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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