Check the listing for details. Cobra 138/139 MOSFET upgrade kit. Condition: New. Listed at 21.99 USD. This modification kit contains all the parts you need to install a MOSFET in place of the original bipolar transistor in your CB radio. Detailed instructions are included. The final transistor installed in most CB and many Amateur radios were discontinued and are rapidly diminishing. They are hard to find and very pricey. If your final has been damaged now is the time to make the conversion to a MOSFET. All high powered devices are changing to MOSFETs. They will be widely available for many years to come. In most cases they produce more power and in all cases they give more modulation. On most radios the time to make the change is under 1 hour. You need only some basic electronics knowledge and good soldering skills. Don't be fooled by those cheap kits. These are not ERF or IRF devices. These unmarked Fairchild MOSFETS are very rugged and forgiving. They were designed for the medical imaging industry and are very tough and may forgive YOUR mistakes. You must specify if you have the XLR version.