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Electronics => Repair => Topic started by: @rt on March 04, 2017, 09:48:35 am

Title: Schema Interpretation
Post by: @rt on March 04, 2017, 09:48:35 am
Hi Guys :)
One of these chip ceramics is reading 43k across it (in circuit, while the other two read 33M across them.
Unfortunately the capacitance measurement function on my meter appears to be broken.

The shorter measuring ceramic is in the path of blue in an RGB video path, and blue is actually the problem.
Everything in the video has a blue tint, so it appears to be the culprit.

For C250_1, C250_3 & C250_5... what is 0.22 a measurement of?

(http://img.photobucket.com/albums/v186/ArtArt/point22what_zpskx1tfjtr.png)

Cheers :)
Title: Re: Schema Interpretation
Post by: Armadillo on March 04, 2017, 10:28:09 am
In uF.
it should be 220 nF or 0.22 uF. Datasheet shows 470nF can be used or 0.47uF, same.

Title: Re: Schema Interpretation
Post by: @rt on March 04, 2017, 10:31:08 am
Thanks, I did find the data sheet example for the ST video DAC,
but three randoms took care of the problem (randoms as in I don’t know their value, but they are the same as each other).

Who would have thought... a chip ceramic failed close to DC short without physical damage...


One fixed $500AU vintage full motion video module for the Commodore Amiga CD32 :)
Title: Re: Schema Interpretation
Post by: Armadillo on March 04, 2017, 10:33:35 am
 :-+
Title: Re: Schema Interpretation
Post by: Kleinstein on March 04, 2017, 11:26:46 am
MLCC failure mode with a dead short is not that unusual.
Title: Re: Schema Interpretation
Post by: @rt on March 04, 2017, 01:18:05 pm
Well not quite short, just a lot more than the others.
Measuring resistance across it while still on the board also includes any path through the PCB from each side of the capacitor, so 43k is really a minimum.
But I take it short enough to be useless for coupling the video.

I have given one of these away with the same problem. Sent it to Sweden and everything, and am pretty sure now it’s one of the same caps.


Title: Re: Schema Interpretation
Post by: SeanB on March 04, 2017, 02:36:42 pm
When you took them off measure again, the resistance should be up into the megohm region again, showing you the fault is a crack that has been contaminated with adsorbed moisture. Common on ceramic caps, and more common is the crack is enough that it falls into 2 or more pieces on removal. Most likely cause is an auto placer that applied a little too much force to the cap in gripping it, or pushed it down too far on the board.
Title: Re: Schema Interpretation
Post by: @rt on March 04, 2017, 03:26:35 pm
All three I removed were in one piece and still looked ok removed.
... and I do have the suspect one separated from the other two.
Still have them, but too late (early morn) here to look again tonight :D