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Thread: KRK V8 Studio Monitor Repair!

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    Post KRK V8 Studio Monitor Repair!

    Hi folks I wonder if anyone can help me to figure out why this 3 LM3886 are getting VERY hot! even without load they are getting pretty much hot. I think its getting over the 165 degres because is distorting the output and the chip is shuting down itself and distorting and shut down again, every 2 seconds.

    The PSU is feeding +/-41VDC to the chips with 200 mV p-p Ripple voltage. Is this too much supply for the chips? too much ripple?

    Already checked all components in the board and everything seems to be fine. All electrolitics are new!
    I dont have too much experience with this chips so a little help would be very apreciated.

    Thanks and sorry for my english!


    there goes the schem...
    Attached Files Attached Files

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    Supporting Member Jazz P Bass's Avatar
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    Try removing the 2.49K input resistors, to see if the heat issue stops.

    The input may be oscillating.

    Either that or all three LM3886 ic's are bad.

    http://www.ti.com/lit/ds/symlink/lm3886.pdf

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    Lifetime Member Enzo's Avatar
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    Two of your 3886s are in parallel, so if one is bad, the other will be affected. The third probably shares the heat sink.

    My first guess is bad ICs.

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    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Jazz P Bass View Post
    Try removing the 2.49K input resistors, to see if the heat issue stops.

    The input may be oscillating.

    Either that or all three LM3886 ic's are bad.

    http://www.ti.com/lit/ds/symlink/lm3886.pdf
    Thanks for the reply! i will check that tomorrow morning! i doubt that the three ics be damaged...

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    Supporting Member Jazz P Bass's Avatar
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    Was the amp worked on before this or did this suddenly come up?

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    suddenly come up... i just pulled up the input resistors of U101 & U201 and U101 stops heating, while U201 remain hot, so i replace U201 with a new one and power up the unit. Now sudenly i got a nice short somewhere. I got a series lamp on the mains that is fully light up.....aggrrrr

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    Supporting Member Jazz P Bass's Avatar
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    Check your work on the replaced ic.

    If any ribbon cables where removed & reinserted, check that they are properly installed.

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    Just my two cents, but make sure those filter caps are discharged before replacing the Ic's. Any charge even a little might causes Ic's to fail when replacing.

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    Quote Originally Posted by diydidi View Post
    Just my two cents, but make sure those filter caps are discharged before replacing the Ic's. Any charge even a little might causes Ic's to fail when replacing.
    And why is that??, isnt the 3886 have a short protection? turns out that the short was U101. Just buy 3 new ICs, would be great to discover the problem before i put this new ones...

    I Discharge the caps before i change the Ic, was a clean replacement, i dont know why the other Ic short then. Any thoughts┐?

    thanks for answer!

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    I dont know where you're buying them from, but- I once bought 14 of them from a store close to me. Only 5 of those 3886's actually worked. The rest were all faulty.
    Be careful who you buy from.
    Use a scope to check the outputs for oscillation and DC. Two of your 3886's are in parallel. Make sure you use two here with the same batch number for close matching.

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    Quote Originally Posted by diydidi View Post
    I dont know where you're buying them from, but- I once bought 14 of them from a store close to me. Only 5 of those 3886's actually worked. The rest were all faulty.
    Be careful who you buy from.
    Use a scope to check the outputs for oscillation and DC. Two of your 3886's are in parallel. Make sure you use two here with the same batch number for close matching.
    Ok, its a trust supplier. I didnt understand why a little charge on the caps might cause the Ics to fail when replacing┐?

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    Supporting Member Jazz P Bass's Avatar
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    When working on any amp, or replacing parts, it is very wise to remove all charge from the system.

    The amp was not designed to take sporadic charges in odd places.

    When you reattach the header for the LM3883, if there is a charge on the power supply, that is exactly what you are doing.

    Introducing odd voltages, at odd times & at odd places.

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    yes i know, i always do that... i just didnt understand very well the answer from diydidi. Lets see what happens when the new chips arrive.

    Thanks for the help...

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    Ok, short was a diode on the PSU.

    i change all the ICs (3886) and the problem persists. Output is distorting, and the ICs getting very hot. I got V+ (44VDC) on OUTPUT pin! How is that possible???? help please!

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    Opamps are all direct coupled to stages without any coupling capacitors.
    Two things, Check +/- 15 Supplies, and also check pin 1& 7 of opamps for DC.
    Also make sure your not using single opamps in dual opamp sockets. If the darn thing has sockets that is.

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    Last edited by diydidi; 02-22-2014 at 04:32 PM.

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    OK, +/-15V supplies are OK. I got 7.5mV on U2 pin1 and 2.6mV on pin7. the rest of the opamps are below 2mV dc on pins 1 & 7...

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    I hate to say it but you've got a tough dog here with this repair.
    Ok. Next step: are the feedback resistors on Lm3886's good and are the trax linking these intact?

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    Yeah, already check those. Im getting crazy with this sh.. the only reason can be the 3886 isnt it? what doesnt fit in my head is that i have the same problem before with the original 3886s.

    got the two boards on the bench, the good one and the broken one. measuring resistance betwen V+ and Output (3886) i get 50Meg on the good one and 4Meg on the broken one. Intresting... but how the ff.. i get 40 VDC on the outputs????? any ideas??

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    Got to be lm3886's

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    Two of your 3886s are in parallel, so if one is bad.

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