Re: Monitor problem

[Allen Bong <allenbsf6502@gmail.com>]

On Mar 31, 10:21 am, "Michael J. Mahon" <mjma...@aol.com> wrote:
> MdntTrain wrote:
> > 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.
>
> I'm glad you said that--I was just about to reply that it is
> quite likely at this point that the problem is not a capacitor.
> And there is some chance that the problem is in the CRT, not the
> associated circuits.  That would be a real problem if this were
> more than just "annoying".
>
> I'd say, either start doing more sophisticated measurements and
> diagnosis or just put up with it.  ;-)
>
> By the time you replace all the components in the monitor, you could
> have bought several new ones!
>
> >     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.
>
> Well said.  Electronic maintenance is a useful (and rare) art
> these days, but very rewarding.  It's a great new "power" to
> posess.  ;-)
>
> -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 -

I think it is just fun to do repairing these days and the result is
always very rewarding.  Of course I'd rather not send a monochrome
monitor to the repairing shop for repair as the labour charge would be
more than the cost of the monitor itself.  But the feeling of
concurring the monitor once you find the fault is so...... very hard
to explain !  ((( ;- )

But if after many trials, the fault is still not located, then it
would be very frustrating.  I'd recommend you just leave the monitor
aside for a week or two and come back later with a fresh mind to
tackle the fault again.  Approaching the problem from a different
angle with a fresh brain would probably make you see clearer the minor
things that you have missed on the last trial.  Giorgio, if you really
think that changing the 21 capacitors would help, you may change them
all if that would clear your doubts.  After all 21 capacitors dont
cost very much.  It's the labour charges that count.   From what I see
from your previous post you have a working monitor of the same model.
Why dont you just compare voltages of the good one and write them on
the circuit diagram on strategic points and compare them with the
voltages of the faulty one.  I am sure you can find the fault in one
way or the other.  Wishing you best of luck and happy trouble
shooting.

Allen