[ICOM] IC-756PROIII No Transmit

[Frank Haas KB4T]

 My ProIII is one of those used on Peter I Island 3Y0X some years ago.
It worked well for nearly 2 years after I bought it. One day, for no
apparent reason, transmit power vanished. A quick check showed the
DRIVER transistors in the PA were both shorted collector to emitter. 

The 2SC1972 transistor is no longer made. I bought a pair from a
fellow in Canada (not Adam) for about $32 each. The replacements have
worked well. There are a limited number of substitute transistors that
can work in the driver circuit. Everyone I spoke to about that option
cautioned me to be sure to check the transmitter output with a good
spectrum analyzer to make sure the spurious output was still in spec. 

I don't know what caused my driver transistors to fail and haven't
seen any autopsy or analysis information posted anywhere. I suspect
the part was discontinued because it proved to be unreliable but there
is no "official" pronouncement I'm aware of. The only real alignment
step having to do with the driver is to check idle current and adjust
to a specific limit. The PA is connected directly to the 12 volt input
without any spike filters. I use the Icom PS-125 power supply. I could
not see any problem with the output of the power supply. There simply
wasn't any other obvious indication as to why the driver transistors
both shorted hard. Does anyone else have any historical info on the
2SC1972's track record? Is the 2SC1972 the typical failure when a PA
quits?

I understand that there is a (more or less) drop in replacement for
the "old or original style" ProIII PA. When I was researching options
for my driver failure, I spoke with Icom. They would not sell me the
transistors or the new PA. They did say that I could send the ProIII
to Bellevue where they would evaluate the failure and get back to me
with options. I presume that meant either using their stock of
2SC1972s or swapping out the PA entirely. Since I was able to get the
actual part, I chose to make the repair myself. 

Just as a matter of curiosity, I wonder what the PA in the 7600 looks
like. I wonder if the PA in the 7600 is the same as the "drop-in"
replacement for the ProIII PA??? I wonder if any spike suppression has
been added to the 12 volt input to these high end rigs???

73,

Frank N. Haas KB4T