Apple Multiple Scan 15AV CRT monitor repair

[old_tv_nut]

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.

[tony359]

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.

[old_tv_nut]

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.

[old_tv_nut]

On ebay:
https://www.ebay.com/itm/23486424495...Bk9SR_6lzMW7YQ

[old_tv_nut]

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.

[Electronic M]

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.

[tony359]

@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!

[old_tv_nut]

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.

[old_tv_nut]

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]

Originally Posted by tony359

@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.

[tony359]

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.

[zeno]

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
LFOD !

[tony359]

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.

[Electronic M]

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.

[tony359]

gotcha - thanks!