :D

Manual degaussing coils all run on mains.
Unfortunately, a quick search on ebay now shows only vintage degaussing coils at prices around $70 or some small plastic wands that I have never seen before, that look like they might not be very effective. The small, less epensive circular coils I have seen in the past don't appear to be available.

Substitutes:
If you have a soldering gun, the large transformer in the body probably produces a strong enough field to do the job. Hold the gun parallel to the tube face, move it in circles across the face for a few seconds, and gradually move away before turning it off.
If you have a hand-held bulk tape eraser, it can be used too.

On ebay:
https://www.ebay.com/itm/234864244950?hash=item36aefff8d6:g:GpQAAOSw-VNjxyXb&amdata=enc%3AAQAHAAAAoPQMljiHvz4JGnOwbD7m9wfh463N0 RFP8O%2FZVPUWWqC3WSnUpdnfzvtm5yUf6pozON3AKgqz7ZbbM YTE%2BsmLAMui4J5ej60KbRAThXLf2nM3QT%2FH6AqOqlcLwTh 2XLqPua2GXpKpoZjScRtYqIgw5zJ%2FHPQ5PBmOZ1FgjWwfA1I dtg%2BGGrasAG%2BgA3Pb5fAWJasWL9aAtoGmjNB18oBEGTQ%3 D%7Ctkp%3ABk9SR_6lzMW7YQ

If you do try the bulk eraser, start with it further away, as right up to the faceplate may be too strong. You want to degauss the tube, but not rattle it.

I think in one of the tube color sub-forums on here I created a sticky on how to make service degaussing coil out of an internal degaussing coil from a scrapped TV. That's an option if you have access to a junker set.

Line voltage isn't that bad...Hell, if you're wearing rubber sole shoes, keep one hand in your pocket and touch a hot wire in a building you'll never even know that you are...
When I was a teenager I did a TON of wallpaper steaming and in one room after steaming close to a wall switch while standing in socks on the concrete building slab I began to realize that the steamed part of the wall had become electrically energized to the point it was unpleasant to touch...Had my dad walk over to feel it and he felt nothing, after some questioning of my sanity I told him to take off his shoes and try again...That time he felt it, and after some discussion we decided that if the wall wasn't electrically hot after it dried we'd pretend we didn't know that something was wrong with the electrical there.

@electronic M
I do have a junker set with a smashed neck tube indeed! I'll go and find the thread, thanks.

@old_tv_nut
Thanks for the link - I am based in the UK, I'll see if I can find one of those but if I could re-use the coil from the other set that would be great as I don't normally work on CRTs.

I think I need to take a look at that 70V line though - PCBs can be inaccurate at times but the IC the line is driving says 80V VCC on the datasheet and I doubt the IC can work as expected with 30V - unless it's just a linear amplifier and that tube only needs an RGB p/p which can be achieved with 30V at the IC.

Also, I do not know whether by any chance there is some variable voltage regulation on the PCB which is tweaking that line - so maybe it's 30V now because it's set to 30V by the way the neckboard is set.
I think the first step would be to ascertain what kind of voltage is going INTO the regulators. If it's, say, 90V, then I know it's either a regulation issue or it's just being regulated to 30V by the various adjustments on the neckboard (which comes from another set so clearly misadjusted).

I'll keep you posted! :)

By the way, doing the manual degaussing while the set is running is a good idea because it will show you when you have moved far enough away to shut off the degausser without affecting the result. If you are close enough to get rainbow effects, you are too close to suddenly shut off the coil.

This video shows one of the small cheap ones that no longer seem to be available:
https://www.youtube.com/watch?v=xboO2JUvtBM

@electronic M
I do have a junker set with a smashed neck tube indeed! I'll go and find the thread, thanks.


I'll keep you posted! :)

I just remembered that you're in the UK where line voltage is double what it is here in the US...my sticky uses line powered degaussing. I'd don't know if European sets use higher impedance degaussing coils to run off the 250V line voltage there. You should check if they did it that way or if they stepped it down in the TVs. If the latter I recommend finding a variac and running the coil at roughly 90VAC.

Interesting. So first of all I was running the set off my light-bulb current limiter so the bulbs were limiting the degaussing pulse!

Today I ran it off mains but still no "boing" noise. I tried measuring the voltage with my meter set to read the max value and I read 68V - the coil reads 18 Ohm.

Checking at the back, it seems that I should expect full mains at the coil, would you agree?


Anyways, I managed to fix the board. I fixed the 70V line and picture came up much better on screen. I did a full white balance and cutoff adjustment and now it's ok - besides those blotches on screen.

I'm aware the de-gaussing is unlikely to fix them but I feel the circuit is not working anyways so I'll look into that regardless.

That sure LOOKS like a soft CRT. BTW If a swap has the same yoke & CRT
change it all along with the neck magnets. Saves at doing set-up.

Cheers
Zeno:smoke:
LFOD !

soft as in unable to run sharp? No, it's pretty good on that side, it's the picture compression.

I tried removing the yoke from the broken tube and I was unable to... somehow it seems to be stuck on the fabric tape they wrapped around the neck.

Anyways yes, the replacement will hopefully be the same from an identical set.

What he means is to not disturb the neck hardware and keep it paired with its CRT. If you swap neck hardware then a bunch of adjustments to it have to be made.

gotcha - thanks!

I made a repair video for this monitor. I mentioned videokarma.org - I said .com by mistake, sorry!

I hope you like it! Thanks for all your help!

https://youtu.be/3u7vCqdEBw8

I made a repair video for this monitor. I mentioned videokarma.org - I said .com by mistake, sorry!

I hope you like it! Thanks for all your help!

https://youtu.be/3u7vCqdEBw8

IIRC the .com is a porn site... That'll spice up certain viewers day.

ahahahaha! Oh No!