FILENAMES: BOSS_5 driver_repair_diodes_front.jpg BOSS_5 driver_repair_diode_rear.jpg DESCRIPTION: These are pictures of the process of repairing a Bridgeport CNC. Posted by LaVerne . LaVerne provided the following description: ================================================================= While using my Bridgeport Series 1 CNC with a BOSS 5 control I ran the machine at high speed into the end stop on the Y-axis resulting in a loss of function in the axis. When the Y-axis was jogged there was only a few thousandths movement but you could hear the stepper driver trying to move the axis. After swapping the X and Y axis boards I determined that the problem was probably with the board as the problem followed the board. After talking to a few people in the newsgroup Don Nichols told me how to check the diodes on the large heatsink at the back of the control. BOSS 5 driver repair rear.jpg shows where to check the diodes on the three assemblies with the diode test function on a multimeter. What you're looking for is different readings on one diode as compared to the others. The test points are indicated with a yellow arrow and if you are not sure compare the readings to another known good board. While I didn't try it apparently the diodes can be checked using just the ohms (resistance) setting. The readings between the screw and the terminals were about 58 ohms and 15 ohms on the good diodes and shorted on the bad. BOSS 5 driver repair front.jpg just shows the front of the assembly. When you replace the diodes make sure to use heat sink compound between the diode and insulator and between the insulator and the heatsink, mine had none on it when taken apart. Make sure you replace the little shouldered nylon insulating bushings on the screws as well. The diodes that were originally used are 2N6547 have been replaced with NTE 53. I had to replace one diode and resolder a poor solder joint on another on th Y-axis. Now, when I moved the driver board the problem still followed the board so I tried the same proceedure on the diodes on the board. One of the small diodes at the connector end had already been replaced but everything there checked out OK but one of the large GM/Delco DTS-411 transistors was shorted. This diode has been replaced with NTE 94. This diode is mounted differently, the outside of the diode requires contact with the trace on the bourd and the terminals are soldered to other traces after the diode is installed. I desoldered and removed the old diode which again had no heatsink compound. When reinstalling this diode, no insulators are required. After replacing the boards everything works as well as before and I figure that it probably only took 2 hours to fix the problem, not counting the time waiting for parts. I now have spare parts so that won't be a problem in the future. While it's probably good practice to limit the amount of heat transfered to the components, I don't think you have to worry much about that here, on the Z-axis board the actual G10 material is brown and the diode got hot enough to melt the solder on the board but still works.