Thursday, October 3, 2013

MFJ 993B Switch Problem

  I own a MFJ 993B antenna tuner and have suffered from the dreaded on/off power switch failure. The antenna tuner functioned properly for about 4 to 6 months and then it would not power on. When the on/off switch was pressed it would have power and then shut off. I did some research on the internet and found that this was a common problem. I emailed MFJ and they sent me a couple of switches to replace the bad switch at no charge.
  Well I have replaced the switch at least four times and this would have been the fifth time. I started checking the internet for a better solution then to just keep replacing the switch. I found some switches at some suppliers on the internet such as Jameco and Digi-Key. I then came across a person that decided to replace the switch with a toggle switch.
  I checked my junk parts and found some on/off toggle switches that are 2amp 125vac, which will easily handle the current and voltage. My next step was to ohm out the six pin power switch to determine what the pin outs were doing. I found that 4 pins were either on or off depending on the switch position. I removed the defective switch from the smaller front board. I looked at the traces and found the two pins were shorted by a trace as well as the other two pins. The last two pins have no trace on either side of the board. I shined a light behind the board and found there are no traces at all going to those pins. They seem to be just for support.
  I should say it is a little tricky removing the small front board, because there is a grey ribbon cable connecting the small front board to the main board. I soldered two wires to the two holes that have the traces shorted. I the drilled a hole in the lower right side of the front panel. I pretty much eyeballed it because the bottom case does slide in under it. I did put some electrical tape on the bottom under the switch just to make sure nothing shorts out. I then soldered the two wires to the toggle switch.
  I powered on the antenna tuner to see if it powered on properly. It powered on and off properly and the meter needles moved properly. I closed it up and connected back in place with my antenna switch and my ts2000 radio. I then tested it in the system and it is working properly. The best thing is if I ever have to replace the toggle switch it will be much easier to replace.
  Thanks for reading 73s from KC2YTI.     

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