Missing phosphor dots - UK Vintage Radio Repair and Restoration Discussion Forum

Hunts smoothing bomb · 13th Apr 2007, 3:09 pm

Hi all,

My Philips K70 has several missing phosphor dots on the screen. When the set is off they don't look any different from the rest but, when the set is on the missing phosphors are just black dots.
How does this happen? Is it something that has always been since the tube was made or is it something that happens to colour tubes when old?

Thanks
Lee

Nickthedentist · 13th Apr 2007, 3:23 pm

Missing phosphor dots, or blocked holes in the shadowmask, I wonder?

Surely if the phosphor itself were missing from the faceplate of the tube, it would look different when it's off, as well as when it's on.

Might be worth turning the tube faceplate-up, giving it a gentle bash, then trying again

Nick.

MALC SCOTT · 13th Apr 2007, 4:05 pm

Missing phosphor dots may be a sign of a regunned CRT fitted. Quite a few CRT rebuilders often used to sell them off as seconds.

brianc · 14th Apr 2007, 9:45 am

Hi all
In my days at the BBC, a colleague of mine was responsible for the appraisal of colour monitors coming into the BBC - it was at the time when quality, not quantity, was the most improtant thing. He often had to rejected Grade 1 monitors because of missing phosphor dots in the tubes. These tubes were relegated to Grade 2 monitors and "complete" tubes installed into the Grade 1s. This was not uncommon and vey often, the holes in the phosphor dots could be seen when the monitor was off. As the dots were deposited using the actual mask, it could be that there were blocked holes in the mask which caused the dots to be missing - chicken and egg?
Cheers
Brian

ekcopyephilips · 17th Apr 2007, 10:15 pm

Hi Guys

When i used to work for a large Japanese TV manufacturer, we used to get brand new tubes with flecks in the shadow mask.

The girls on inspection were so good they could spot a fleck as small as 0.3mm on a red purity screen in seconds.

Aparently they are caused by the final anode voltage attracting any c**p that may be in the neck at first switch on.

We were sometimes sucsessfull at knocking the blockage out of the mask by banging the faceplate quite hard (hard enough to make an onlooker think it was going to go bang) with the handle of a screwdriver. Sometimes it worked, if we were lucky. Unfortunately though even if you did clear it, as soon as you switch the set off then on again, the offending piece of c**p would usually get attracted to the shadowmask again.

Mike Burton

13th Apr 2007, 3:09 pm	#1
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Missing phosphor dots Hi all, My Philips K70 has several missing phosphor dots on the screen. When the set is off they don't look any different from the rest but, when the set is on the missing phosphors are just black dots. How does this happen? Is it something that has always been since the tube was made or is it something that happens to colour tubes when old? Thanks Lee __________________ Lee

13th Apr 2007, 3:23 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Missing phosphor dots Missing phosphor dots, or blocked holes in the shadowmask, I wonder? Surely if the phosphor itself were missing from the faceplate of the tube, it would look different when it's off, as well as when it's on. Might be worth turning the tube faceplate-up, giving it a gentle bash, then trying again Nick.

13th Apr 2007, 4:05 pm	#3
MALC SCOTT Octode Join Date: Aug 2006 Location: Willington, County Durham, UK. Posts: 1,499	Re: Missing phosphor dots Missing phosphor dots may be a sign of a regunned CRT fitted. Quite a few CRT rebuilders often used to sell them off as seconds. __________________ Malc Scott Last edited by Mike Phelan; 1st Jun 2007 at 7:47 pm. Reason: Typos

14th Apr 2007, 9:45 am	#4
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: Missing phosphor dots Hi all In my days at the BBC, a colleague of mine was responsible for the appraisal of colour monitors coming into the BBC - it was at the time when quality, not quantity, was the most improtant thing. He often had to rejected Grade 1 monitors because of missing phosphor dots in the tubes. These tubes were relegated to Grade 2 monitors and "complete" tubes installed into the Grade 1s. This was not uncommon and vey often, the holes in the phosphor dots could be seen when the monitor was off. As the dots were deposited using the actual mask, it could be that there were blocked holes in the mask which caused the dots to be missing - chicken and egg? Cheers Brian

17th Apr 2007, 10:15 pm	#5
ekcopyephilips Hexode Join Date: Dec 2006 Location: Norwich, Norfolk, UK. Posts: 440	Re: Missing phosphor dots Hi Guys When i used to work for a large Japanese TV manufacturer, we used to get brand new tubes with flecks in the shadow mask. The girls on inspection were so good they could spot a fleck as small as 0.3mm on a red purity screen in seconds. Aparently they are caused by the final anode voltage attracting any cp that may be in the neck at first switch on. We were sometimes sucsessfull at knocking the blockage out of the mask by banging the faceplate quite hard (hard enough to make an onlooker think it was going to go bang) with the handle of a screwdriver. Sometimes it worked, if we were lucky. Unfortunately though even if you did clear it, as soon as you switch the set off then on again, the offending piece of cp would usually get attracted to the shadowmask again. Mike Burton