On Wednesday 16 July 2014 02:47:55 Gregg Eshelman did opine And Gene did reply: > On 7/15/2014 5:59 PM, Leonardo Marsaglia wrote: > > There are some big electrolytic caps inside the cabinet (80 mm > > diameter and 180 mm long approx). I can replace them easy but is > > there a way to know if they are faulty before replace them? > > Physical examination is the first step in diagnosing electrolytic > capacitors for faults. Check that the tops of the capacitors are not > bulged or split open. If they're not flat, they're bad. If they're > split open with goo coming out, yup, bad. Also look for evidence of > leakage under the capacitors onto the circuit board. That can be quite > nasty as the electrolyte can corrode through traces on the board. > Capacitors can leak from their bottoms without bulging or splitting > the tops. > > Bottom leakage can be hard to detect. Look for dust that won't blow off > or areas that are darker than the rest of the board surface. Bad cases > can look like dried cat pee on the board, and just as corrosive. I once > had a cat that decided the CRT on a Xerox 820-II was "his" and needed > "watered". The pee ran down the face of the CRT then dripped onto the > main circuit board, eventually damaging it. > > > My brother has a digital oscilloscope that can measure capacitance > > but I don't know if that's ok for this kind of capacitors. But I > > guess if they are in the range the oscilloscope can measure I might > > work.
Does this scope also measure "ESR"? That is an often overlooked characteristic, but very real fault in electrolytic capacitors, much more prevalent in the lower voltage stuff. The higher voltage stuff can usually fix itself by re-welding the connections internally. Lower voltage, under 150 volt stuff can't. ESR stands for equivalent series resistance. Its caused by poor connections between the terminal leg you see, and the actual alu foil and electrolyte soaked kraft paper that is the capacitor itself. The only test tool I ever found that does a decent job of measuring this is called a "Capacitor Wizard", and sells for about 180-200 dollars on this side of the pond. Google for it. It uses an 85 millivolt signal to measure the capacitors resistance to a 100 khz signal, and anything over 2 ohms was either bad or going bad next week, change it. 85 millivolts so it didn't "fix" the bad cap. 100 khz so it largely ignored the size of the cap until you were down to less than 2 u-f. When I was the engineer fixing stuff at the tv station, it found bad caps, in circuit, in digital circuitry, where the average cap is the size of a pencil eraser, surface mounted, by the 3 lb coffee can full. It was the single most valuable tool I had to keep about 20 DVC-PRO video tape machines running well. That DVM that can measure capacitance, may well tell you that cap labeled 47 u-f is actually 44 u-f, but it won't tell you its pure crap because its internal resistance is well north of 10 ohms. [...] Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS ------------------------------------------------------------------------------ Want fast and easy access to all the code in your enterprise? Index and search up to 200,000 lines of code with a free copy of Black Duck Code Sight - the same software that powers the world's largest code search on Ohloh, the Black Duck Open Hub! Try it now. http://p.sf.net/sfu/bds _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
