Re: Monitor problem

[Giorgio Morocutti <giorgio.morocutti@alice.it>]

On Fri, 29 Feb 2008 01:31:54 -0800, "Michael J. Mahon"
<mjmahon@aol.com> wrote:

>Giorgio Morocutti wrote:
>> On Sat, 23 Feb 2008 19:25:49 +0100, Giorgio Morocutti
>> <giorgio.morocutti@alice.it> wrote:
>> 
>> 
>>>On Wed, 20 Feb 2008 21:03:23 -0800 (PST), Allen Bong
>>><allenbsf6502@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.morocutti@alice.it
Apple II & Apple III forever !