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enter image description here

This is in the right hand power amp in a Nakamichi AV-10 receiver. Whilst watching the Olympics opening ceremony at considerable volume, the unit overheated (hot pcb smell) and cut out.

Having cooled it now works up to a point.

The main problem is that now cuts out at moderate volume. Using an Infrared Thermometer I have identified this capacitor running at 63C. It's mate on the other amp is at 30C. Cooling it with a fan allows the amp to operate at higher volume.

The other slight problem is that there is a faint crackle on the right speaker.

So, my question.

Do you think that the capacitor is the problem, or is it merely the indicator of a different failure?

markrages
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John
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  • The link is not readable. – Dave Tweed Dec 13 '12 at 15:34
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    I'm not going to create a Google Account just to follow this link! –  Dec 13 '12 at 15:44
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    It's not readable even if you have a google account. – pjc50 Dec 13 '12 at 15:57
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    I get error 403 even *with* Google account. – Turbo J Dec 13 '12 at 16:25
  • Well I'm definitely not going to get a Google account just for a 403! :-) We're all working on the assumption that it's a big fat electrolytic here ... but what if it isn't? In any case, 0.15pf sounds a rather unlikely value... –  Dec 13 '12 at 16:32
  • Where in the picture is the part you are talking about? The center of the pictures shows only a few resistors and bare board. In any case, the obvious answer is to replace it and see what happens. This is a el-cheapo single sided punched phenolic board, so replace the part shoul be easy, even if it is thru hole. – Olin Lathrop Dec 13 '12 at 20:18
  • I doubt that it's the capacitor that's overheating considering that it's not an electrolytic one. Have you checked any neighboring devices? It is possible that the capacitor is just radiating the heat from a nearby source (conducted to it through direct contact or the PCB trace). – shimofuri Dec 14 '12 at 10:48
  • That faint crackle is not insignificant. It may be related to this problem, or it may develop into your next problem. I have found noisy resistors, and noisy transistors as well. By all means change the cap, and see how things work after that. – gbarry Dec 14 '12 at 19:43
  • Closed until you can update @john. – Kortuk Dec 16 '12 at 03:41

2 Answers2

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This question gives some of the background here. The capacitor may or may not have developed excessive leakage but its electrolyte is drying out - its ESR (equivalent series resistance) is increasing, so any ripple voltage dissipates more power in it, heating it up and drying it out faster. If you can measure its capacitance, you'll probably find it's lost most of that too... It's not totally unexpected, they are rated for a few thousand hours at a certain temperature - say, 85C.

Dave is right - replace it, paying attention to its value, voltage, and ripple current rating. And expect to replace its twin in a couple of months... better to do both at once.

********* EDIT *******

Now that the photo is visible, the above is wrong! (do I have to downvote it? )

This is not an electrolytic capacitor but some plastic film type, at a lower value (0.15 uF), and without the electrolytic's problems! The most likely reason for its temperature is that it is resting against a (probably rather hot) driver transistor. Is its mate on the other channel clear of the transistor? It should be safe to (gently) straighten it, but I think you have to look elsewhere for the real problem. The equivalent of these capacitors would be suspect in a 1940s valve amp, but not in a modern amp...

Check temperature of the output transistors first (one suspect is the bias current, if there is an adjustment for it) and check the power supply voltages if you can. Oh, and if you can measure capacitance, check the big electrolytics :-)

Community
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Yes, it's very likely that the capacitor in question has developed excessive leakage and/or series resistance. It would explain both the heat and the noise. Replace it.

You might consider replacing both capacitors; if one has reached its end of life, the other one is probably close, too.

Dave Tweed
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  • Dave, you have made my day! I bought this lovely thing for over £1k nine years ago, and at a time when I could afford to. Things could not be more different today. the amps are attached to a monster heats sink and fan assembly, but it's all plugged and socketed, and the thermal paste looks very much re-usable. What type would be the very best (I know that it's a 0.15pf) but now the type. – John Dec 13 '12 at 15:48
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    Can you fix the picture? Then we can give a more definitive answer. – Dave Tweed Dec 13 '12 at 16:54
  • I would suggest that you even increase the rating on the capacitor so that the heat will not cause it to fail as quickly (rule of thumb is life is halfed for every 10 degree increase). – Gustavo Litovsky Dec 13 '12 at 17:16
  • Done. Didn't realise that Picasa web albums were Google only. How silly. AND TYPO I meant mF not pF - sorry. – John Dec 13 '12 at 18:16
  • You're saying it's the little blue capacitor that's getting hot? Very unusual. BTW, the value is 0.15 uF (150000 pF). This is NOT an electrolytic, like we've all been assuming. In any case, if it's the *only* thing getting hot, replace it. – Dave Tweed Dec 13 '12 at 18:31
  • Sorry about the typo, but yes, it's the blue one. Now that it's been running for a while, it isn't the only thing getting hotter than its' oppos. Don't waste any more time on this until I've dismantled it a bit. It's very hard to see the other amp due to a wiring harness. I'll be bsck, thanks for help given so far. John – John Dec 13 '12 at 18:55
  • @John: I'm just being pedantic, but .15mF is 150uF -- huge for a cap of that type. Metric units are a bugger :) I think you meant 0.15uF. – Bryan Boettcher Dec 13 '12 at 20:05
  • @insta : let's see you work it out in imperial units then ;-) About 130 jars, I think... –  Dec 13 '12 at 20:36
  • Sorry, I used mf meaning micro farad! Need to use proper symbols. Tail between legs. J – John Dec 13 '12 at 20:45