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EE help please for plate amp!!

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  • EE help please for plate amp!!

    I have had three PE sub plate amps fail. They are over ten years old but I am still disappointed. I was able to repair one by replacing a visibly blown tiny capacitor.

    The other two are different amps, but share very similar (but not identical) power supply boards. We have an electronic waste collection in town tomorrow, so I took one more look at them before pitching them.

    One amp would light the power light but no sound or even a turn on thump. I found two 6800uF 80V caps in what I think is the power supply board. One was slightly swelled on top and rattled softly inside when shaken, but when removed from the board it tested OK--tested just like the other one with no visible swelling.

    The second amp had blown fuses in the power cord receptacle fuse holder. I borrowed a fuse from the other amp. When I powered up this amp, some pilot/function lights did light but the amp hummed louder than it should. Within a few seconds the same pair of 6800 uF caps began to cook and one showed the same case swelling on the top. Time to yank the cord out!

    Anything is possible, of course, but I'm wondering if these caps failed solely because of age or did something else fail and consequently damage the caps? The second amp was new in box--never used.

    If it is likely only these two caps, then it may be worth replacing them, but then I wonder how long the smaller caps on the board will last.

    What do you think?

    Thanks,
    Marvin


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  • #2
    If you saw them begin to get warm and swell up, you're likely either getting reverse voltage or excess voltage on them.

    When you say they measured correctly, was that with an LCR meter or DMM? If so, those are only testing at low voltage. There are times where faulty caps can measure just fine with low voltage from an LCR meter, but under full voltage, will show that they are faulty.

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    • #3
      The capacitors were probably marginally rated to begin with. Heat is always an enemy of most any and all electronics. I would imagine that the temperature inside the subwoofer enclosure was always quite warm maybe even hot when the subwoofer was being used. Get larger, higher voltage capacitors. There may be other problems too.

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      • #4
        I've had, >5 failures. The disappointment drove me to to separate amp, Crown drive core 2 with DSP bridged. With adjustable input sensitivity it runs great from my old AVR LFE connection.
        John H

        Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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        • #5
          My plate amp has been running 24/7 for about 15 years. One reason why is that it's not in the speaker cab, so it can breathe. The other is because my sub is horn loaded, so what most subs need 100 watts to produce mine will with 10.
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

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          • #6
            The problem is definitely not one of overheating since the second amplifier was really never used. It's been sitting in the box for a long time.

            I did my testing with a digital meter which has a capacitance tester function but I don't think it is designed for this large of a capacitor. So I'm not real confident in the test. When the cap fails the shake test and I can hear it softly rattling inside I know there's something wrong.

            With the track record of these amps and Jon track record as well, I don't feel too confident and trying to repair these. I think they will go to the recyclers tomorrow. Now I just have to decide what to replace them with.

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            • #7
              Originally posted by billfitzmaurice View Post
              My plate amp has been running 24/7 for about 15 years. One reason why is that it's not in the speaker cab, so it can breathe. The other is because my sub is horn loaded, so what most subs need 100 watts to produce mine will with 10.

              I was not using a sub for a while in my Ht and one day I walk into the room and the sub is humming. Best that I could tell the auto on circuitry had been cooking the amp. Then there was the time I thought the woofer had blown because of huge distortion on bass notes. Nope plate amp. The 5-year warranty from PE is tempting but the extra $ seems to me to be well spent for a separate amp.
              John H

              Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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              • #8
                Originally posted by marvin View Post
                Anything is possible, of course, but I'm wondering if these caps failed solely because of age or did something else fail and consequently damage the caps? The second amp was new in box--never used.

                If it is likely only these two caps, then it may be worth replacing them, but then I wonder how long the smaller caps on the board will last.

                What do you think?

                Thanks,
                Marvin
                ​Electrolytic capacitor reliability has been studied extensively, and for devices that used quality components and good manufacturing processes, the failure rates are usually predicable. Capacitors that are used in power supplies are subject to a certain amount of AC ripple that causes some internal heating. That, combined with being in a sealed box with no airflow can result in high failure rates. But if you use a part specified with a long life at 105 degrees, the life should be longer than your own. Most plate amps that I've seen use parts specified at 85 degrees, which will result in less reliability. If you replace those parts with a long life parts specified at 105 degrees you should have a reliable amp.

                ​But as you note, your failure wasn't due to internal heating because the second amp hadn't been used. That means the failure was due to either poor components (electrolyte material, internal contacts, etc) or poor processes (improper handling or cleaning or some other contamination). So these are probably just poorly made caps. Replace them and all should be well.

                Usually the lower voltage smaller capacitors are more reliable, so there is probably no reason to replace those as well as the large pair in the power supply section. But do a search on the manufacturer name and see if there is a history of failures due to poor components or processes.
                Free Passive Speaker Designer Lite (PSD-Lite) -- http://www.audiodevelopers.com/Softw...Lite/setup.exe

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                • #9
                  Originally posted by marvin View Post
                  The problem is definitely not one of overheating since the second amplifier was really never used. It's been sitting in the box for a long time.
                  That could indicate a high degree of engineering expertise from the standpoint of planned obsolescence: the amp will die just beyond the warranty period, whether you use it or not.

                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    Thanks to everyone for their comments!



                    This may explain the failure of the un-used caps:

                    Parts kept in long-term storage (2-3 years or more) may have very high leakage when first used and should not be subjected to full rated voltage (and should be current limited) until the leakage falls to normal. Reforming of the dielectric can be done by applying a voltage that is slowly increased to maximum over a period of several hours, with current limited to rated leakage current. Some manufacturers have published such elaborate recommendations for reconditioning capacitors after long storage that it seems easier to just buy new ones. However, while common wisdom has it that storage is very bad for aluminum capacitors, manufacturer's literature does not always support this. One manufacturer has a 1000 hour/85C shelf-life test that specifies that leakage will not exceed that of a new capacitor. That's equivalent to about 7 years at 25C. It may be that modern capacitors are less susceptible to storage problems than older ones.


                    Source: http://iequalscdvdt.com/Reliability_1.html

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