Re: Monitor problem

On Mar 31, 10:21 am, "Michael J. Mahon" <mjma...@xxxxxxx> wrote:
MdntTrain wrote:
On Mar 29, 6:00 am, Giorgio Morocutti <giorgio.morocu...@xxxxxxxx>
wrote:

On Fri, 29 Feb 2008 01:31:54 -0800, "Michael J. Mahon"

<mjma...@xxxxxxx> wrote:

Giorgio Morocutti wrote:

On Sat, 23 Feb 2008 19:25:49 +0100, Giorgio Morocutti
<giorgio.morocu...@xxxxxxxx> wrote:

On Wed, 20 Feb 2008 21:03:23 -0800 (PST), Allen Bong
<allenbsf6...@xxxxxxxxx> 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...@xxxxxxxx
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
.

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