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Lightening Strike Repair by K1DEU

One memorial day morning, I turned on my Yaesu ft-1000 MP to find I had no more than 20 watts output on any mode or band. Next, I noticed that the built in reflected SWR metering was full scale even into my dummy load. I f I used my antenna tuner into my dummy load and went off one side of resonance ( 3:1 ) , I could null my reflected reading on the FT-1000 MP internal SWR meter. Oh No ! The previous evening I had forgotten to ground my 80 meter antenna system on the 120 foot tower, and there was lightning in the area.

With the above symptoms, I assumed the internal SWR bridge was kaput. Looking at the schematic's which came with the rig, I finally found the SWR bridge on a low pass filter board located near the RF output finals. I knew these finals had to be bolted to the bottom of the massive heat sink, and sure enough, I found the 4 diode SWR bridge nearby. The schematic showed the diodes as 1SS101 , and by using Google I downloaded a PDF application file, that showed the diode characteristics as Shottky Barrier type. My trusty Radio Shack Ohm meter ( 22-163 ) revealed one diode ( in circuit ) having no front to back ( 85 ohms in both directions ). The 3 good diodes read 85 Ohms back and .5 Ohms forward. I called the people at Yaesu Tuesday morning, and they suggested I double them up. This sounded a little strange, but the bottom line was " they don't seem to order a second time. " Therefore, I ordered 10 diodes ( two spare ). And when they ( part # change from 1SS101-T1 , G20902 to G2060018 ) arrived two days later, I first made up 4 diodes , with two doubled up in parallel to replace each of the original four diodes. Sucked and soldered away, I fired it up and I didn't even have to touch the null trimmer. And away I go until "The next time. " Whew ! No chance for shipping damage is a relief ! BTW: diodes are long wire, leadout types, very easily changed !

Technical service said this SWR bridge problem has happened many times before. For the future, I also ordered a 3 tab +9 volt voltage regulator ( PN: KIA7809PI / G1092163 ) to put on the shelf, in case my Audio Output IC , shits the bed some day ( Vermont dialect ) . Time to say thanks to Vertex / Yaesu for their parts stocking and time to look into transient voltage clamps ( back to back zener diodes ) for antenna input protection. The original 1SS101's in the SWR bridge are rated at 70 volts and 15 Mils.
 

 

 

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This site was last updated 08/21/07