BDS1451UK - CRT - Bad colours

[tmcw]

Have this old 14" Black Diamond (Mitsubishi) TV that is displaying some bad colours. The attached photo is a snap of the TV showing SCART input, with no SCART device attached, so should be a fully blue picture, but you can see the red/pink along the top, top left, and left of the screen.

There was another issue with the TV set, where it was making a clicking noise, and not starting up, but I found a few suspect bad solder areas, reflowed and added solder, and the TV is reliably functioning now, except for the colour problem.

What I'm wondering is if this is something that can be fixed, and if it can, where is the problem likely to be?

I can take photos of any boards or other parts anyone wishes to see, if they think it would help.

[tom66]

Re: BDS1451UK - CRT - Bad colours

Looks like the shadow mask is bent; someone has taken a rather large magnet to it. Can't think of an electronic failure that would cause that!

You can fix it with another very strong magnet - try and bend the shadow mask back - or use a degaussing coil external to the TV.

[tmcw]

Re: BDS1451UK - CRT - Bad colours

Not sure about the magnet, the TV was originally mounted in a bedroom near the ceiling, or in a kitchen under cupboards with a light fitting installed underneath the cupboard (there were 2 identical TVs in the house, so it was one or other location). Maybe electrical cable fields could have done this?

Anyhow, I tried a small magnet and was able to improve the picture somewhat, but couldn't get it out fully. There are 2 red spots at the top, one on the left, and a little near the centre. Is it likely I'll never get it right? Or should a degaussing coil sort it?

The other thing I was thinking about is the degaussing coil in the TV, should it be able to sort it out, or if that isn't functioning correctly, another reason why it went the way it did? Is there any safe way to test the degaussing circuit?

[Th3_uN1Qu3]

Re: BDS1451UK - CRT - Bad colours

The other thing I was thinking about is the degaussing coil in the TV, should it be able to sort it out, or if that isn't functioning correctly, another reason why it went the way it did? Is there any safe way to test the degaussing circuit?

That's it. Bad PTC thermistor in the degaussing circuit, no need for testing, just replace the PTC. It is in series with the degauss coil and this series group is in parallel with the mains. It's usually white, albeit i've seen a few black ones. Not sure where you can get a replacement as the part codes on those things are always cryptic, i just scavenge them from sets that have died for another reason.

Replaced a few myself, they go bad in cheap sets after a few years because they run hot, this is their normal operating state. When the PTC is cold it has little resistance so all the mains current goes thru the degauss coil, and then it heats up quickly and goes into a high resistance state, removing the power from the degauss coil. This circuit is the reason for the high current spike at startup of CRT sets, and the accompanying buzzing sound.

[tmcw]

Re: BDS1451UK - CRT - Bad colours

Thanks for that, you were spot on.

I pulled the schematics (board is a Vestel 11AK36-2 from about 2001), and as luck would have it, I have a scrap board which I didn't realise is a similar 11AK36-A8 (2003 vintage), I think I'd pulled that out of a discarded Beko I was unable to revive some time back.

The thermistor had the same markings (MZ72AL 9RM), and when I swapped the one from the scrap board in, I now get a perfect blue picture.

Nice result! Thanks again for sharing your knowledge, I've learned a little more today.

EDIT: I had a google of the part number of the thermistor, and the first couple of pages of images have pictures of TVs with messed up colours. I suppose in time, these pictures might join them.

[Scenic]

Re: BDS1451UK - CRT - Bad colours

EDIT: I had a google of the part number of the thermistor, and the first couple of pages of images have pictures of TVs with messed up colours. I suppose in time, these pictures might join them.

They won't, because you can't view any of the forum attachments without having an account and being logged in, so google's bots won't see them either. I can understand why topcat did this (traffic), but it's still pretty annoying..

If you log out and view this thread, you'll see that there are attachments, but you can't view them. Same happens when google's bots parse the page.