Re: Monitor problem

[MdntTrain <js@cimmeri.com>]

On Mar 29, 6:00 am, Giorgio Morocutti <giorgio.morocu...@alice.it>
wrote:
> On Fri, 29 Feb 2008 01:31:54 -0800, "Michael J. Mahon"
>
>
>
>
>
> <mjma...@aol.com> wrote:
> >Giorgio Morocutti wrote:
> >> On Sat, 23 Feb 2008 19:25:49 +0100, Giorgio Morocutti
> >> <giorgio.morocu...@alice.it> wrote:
>
> >>>On Wed, 20 Feb 2008 21:03:23 -0800 (PST), Allen Bong
> >>><allenbsf6...@gmail.com> wrote:
>
> >>>>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.
>
> >>>Good news for my monitor !
>
> >>>I have already changed C206 and C501 ( C901 !?), now I will change
> >>>C417 1000uF-16V.
>
> >> Hallo Michael and Allen
>
> >> I have changed C416-100uF 25V and C417-1000uF 16V, but no result :-(
>
> >I went back to the schematic, and I have one more possibility, based
> >on the theory that the monitor *always* unblanked during powerdown,
> >but the power supply voltage held up long enough to keep the scan
> >going until the HV started to drop, causing a bright, shrinking
> >*raster* at powerdown instead of an instant collapse to a dot.
>
> >The likely electrolytics (!) are both in the +15v power supply:
> >Cx01 and Cx03 (x=6?--hard to read...), which are 3300uF @ 35(?)v
> >and 47uF @ 25v.  Of the two, the 3300uF is much more likely to
> >be the culprit.
>
> >Because of the way the power supply is designed, a short "holdup"
> >time could also be caused by low input voltage to the 15v regulator.
> >The most likely causes of this are 1) one or two diodes (or their
> >leads) open in the bridge rectifier (D401,2,3,4) or, 2) excessive
> >load current on the 15v line (which could be almost anywhere, but
> >should cause something to be running too hot).
>
> >Some voltage measurements at the input and output of the 15v regulator
> >would cast light on these possibilities.
>
> Hallo Michael
>
> I have changed C601 and C603 ( 3300 and 47 uF), but with no result.
> :-((((((
> I must change ALL 21 electrolytic cap !!!
>
> Ciao
>
> Giorgio
>
> Giorgio Morocutti
> giorgio.morocu...@alice.it
> Apple II & Apple III forever !- Hide quoted text -
>
> - Show quoted text -

   I too have been working on monitors lately, kinda from a novice
level.  I have discovered that not all problems are capacitor-related:
they're just the most *likely* cause these days.   A brute force
approach of just replacing all caps may or may not work, and may not
address other issues going on.   I have also discovered that you must
be properly equipped, and that you must have a real working knowledge
of how the whole system works.. otherwise, it's all hit or miss, and
temporary repairs.

    For best results, my advice is to either learn video electronics
(as I am trying to do), or to give the monitor to a person who is
already expert for a complete overhaul.  They probably could use the
work.

  JS

   JS