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Re: It's capacitors, stupid! :)
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I suffered from
the same problem with Aeroflex IFR120 units that I recovered by
replacing several electrolytic capacitors, unfortunately it is a case
that is repeated in many instruments
Em segunda-feira, 5 de novembro de 2018 02:30:48 BRST, Sergey Kubushyn <ksi@...> escreveu:
Electrolytics strike again... Got 8751A VNA failing power-on test some time ago and finally got some time to fix it. It had ALL THREE receivers failing self-test. Shuffling them didn't change anything (it is three IDENTICAL boards). Pulled them from the unit, removed covers and visually inspected them. On the first glance everything looked perfectly neat -- no obvious signs of something blown, very good assembly job, high quality components -- so I prepared for a difficult fight getting in the very guts. However, on the second, more thorough visual inspection something caught my eye. There was a barely visible kinda like oily film and tiny droplets on part of the board around those AK9201A-VP Digital Filter chips. Barely visible, very easy to miss, as if boards were not washed thorougly enough. Closer look revealed some slight solder joints corrosion in that area. Nothing major, almost natural but anyway... All alarms went off immediately, reached for my desoldering station gun and pulled all electrolytics from one board (there are 9 x 10uF50V, 2 x 47uf35V, 7 x 100uF35V, and one 470uF16V, all genuine KME type, looked perfectly OK) and found that all those 10uF50V around DF chip failed miserable with 100+ Ohm ESR and their internal liquid totally gone. That was that oily film around then covering quite a big area. All others also leaked some but not that bad so they were still within specs (on the margin of failure) and they also produced that stinky odor when desoldered and removed. Luckily enough those boards were of way better quality than infamous Tektronix 2465B A5 board. Also the genuine through-hole Japanese Nichicon KME seem to be filled with less aggressive blood than those SMD caps on 2465B's A5 so after thorough washing with all electrolytics removed boards looked pretty fine, no need for restoration job. New Panasonic FR capacitors were installed then, boards assembled, put back in the 8751A and bingo! -- No Errors Found, works like a charm... Will probably replace all remaining electrolytics before calibrating and putting it for sale, then repeat for my primary one sitting on the workbench and used from time to time... Resume: one should _ALWAYS_ replace _ALL_ aluminum electrolytics in _ALL_ instruments that mature before calling it refurbished and taking on calibration. No matter how nice those electrolytics look, where they were made, where the entire instrument was assembled (8751A are fully Japanese so one would expect high quality parts and assembly) -- _REPLACE THEM ALL_ It is not worth a bother checking them one-by-one and only replace ones that already failed -- they will _ALL_ fail soon. And it is actually worse with high end instruments -- those failures are _GRADUAL_ so it is not like something goes BOOM! and instrument goes dead. They start drifting out of specs instead, becoming flakey and unreliable way before they finally fail. In my early years, 40+ years ago, we were taught that electronics is all about contacts -- if it didn't work some contact was missing where it's needed or present where it is NOT needed. These days that sacral knowledge should be probably replaced by new one, about aluminum electrolytics. 99.99% failures I see for last 10 years or so were caused by bad capacitors... --- * *? KSI@home? ? KOI8 Net? < >? The impossible we do immediately.? * *? Las Vegas? NV, USA? < >? Miracles require 24-hour notice.? * * |
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