Re: Monitor problem

[Allen Bong <allenbsf6502@gmail.com>]

On Feb 21, 6:26 am, Allen Bong <allenbsf6...@gmail.com> wrote:
> 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- Hide quoted text -
>
> - Show quoted text -

Hi,

Michael was right!  I found 3 caps having some kind of electrolyte
leak at the legs of the caps at C417 1000uF 16V, C901 10uF 100V and
C206 220uF 90V.  I changed them one by one and tested the monitor
after each cap was changed.  After changing the 3rd cap, the bright
dot was gone.

I guess the cap must have become open circuit but I have no capacitor
meter to confirm.  I used caps with smaller footprints but higher
voltage rate as 90V and 100V caps were not available here.

Allen