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One of my clients has a number of SWR SM900 bass amps. When the company had created their last version of the amp, now having two power transformers and much better heat sink amp assemblies with enough power transistors to survive, they also got clever in the sheet metal shop and folded up their new aluminum top cover with folded rack flanges off the top cover. Seemed like a clever idea to stiffen the 1/8" front panel, only it wasn't very well thought out. All the weight of that 2U chassis is hanging off the folded rack flanges of the top cover and the folded-down tabs that pick up teh front top span of the front panel.
Two of these amps kept continually coming into the shop with hum problems that kept re-occurring. The wiring harness from the two amp modules to the rear panel was very sensitive in wire placement, as well as the multi-wire harness off the front panel to each module. In removing the top covers each time, I began to see stress fractures developing on the rack flanges of the top cover, as well as on the fold-down tabs along the front edge of teh top cover, where it gets picked up by screws thru the front panel. As the two circuit board assemblies are tied together with long standoffs, and anchored to the chassis floor, it became apparent that at some point, when the rack flanges finally shear thru the fold in the top cover, and then the the tabs break away from the front edge of the cover, now all the weight of the amp is being supported by the bottom edge of the panel/chassis AND the control bushings of the PCB 's.
You can see the complete fracture now on the top cover's rack ears in the last two images. I knew what I was going to have to do was to fabricate new rack ears from 1/8" Aluminum extrusion, cut to length, then machine them for mating mounting holes to mate the rack panel openings, as well as provide mounting holes for #10-32 hardware to attach them to the front side covers. On one, where the top folded flanges had sheared away, I installed 1/2" square aluminum bar stock to pick up the top of the front cover, while on another, the brackets hadn't yet failed, so just the side brackets added was enough to save it.
The outcome on the two SWR SM900's with the damaged top covers I've revised solved the problems caused by the weight of the chassis inducing problems to the wiring and the PCB assemblies. Fender had taken over SWR, and since discontinued the products. No replacement top covers was available, leaving it to this sort of repair.
When I was working for BGW Systems as a product engineer, we had done a similar top cover design on the Millenium Series amps, which I saw placed all the weight on that folded rack-ear flange joint, as well as on the side cover screws in supporting the weight of the amplifier. We were at least building the covers out of steel, and had also gone to a heavier 14AWG steel for the top cover, if memory serves.
Two of these amps kept continually coming into the shop with hum problems that kept re-occurring. The wiring harness from the two amp modules to the rear panel was very sensitive in wire placement, as well as the multi-wire harness off the front panel to each module. In removing the top covers each time, I began to see stress fractures developing on the rack flanges of the top cover, as well as on the fold-down tabs along the front edge of teh top cover, where it gets picked up by screws thru the front panel. As the two circuit board assemblies are tied together with long standoffs, and anchored to the chassis floor, it became apparent that at some point, when the rack flanges finally shear thru the fold in the top cover, and then the the tabs break away from the front edge of the cover, now all the weight of the amp is being supported by the bottom edge of the panel/chassis AND the control bushings of the PCB 's.
You can see the complete fracture now on the top cover's rack ears in the last two images. I knew what I was going to have to do was to fabricate new rack ears from 1/8" Aluminum extrusion, cut to length, then machine them for mating mounting holes to mate the rack panel openings, as well as provide mounting holes for #10-32 hardware to attach them to the front side covers. On one, where the top folded flanges had sheared away, I installed 1/2" square aluminum bar stock to pick up the top of the front cover, while on another, the brackets hadn't yet failed, so just the side brackets added was enough to save it.
The outcome on the two SWR SM900's with the damaged top covers I've revised solved the problems caused by the weight of the chassis inducing problems to the wiring and the PCB assemblies. Fender had taken over SWR, and since discontinued the products. No replacement top covers was available, leaving it to this sort of repair.
When I was working for BGW Systems as a product engineer, we had done a similar top cover design on the Millenium Series amps, which I saw placed all the weight on that folded rack-ear flange joint, as well as on the side cover screws in supporting the weight of the amplifier. We were at least building the covers out of steel, and had also gone to a heavier 14AWG steel for the top cover, if memory serves.
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