KRK V8 Studio Monitor Repair!

[angelothewolf]

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...

[Jazz P Bass]

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

[Enzo]

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.

[angelothewolf]

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...

[Jazz P Bass]

Was the amp worked on before this or did this suddenly come up?

[angelothewolf]

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

[Jazz P Bass]

Check your work on the replaced ic.

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

[diydidi]

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.

[angelothewolf]

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!

[diydidi]

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.

[angelothewolf]

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¿?

[Jazz P Bass]

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.

[angelothewolf]

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...

[angelothewolf]

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!

[diydidi]

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.