Strange electrolytic capacitor failure,

[CaptDon]

Just wanted to share this experience with you troubleshooters. We have a Carver PM-350
stereo amplifier used as a monitor amplifier in our band. The left channel started to sound
distorted and while it was on my bench for repair it kept getting more distorted. I found
when I placed my finger on top of a TL072 the audio instantly cleared up. Checking for
solder voids and cracks and finding none I replaced the I.C.  Seemingly and instant fix!!
The unit was powered on and off several times and played for over an hour with no problems.
Next morning the very same channel was out again!! Turns out this time, touch the I.C.
and no joy, but touch the electrolytic beside the I.C. (470uf@6.3vdc) and presto, amp plays!!
Bottom line, the cap was bad but in a strange way. What I found was when the amp was
turned off the capacitor would end up with about -1vdc on it and we know electrolytics hate
reverse voltage. Apparently the negative voltage would cause the capacitor to develop
low internal resistance when left to rest for 12 hours or more and then the amp wouldn't
play when powered back up. Physically disturbing the capacitor would temporarily cure it!!!
Drove me crazy trying to nail down why the amp was dead each morning and knowing
it wasn't thermally related!! I replaced the cap with 470@16vdc to be more tolerant of the
reverse voltage during the shutdown/bleedoff phase.


 

[David Hess]

How old was the capacitor?

That failure seems more like one lead of the capacitor was open inside.  When they get old, the seal may leak allowing water in which corrodes the aluminum where it connects to the lead eventually causing an open.

[HighVoltage]

I just had to fix a high voltage power supply and one of the electrolytics failed in a similar way.
The quick test of the cap showed it to be OK.
They fail in many different ways.

[bob91343]

The problem is that an electrolytic capacitor isn't a real capacitor.  It's an electrolytic cell that just happens to have substantial capacitance.  Devices like this have been causing trouble for decades.  They have short shelf life and develop leakage for no obvious reason.  Temperature is their enemy as well.

Unfortunately there is nothing available to replace it.  That's why people who work on old gear look at the capacitors first, and 'recap' has been a watchword.  Of all the parts that can fail, it seems that capacitors are the worst.  And there are many of them in most devices.