One last quirk about switch-mode
power supply capacitors: Besides usually having a higher temperature
rating on the case, caps used in these supplies are designed to have a very low self-inductance. See, capacitors can really be thought of as a
capacitor,
resistor, and inductor wired in series. Just the nature of the beast, what with the long cold of metal film. For switch-mode supplies, the caps are designed to have very little
inductance and resistance as the high-current at high frequency would contain a lot of ripple. When in doubt use
OEM caps as the off-the-shelf ones could leave you with unexpected problems.
@JD @FMEng Writing in support. When I moved to Stratford, Ontario's, Stratford Shakespearean Festival's main
stage to become their IA Head Of Sound, the SFX playback
system I inherited included a 44RU rack housing twelve
Crown D150 stereo
power amplifiers driven by their custom Ward Beck 12 x 24 matrix effects
console and powering 24 effects speakers, mostly Tannoy Belvederes, throughout their
auditorium,
stage, and backstage areas. Prior to my arrival, it had been their standard policy to
power all of their amplifiers off between performances and rehearsals, even on matinee days. Coming from a 24 / 7 / 365 commercial AM broadcast background, I chose to leave the amplifiers on 24 / 7 / 365. Prior to my arrival, it was common for them to lose one
channel several times per season whereupon they'd pull the stereo amp and ship it to Toronto for repair leaving them minus two channels for the duration. My approach was different; I purchased a 13th
Crown D150 as a spare and installed it on the rear rails of the same rack with a pair of input extension cables and a pair of output extension cables coiled adjacent on hooks allowing me to swap the spare amp into service between cues if / when required. I learned the most common failure mode was one of two identical capacitors in each D150's
power supply. At the end of my first season when it had been determined I'd be kept on by the Festival, I ordered two
OEM capacitors from
Crown's Canadian rep' in Toronto for
stock and that was the end of shipping
Crown D150's to Toronto for repair. What I learned was if the amps were powered for the season, they rarely failed. I'd
power them down for the winter months and re-power them upon my
return. If they re-powered successfully, they were most likely good for another season. If one
channel was humming loudly upon powering up, I'd determine which amp it was, swap in the spare amp, pull the faulty amp, replace both of its capacitors, test it, reinstall it as the spare and order two more
OEM capacitors for
stock. The
Crown amps continued to percolate happily for the remainder of my time with the Festival.
Toodleoo!
Ron Hebbard