SMD electrolytic capacitors are a tricky problem in several ways:
First, they are easily damaged by the heat of the reflow oven,
and solvents that may be used in cleaning flux,... though cleaning
flux isn't done much if at all, on consumer grade equipment.
Second, it is almost impossible to find out what was originally
installed. There are no standardized markings to identify the
manufacturer, or what grade capacitor was installed.
Third, it is hard for a re-worker to find a complete spectrum of
ESR values from which to select replacement capacitors.
When you grab an assortment of SMD electrolytic capacitors off
of ebay, you are unlikely to be able to match more than just the
capacitance and voltage of the original. Never the ESR or lifetime
rating.
When you buy from Mouser, or DigiKey, you will find that cute
little 20uf, 16V cap you need to replace, (because its ESR is 3 ohms
while its cohorts are all 0.5 ohm), can only be had with ESR's of
0.7 ohm, 3 ohms, and 12 ohms! Replace the 0.5 ohm ESR caps with a
0.7 ohm cap, and you have taken the expected life of the cap and cut
it in half (or more). Put in a 3 or 12 ohm version, and you will have
improved nothing.
Tricky indeed!
-Chuck Harris
Kevin Oconnor wrote:
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I have to agree with Chuck. Most test equipment with imbedded/integrated switchers are going to be very difficult to qualify parts if working. Their functionality can be extremely subtle. Blanket replacement of components will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular switchers. (I forget the HP model but 5digits). Switcher went bad. No docs, no schematics. Really hopeless to diagnose. Best option was an eBay replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s where “replace em all” will serve u well! Lots of crap used in those devices.
K
Sent from kjo iPhone
