CRT displays

Gentlemen,

Would I be correct in thinking that if you have an item of test equipment with a CRT display and it's dim and when you try to turn up the brightness it actually gets dimmer for the last bit as you get towards maximum? And there's nothing you can do about it; it's only fit for spare parts?

tia

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Reply to
Cursitor Doom
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Lots of circuit maladies will simulate a weak CRT. Start with voltage readings, move to waveforms. Low high voltage will cause the picture to dim as the drive to the tube is increased for instance.

Reply to
John-Del

I was given to understand elsewhere (which is why I'm double-checking here) that a CRT which has gone dim and reaches its peak brightness before dimming again towards the end of the intensity control sweep is irretrievably f***ed and not worth further investment in time. It would be nice if that advice is indeed incorrect!

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Reply to
Cursitor Doom

Hardly.

No. You would NOT be correct. Find the service manual of your unspecified model piece of antique test equipment, and check the voltages in the CRT and Hi-V section. Use a Hi-V probe if you value your life and your DMM. My guess(tm) is that you'll find some form of component failure or deterioration, such as a burnt resistor, leaky capacitor, arcing flyback, sick hi-v diode, or the usual high ESR electrolytic capacitor. Plenty of possibilities.

Now, I have question. Is there some reason you chronically fail to provide the maker and model number of what you are working on? I'm wondering if you have a pathological aversion towards numbers, or perhaps you might be embarrassed by your choice of test equipment.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Also, some of us have CRT test fixtures that can also restore/rejuvenate almost any picture tube/CRT - B&K model 490 for example.

If the image grows/shrinks when you change the brightness, then there is usually a fault with the HV diode, especially in B&W CRTs.

I'd first look at the electrolytic caps in this unit - most are past their best before date... When replacing the caps take care to use caps specced for the circuit they are being used in. And try using a good quality cap like Panasonic from Digikey or Mouser. Not crap from eBay...

That might be considered to be cheating - actually providing useful background information...

John :-#)#

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Reply to
John Robertson

I didn't want to limit it to one specific item, Jeff; just wanted to make the question as general to CRO test equipment as possible. Someone opined thusly about the matter on some discussion forum and I thought it might be useful to keep it in mind for future eventualities, that's all! Anyway, it was bad advice, clearly.

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Reply to
Cursitor Doom

Since you didn't read the question in the first place I'll excuse myself on that one. The issue was *not* about the image shrinking/growing during intensity adjustment!!

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No deal? No problem! :-D
Reply to
Cursitor Doom

But it's not uncommon for a low High Voltage circuit to cause what we call "blooming" when the brightness is advanced. If your example was also doing that and you didn't initially notice it, you would say "hey, mine is doing that!!! Thanks!!".

Look, it's still likely a weak CRT (depleted cathode), but it could be many other things as well. I remember 1980s era NEC projection TVs used a DC su pply for the filaments, and a lazy bypass electro dropped the filament to a bout 4 volts. The three tubes looked like they were shot, but it would be unusual for all three to go that weak at once. New cap, filament back to 6 .3, TV looked like new.

Reply to
John-Del

eadings, move to waveforms. Low high voltage will cause the picture to dim as the drive to the tube is increased for instance.

That is very incorrect.

  1. Check all your electrode voltages, using a true rms meter on the heater if it's LOPTF driven and you have one. Also check video drive amplitude on all relevant electrodes (many TVs put different signals on different electr odes, using the tube to effecively calculate the resulting output).
  2. If still not upto snuff, there are 4 reasonable ways to improve it a) Zap type rejuvenation. Used to be popular, but don't do it. It doesn't l ast long & causes severe smearing as emission drops again, making the tube unusable. Not all tubes can be helped with zapping. b) cook type rejuvenation. Just wind the heater voltage up eg from 6v to 9v and let it sit a while, maybe 15 minutes. Often the tube then works nicely . c) voltage boost: increase heater voltage. 10% gives slight improvement, I' ve used upto 66% as an experiment in a severe case (zero emission was detec ted, worked fine for the next 10 years I kept it) d) A sort-of obvious one that sometimes is overlooked: if it's an old tube with plate glass guard, often there's a great deal of filth in that gap, cl ean it. Sometimes nicotine on more modern screens can be less than obvious too. f) Finally, and this is usually to be avoided: increasing EHT. May be safe to do on historic low EHT tubes, but 1950s/60s onward tubes (typically) hav e high enough EHT already that x-ray output would be increased problematica lly. X-ray output level must be checked if this approach is used. f) Really finally, re-gun it if worthwhile & everything else failed. A majo r rebuild job, and very few are done now. But Hawkeye's equipment was bough t and is still getting used.

Oh, and there's one superbasic approach that can get old tvs just good enou gh when they're not quite there: turn down the black level to ensure all bl ack areas are fully black.

And note that when colour tubes are improved by most of the above methods, you can expect poor colour tracking. The worse it was, the poorer the track ing will be. Usually not a big deal, but can be.

There's more. Obwarning: don't do any of this unless you know how to do it all safely.

NT

Reply to
tabbypurr

Might add, resistors that have increased resistance. Not open, just high resistance. Mikek

Reply to
amdx

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