30 January 2011

The Leaking Electrolytic Capacitor Plague and my Tek TDS-460

There are many good things in life, but bad ones tend to take more of humanity's time. There is compassion, love, science, understanding, but people spend most of their life - if not all of it - in greedy bellicose hunger, hating, believing in ghosts (holy or otherwise), and missing the - equally valid - other guy's perspective. The news every day provide ample proof.
Having put that off my chest, I had better now focus  my "elegant tapestry of quotations, musings, aphorisms, and autobiographical reflections" to the more mundane matter of  The Capacitor Plague, which, although completely unknown to the Medical Science, has caused many a frustrated consumer's tear to flow. In short, electrolyte leakage from bad (bad, bad) electrolytic capacitors destroys thousands of electronic devices every day, by corroding copper traces and creating parasitic conductivity on printed circuit boards. The plague doesn't discriminate and victims appear in every class of electronic devices. I have seen my car's ECU, personal computers, power supplies, test instruments, transceivers (and my friend's Stavros Sony ICF-80) die a gruesome death by the infamous Capacitor Plague.
In all godlike modesty, I have been able to resurrect most of the victims, save a few that were truly beyond redemption, as the time and cost to fix them was more than getting a new one. The latest unfortunate victim in my troubled experience was my beloved Tek TDS-460 digitising oscilloscope, a true work of four-channel art, which suddenly (and scaringly) started failing the self-test and showing erratic triggering (quite blasphemous for a prestigious Tek stallion). As the original SMD electrolytic capacitors, with a tarnished history of leaking, had been very wisely replaced with new ones before I acquired the instrument, I was almost certain that the malfunction couldn't be attributed to the electrolytics - although the symptoms - in an eerie way - pointed straight to that direction. Turns out that the Plague is like a time-bomb, and a very delayed one sometimes.

After thoroughly cleaning the (amazingly beautiful) acquisition board with isopropylic alcohol around the electrolytics, the symptoms went away (to my great joy), but only to come back to haunt me a couple of days later. I immediately repeated the cleaning and drying - same story. I replaced some of the caps in despair - nada. So I was clearly missing something. My trusty 4X Russian magnifying glass in hand, I started examining the PCB in detail around the electrolytics - and I finally spotted what you see in the pictures (click to enlarge), near the pins of U82 and U140. The discoloration was well concealed UNDER the conformal coating of the PCB and very hard to see, due to the light reflecting off the epoxy coating and the glare obscuring the surface under.
As there were no signs of electrolyte leakage from the new capacitors, the damage must have started with the old, leaky caps, and it took years to finally manifest itself as a malfunction of the instrument. The electrolyte had crawled under the conformal coating and gradually compromised the insulation, eventually disturbing trigger control potentials and making the self-test fail (it logged the error message: "trigComparatorTest, TRIGA status after trigger: exp(ected) = 1, act(ual) = 0).  The dried-up murderous electrolyte residue shows best in the second photo, of U82 (the first is of U140).
After scraping off the coating and affected spots under it with the point of a scalpel, the symptoms vanished, and this time I think it will be for good. I couldn't find any more such discolorations elsewhere on the board, but there's always the inaccessible area under the SMD  integrated circuits, so I shouldn't be unduly surprised if...!

Well, there's a nice thought to keep a test instrument lover twisting and turning in his bed at night...