boosting CRT heater voltage

Tony Duell ard at p850ug1.demon.co.uk
Fri Nov 19 15:14:30 CST 2010


> 
> --- On Thu, 11/18/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
> 
> > My desktop CRT (over 10 years, but not what I'd consider
> > vintage!) has been ailing for some time, with the picture
> > getting darker and darker despite having the brightness at
> > 100%. It's still good for high-contrast stuff such as black
> > text on white background, but forget trying to pull details
> > out of most photos, for instance.
> 
> A couple of things can cause this, but yes, most common is a weak 
> picture tube. The tube in my monitor here at work is pretty worn out too 

Is it? I am not convinced. I think in all my classic computer repairs 
I've had one low-emission CRT, and that was in a Mac+. The picture coul;d 
be made bright enough, but there was a curious shaddowing to the right of 
objects on the screen. There as nothing wrong with the video amplifier 
circuit, and a new CRT did cure it.

But many times I've had CRT electrode voltages drift, upsetting 
brightness or focus.

> It'll help a lot, usually. Start with maybe 10% increase. You can 
> create a higher heater voltage by making your own turns around the 
> flyback core - a couple of turns of insulated wire should be enough. 
> Filament voltage does not have to be DC. Another thing to check before 
> boosting the filament would be the monitor's main B+ voltage. If it's 
> sagged too much due to worn out electrolytics, you'll have a dim, 
> shrunken picture. Similarly, faulty electrolytics in the signal path 
> will cause smearing and dim colors. So, start by checking voltages - the 
> main B+ and the supply voltage to the video amplifiers. One thing I have 
> seen is a monitor where the video signal from the computer was dim and 
> faded, even with the brightness cranked up - but the OSD menus were 
> really bright and clear! The problem was related to the video amplifier 
> for the signal input. 

The other thing to distinguish betwene is low brightness and low 
contrast. They are not the same. Contrast problems are almost always due 
to video amplifier problems.

Oh, and clean the screen :-). Seriously, I once spent a day tracking 
down a nonexistant video amplifier fault in a VT100 with a dim picture 
which turned out to b caused by a filthy CRT screen...

> 
> If that all looks good, then crank up the heater voltage a bit. Worse 
> case, you burn out the tube, but hey, it was already unusable when you 
> started, you haven't much to lose. You should be able to get several 

Hmm.. Sure, if the CRT is defective, there's no harm in trying to boost 
it a bit. But if the fault is elsewhere, then boosting the heater voltage 
will not help, and may damage the CRT. Then you have both the original 
fault to find and cure, and a CRT to replace.

-tony



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