Brother having APC Smart UPS 1000VA and I noticing high pitched buzzing noise coming from UPS: I think it from power filter capacitor - tall one you see on LHS of picture: http://www.elektroda.pl/rtvforum/files-rtvforum/im006101_1864.jpg

(Picture all sourcing from http://www.elektroda.pl/rtvforum/topic843513.html, as brother not allow me open UPS)

I say this because UPS model 2003 and I know when electrolytic caps age, sometime their electrolyte dry, and hence making pitched buzzing noise?

  1. Am I correct, or there be possibility transformer itself making high pitched buzzing noise (unlikely as noise intermittent and goes away for few seconds now and then)

  2. That cap is 2700uF, 40V and I having hard time finding this cap locally. But I have 2200uF, 35V caps - so I thinking:

  3. Connect two 2200uF, 35V caps in series to get theoretical 1100uF, 70V caps: calculation correct?

  4. Connect two theoretical 1100uF, 70V caps in parallel to get theoretical 2200uF, 70V cap: calculation correct?

  5. I guessing that tall cap 2700uF, 40V is power supply filter likely, so replacing it with theoretical 2200uF, 70V cap for confirming suspicion not bad?

  6. I also have ESR meter: what value bad cap vs good cap read to confirming suspicion?

  7. What other component possible make such high pitched buzzing noise from UPS? (UPS out of warranty since last 5 years and now obsolete model also, so APC not interested with feeedback)

  • 1
    \$\begingroup\$ Well first step would be to actually open the UPS. How are you going to repair it if your brother doesn't let you open it in the first place? The actual problem will be much more obvious (and you could also have say capacitor plague or some other situation). Also, I disagree with the transformer conclusion in the question. The amount of noise transformers emit is related to the load on the transformer which is not going to be constant. \$\endgroup\$ – AndrejaKo Jul 5 '12 at 15:50
  • \$\begingroup\$ @AndrejaKo: I destroy his stuff by opening before, do now I need to prove I know things now. The noise comes always even if UPS not loaded \$\endgroup\$ – sekharan Jul 5 '12 at 17:19
  • \$\begingroup\$ Well then good luck with repairs. Do take care not to kill yourself! Also can you document the noise a bit better? Can you measure for how long the noise lasts and how long the pauses between the noise periods? Is the noise rhythmical? Also coils outside of transformers, such as ones on the top right part of the picture, can cause noise. Also I've seen ceramic capacitors also making noise. For the capacitor ESR, once you open the UPS, you should find the exact model and see its datasheet for ESR (if it's specified). \$\endgroup\$ – AndrejaKo Jul 5 '12 at 17:35
  • \$\begingroup\$ I'd check the coils for a whistling sound. As AndrejaKo states, be careful with the high voltages in the UPS. \$\endgroup\$ – jippie Jul 6 '12 at 20:15
  • \$\begingroup\$ why would the coils be whistling sound? and importantly, how do I fix? do it need fixing then? \$\endgroup\$ – sekharan Jul 7 '12 at 3:21

For 2 use a 3300 uF, 40+ V. That is a more standard capacitance value than 2700 uF. And you may find others that are more than 40 V are available and those at 40 V.

For 2 your calculation is correct. However this reduces the capacitance so I wouldn't go that way. I would go with 4 or 5 instead.

  • \$\begingroup\$ Thanks Brian, but can you comment on others also? \$\endgroup\$ – sekharan Jul 5 '12 at 17:18

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