Re: Monitor problem

[Allen Bong <allenbsf6502@gmail.com>]

On Feb 13, 7:29 am, "Michael J. Mahon" <mjma...@aol.com> wrote:
> Giorgio Morocutti wrote:
> > On Wed, 06 Feb 2008 01:02:39 -0800, "Michael J. Mahon"
> > <mjma...@aol.com> wrote:
>
> >>First, a few questions:
>
> >>1) When the monitor is warmed up, about what percent is the
> >>   brightness control turned to in order to begin to see the
> >>   "background" (black) raster?  10%?  60%?
>
> > Circa 50%
>
> >>2) If the brightness control is set to just below the point
> >>   where the background raster is visible, what percent of
> >>   the contrast control is required to get a good display?
>
> > circa 25%
>
> >>3) After setting the monitor up as in question 2), what setting
> >>   of the brightness control is sufficient to make the entire
> >>   screen (including the text) black?
>
> > circa 33%
>
> Al of that looks typical.
>
> >>4) Did you notice any change in the normal settings of the
> >>   brightness and contrast controls when the problem began?
>
> > unfortunately, no !
>
> Hmm.
>
>
>
> >>Without knowing the answers, there is one more capacitor that
> >>is a candidate for your problem:  C501, a 10uF @ 150v electrolytic
> >>near the horizontal output transformer.  It's the filter capacitor
> >>for the negative grid bias.  Note that the positive side of the
> >>capacitor is connected to ground, and the negative side (usually
> >>the "can") is at about -100v relative to ground.
>
> > I have made it, but no change.
> > (Only the new cap is 350VL rather than150VL)
>
> > Thanks Michael!
>
> I'd feel better if we found the problem--but at least we've ruled
> out a couple of things.  ;-)
>
> I'm running out of ideas, in the sense that from here on, the
> failures that cause the problem become weird, and that opens up
> a lot of possibilities.
>
> I don't know what kind of test equipment you have, but if you
> have a voltmeter with at least 1 MegOhm input impedance, it would
> be a good time to start measuring the voltage at the wiper of
> the brightness control, both during normal operation and just
> after the monitor is turned off.  (This voltage will be changing
> pretty fast, so a digital voltmeter is at a disadvantage.)
>
> (BTW, I'd recommend using test clips so you don't have to hold
> probes while making the measurement.)
>
> The other useful measurement under the same conditions would
> be the voltage at the cathode of the CRT, noted on the schematic
> as the RED wire at the top of the schematic near the CRT symbol.
>
> Either the cathode voltage is dropping too fast at power down
> or the grid voltage is rising too fast.  (It's actually the
> difference between them that is significant.)
>
> I've even run across a few reported cases where the CRT itself
> was causing this problem--which would be a good reason to
> either ignore it or engineer a special solution.  ;-)
>
> -michael
>
> NadaPong: Network game demo for Apple II computers!
> Home page:  http://members.aol.com/MJMahon/
>
> "The wastebasket is our most important design
> tool--and it's seriously underused."- Hide quoted text -
>
> - Show quoted text -


Hi Michael & Giorgio,

I have come across the same problem on a 14" Sanyo B/W TV many years
ago.  And I remember changing an electrolytic capacitor near the Video
Amp stage next to the Cathode of the CRT as taught by my friend.  But
I cant remember which cap it was.  I'll try to dig out all my notes
and cct diagrams and see if I can find it.

Allen