In fact, in some cases, capacitors are the last thing to look at.
I picked up a 'tech special' O-scope last month, a LeCroy LT342L. Nice unit, 500MHz bandwidth, 2-channel, LCD display, lots of nice math and analysis features, etc. The 'tech special' part came in the form of it being (apparently) completely dead when plugged in and power-up attempted.
I was on my annual scrounging road trip at the time (just after the Labor Day weekend, September), staying with a fellow techie in San Jose (SF Bay Area). After we brought the beastie back to his place, we spent a good chunk of the evening troubleshooting the power supply. We both assumed, at first, the thing was plagued with the usual leaky electrolytic caps.
To paraphrase Johnny Carson, "Wrong, isopropanol-breath!" Every single electrolytic in that scope was completely intact, no signs of leakage or damage at all. Not only that, each and every one bore two manually-applied color ink dots on top (see the photos), which told us the caps had likely been hand-selected and burned in by the manufacturer (no surprise with LeCroy).
To add to the mystery, we could hear the upscaling whine of a switching supply firing up every time we plugged in the AC cord. And, when the front panel power switch was engaged, the fan would twitch in its proper running direction.
Thanks to the service manual (which I found online, my eternal thanks to whoever scanned it), we found the power supply was actually two isolated supplies on one board, and the primary one (which supplied 27VDC for control and startup purposes) was perfectly fine. It was the secondary supply (the one which provides power to the scope's guts and the fan) which wasn't running.
Further troubleshooting led us to suspect the PWM controller chip, a hard-to-find Mitsubishi part, had gone bad. I found some from a China-based source on Ebay and ordered them, then put everything aside for future work.
Fast forward to early October (yesterday, in fact). The chips finally arrived, and I replaced what I thought was the bad one. And, as you probably already guessed, I had the same symptoms.
After some creative contributions to the world of invective, I had a brainstorm: Neither myself, nor my friend, had done a comprehensive check of ALL the power supply output points at any point during the initial troubleshooting run!
I wasted no time in doing so -- And that was when I found everything was working EXCEPT for the +12V output which supplies both the fan and the +12_DIGITAL rail in the scope's guts.
The culprit turned out to be the component in the middle photo: A 7812 three-terminal regulator which had, for whatever reason, a dead output side. Three bucks and a bit of soldering rework later, I had the result in the final photo: A now-happy scope, displaying the output from a Tektronix quick-test board.
Lessons learned: ALWAYS check every single output from a suspect power supply FIRST. And don't automatically assume the problem is bad caps!
Happy tweaking.