10/18/07, After throughly giving it consideration, I decided to get an additional Astron DC power supply to power my station (VS-70). I had reservations about purchasing another Astron because of a noise/vibration issue I had with an RS-20M which I already own. However, after reading multiple reviews, it seems that this problem is more the exception than the rule, so a new supply has been ordered, and it should be here soon. In regard to the RS-20M, I did find a way to quiet down the supply to a very slight hum, with no vibration. To address this issue, I installed a silicone pad under the transformer, which resolved about 95% of the noise problem.
Here is the modification that I performed on my power supply. If you choose to perform this modification, you are doing so at your own risk. For materials you will need a supply of silicone pad material (I purchased mine in the form of silicone pot trivets for hot pots sold at Target), you will also need four #10 X 3/4 inch screws with lock nuts, and one #8 X 3/4 screw with a lock nut, a short length of insulated #14 wire, two ring terminal for the wire, and some masking tape. For tools you will need screwdrivers and nutdrivers for you screw/nut combination, and a drill with drill bits. A vacuum cleaner is also useful for cleaning up metal shavings caused by removing the rivets which secures the transformer to the case. First, unplug the supply from the AC wall socket, turn the main switch on, and short the positive and negative terminals. This will drain any residual charge out of the filter capacitors. Second, remove the screws from the bottom and sides of the case and slide the cover off. Once open, you will see that the transformer is riveted in place with two washers on each rivet, however one actually has three washers, two flat washers and one star washer, don't discard the washers, you will use some of them later. Take the masking tape and temporarily seal the windings on the transformer to prevent metal shavings from getting into the windings. Turn the unit upside down and drill out the rivets from the bottom. Once all the rivets are out, you should have eight small but thick washers that you can pull off of the rivets, and one star washer. Next, measure the transformer and mark the transformer foot print and screw holes on the silicone pad. Cut the silicone pad to the appropriate size. For the screw holes, I used a simple paper hole punch to make the holes in the pad. Once the pad is cut to the right size, slide it under the transformer, lining up the screw holes. Take the excess cut off material from the silicone pad, punch four equally spaced holes about a 3/4 of an inch apart in the material, and cut out four "silicone washers" of equal size. Next, install three screws up from the bottom of the case, through the silicone pad, the transformer frame, a silicone washer, and one of the thick metal washers, then secure each screw with a lock nut. The first installed screws should be the two in the rear and one in the front, closest to the side. Next, locate a convenient hole in the bottom of the case near the remaining transformer hole and drill it out to accept the #8 screw, now make a jumper of wire using the two ring terminals to go from the #8 screw to the remaining transformer mounting hole. Now install the final transformer mounting screw through the case, silicone pad, transformer frame, jumper ring terminal, silicone washer, thick metal washer, then the lock nut. Finally, install the star washer first and then the other end of the jumper onto the number eight screw and secure it with the #8 lock nut. At this point you should check continuity between the transformer frame and the power supply case for ground. Remove the temporary tape from the transformer, vacuum out the unit, and reinstall the case cover. The modified supply should now be much quieter when it is on.
73, Jim AB4D