CRT displays

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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
--  

No deal? No problem! :-D

Re: CRT displays
On Sunday, January 19, 2020 at 1:18:51 PM UTC-5, Cursitor Doom wrote:
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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.

Re: CRT displays
wrote:

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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 fucked and not worth further investment in time. It
would be nice if that advice is indeed incorrect!

--  

No deal? No problem! :-D

Re: CRT displays
On Sunday, 19 January 2020 22:40:18 UTC, Cursitor Doom  wrote:
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eadings, move to waveforms. Low high voltage will cause the picture to dim  
as the drive to the tube is increased for instance.
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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

Re: CRT displays
On Sun, 19 Jan 2020 18:18:49 +0000, Cursitor Doom

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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.

--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: CRT displays
On 2020/01/19 3:02 p.m., Jeff Liebermann wrote:
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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...

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

John :-#)#

--  
(Please post followups or tech inquiries to the USENET newsgroup)
                      John's Jukes Ltd.
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Re: CRT displays
wrote:

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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!!
--  

No deal? No problem! :-D

Re: CRT displays
On Monday, January 20, 2020 at 4:59:07 AM UTC-5, Cursitor Doom wrote:
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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.

Re: CRT displays
wrote:

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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.

--  

No deal? No problem! :-D

Re: CRT displays
On 1/19/2020 5:02 PM, Jeff Liebermann wrote:
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  Might add, resistors that have increased resistance. Not open, just  
high resistance.
                                              Mikek

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