In early November 2015, we received this Roland KC-880 keyboard amplifier for repair. The right rear removable caster had suffered some sort of heavy impact that destroyed the surrounding MDF and bent the caster mounting socket, which had been pulled out of the cabinet altogether. In this article we show how this cabinet was repaired and brought back to full functionality.
We began by removing anything near the site of the damage which would be in the way, such as the metal corner and rubber foot. Some of the work would need to be done from inside the cabinet, so the grille, 12” drivers, and power transformer were all removed as well. The power amplifier board was located far enough from the work area, so it was left in place. The damping material was unglued from the cabinet walls in the work area and tucked up out of the way, then plastic sheeting was used to tent off the repair area from the inside to keep debris out of the rest of the cabinet interior.
A small rectangle of carpet was removed from the work area using a sharp knife. It was extremely important to make cuts square to the cabinet walls so that there would be no gaps when the new piece of carpet was installed. A clear, flat area considerably larger than the actual damage was needed for the router and jigsaw to be able to make accurate cuts,
The remaining carpeting was masked off with clear plastic sheeting so that sawdust would not become embedded in it as the work progressed.
As with the carpet, an accurate, square hole had to be cut around the perimeter of the damage so that a plywood plug could be tightly installed with no gap. Pencil lines were used to position a straightedge which was clamped onto the cabinet to guide the jigsaw for two perfectly straight cuts, one opposite each of the cabinet walls that form the right rear corner of the enclosure.
The bulk of the waste material was then taken out with a freehand cut using the jigsaw, up close to the two adjacent cabinet walls. A very sharp chisel made it easy to clean up the opening, right to the cabinet walls without damaging them.
Once the damaged area was cut out square to the cabinet walls, a ¼” plywood jig was made to fit snugly inside the cutout and guide the router bit around the perimeter of the hole as the rabbet was cut. The rabbet provided a shoulder for the plywood patch to bond to and helped to increase the load bearing capacity of the repair area.
A patch was made for the cutout using high quality, void free ¾” birch plywood. After cutting the patch to the correct outside dimensions, a rabbet was cut along the four sides to match the one made earlier in the cabinet, and then the rounded corners were cut slightly oversize on the scroll saw. Final fitting to the cabinet was done by sanding the edges until a perfect fit was obtained.
Before gluing the patch in place, holes were made for the caster base. Originally, Roland used wood screws to fasten the caster to the cabinet. We use 10-32 T-nuts and machine screws instead, so holes were made for these as well. T-nuts are superior to wood screws in this application because they sandwich the wood between them and the screw, allowing high clamping force without damaging the surrounding wood, and can be easily replaced if the screws or threads are ever damaged.
The patch was glued in place with high quality waterproof wood glue. ¾” wide reinforcing strips were glued from the inside of the cabinet over the seams between cabinet and patch. This both increases the strength of the repair and provides an airtight seal of the repair area. Even a ported cabinet must be airtight (except for the port, of course). Air leaks affect box tuning and can have an audible effect on distortion levels.
The edges of the carpet were pulled tight and glued back on with contact cement, then the replacement carpet was carefully trimmed and glued in place as well. Unfortunately, the color of the new carpet was a slightly different shade of black; this is usually the case due to different manufacturers, suppliers, and dye lots. The only solution is to completely recover the cabinet with new carpet, but that was not part of this job. Overall, the carpet patch came out very tight and neat, with no gaps or wrinkles.
This picture was taken just before the drivers were remounted. The caster mounting base and the power transformer for the amplifier have all been mounted to the cabinet with 10-32 machine screws, lock washers and locktite thread locking compound. Without the locktite, the screws would eventually vibrate loose despite the lock washers, and that transformer would cause some pretty serious damage inside the cabinet. The white polyester damping material was re-fastened to the cabinet walls using stainless steel T50 staples applied with a pneumatic stapler. If you look carefully, you can see the repair area with the ¾” reinforcement strips and the caster socket in the right rear corner.