View Single Post
Old 2nd December 2017
  #3
CAUTION: Repair at your own risk. Always be aware of high-voltages which could shock you, i.e. be especially careful around "mains" voltages, amplifier rails, switching power supplies, and high-voltage capacitors. Obviously, you don't want to be touching these while energized, and some capacitive circuit retain a charge.
____________________________________________________

In Post #2 , we've already seen the PCB's up close, so it was assumed that the speaker was already taken apart. However, in this post, we'll start from scratch, but focusing on component replacement.

NOTE: The following guidelines were created on a set of KRK Rokit 6 speakers, but the general disassembly, troubleshooting, and reassembly may be used on all Rokit models/sizes. See next post for the schematic for KRK Rokit 6 (G2) for more in-depth evaluation, component selection, and repair.

First Steps

Turn monitor on it's side as shown and remove the 8, Philips pan-head screws around the perimeter of the backplate.



Take a small flathead screwdriver and pry the backplate from the cabinet. Drop it down to the table.




Replacing Bulged Capacitors and Burnt Resistors

Check out the capacitors and the resistors for trouble. I've included pictures of the particular trouble that plagued my speaker. NOTE: KRK put this awful, crusty black goo all over everything on the amplifier board. Basically, it's glue to hold various components and connectors onto the board, preventing vibrations from disconnecting or breaking them.




The capacitors are used for positive and negative rails (±30Vdc) for the amplifier circuitry, perhaps in an AB class design. The bulging capacitor means that it has failed. The dielectric either dried out and/or the rail voltage exceeded the rating of the capacitor, causing it to fail. In my haste, I did not pre-measure the failed capacitor's voltage while powered on. This would have been a good tidbit of information. However, I surmise that the root cause of the failure is the capacitor, which caused a high leakage-current which in turn raised the rail voltage thereby causing overheating of the 2.2kΩ. Whew! Anyway, we gotta get these garbage components out of here!

Rotate the backplate counter-clockwise into the cabinet, being careful not to overstretch the wires. You could disconnect some of the leads, but why bother. The goal here to to access the solder side (back) of the amplifier board.



Using the soldering iron at the highest setting and the desoldering braid (with a little flux), remove the two, 3300µF capacitors and the failed resistor(s). I chose to not replace the resistor that was OK. No reason to do it. The capacitors, however, are crappola and need to be removed regardless of how they look!



Install and solder the new components to the circuit board. Hopefully you noted the orientation of the capacitors before you removed them! Either look my pictures and/or note the silk-screening on the component side of the PCB. Here are the new components soldered to the board.




Reassembly

Reassemble the speaker by reattaching the backplate. Do not not over-tighten the 8 pan-head screws as the speaker cabinet is made of MDF. The material is soft and the screws can strip easily.