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KRK RP8 Autopsy

I looked closely at the Active Filter board because my hunch says the problem is on this board. I saw a "suspicious" looking short and I fixed that. See picture below.

This is the photo of the Active Filter board separated from it's monitor. This board will be scrutinized by me.

First, we need to hook this up to the amplifier board, and for curiosity's sake, let's look at the output waveform. Here, you can see me connecting the output of the amplifier to my oscilloscope and feeding the active filter board signal from my Signal Generator.

Turning on the Volume knob towards max, I reached clipping. Not Good! Of course, at maximum volume setting, it is getting close to a square wave.

The Moral of this lesson is do not turn the volume knob to Max or halfway to Max. Just leave it at 0dB. okay? Unless you enjoy the sound of clipping square waves.

Another shot of the Active Filter board. It looks like a patient on an ER operating table with wires sticking in and out from the patient.

I am disappointed in the workmanship of the RP8 electronics. It's just lazyness! Plain lazyness. Look at some of the things I've found. Components not inserted fully, components soldered crookedly, resistors installed at an angle... like a ramp. Just plain lazyness!!!! Whoever the guy in China that worked on this should be fired!

I'm thinking, maybe those signal diodes are causing the noise... so I desoldered all of them, and put in 1N4148.

But upon powering on, there was no sound. Totally Dead.... this is weird... it looks like I made the problem worse. I checked if any of my diodes were put on backwards compared to the PCB marking. They're not.

Then I notice that Stand By pin was at 0.1 Volts... almost 0 volts. This is not good. If the STD BY pin is 0V, the amp is effectively turned off.

So.... I found a diode connected directly to the Stand By pin. And I'm thinking... we don't need a diode there, so why don't I just jumper it directly? And that's what I did. See picture below...

Guess what???? The monitor is quiet, and it works now. No more static noise, no more noise of any kind! It works just like new. I'm happy!

Conclusion: I don't really know what made the static noise, but me changing all signal diodes on the board with 1N4148 and replacing Z1 diode (pictured above) with a straight jumper made the problem disappear. No more noise!!!!

That's all folks! I hope somehow this short RP8 article has been of interest to you.

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