[ICOM] Thank you, Adam

[Frank Haas KB4T]

I appreciate the quick response and the advice. 

I understand the design considerations and the practical aspects of
leaving high current stuff energized even when the radio is shut down.
Now I know I need to shut down the power supply AS WELL AS the radio
when shutting down the station.

In Florida, as some pointed out, we have many lightning induced
surges. Quality surge protectors and battery back up units protect all
the gear in my ham shack. Quality lightning protection devices are
installed on all RF lines into the shack. But I admit my lightning
protection does not possess the same industrial strength as that used
at commercial tower sites and it didn't keep all of the DIRECT
Lightning strike that hit my tower last year out of the shack.
Fortunately, the measures I take limited the damage to a level far
less than it could have been. It's a matter of cost vs. benefit. I
live in Florida. I accept the gamble.

Lightning can stress components enough that they will fail sometime
after the strike. My failure occurred a full year after my strike
which leads me to believe that the failure of the two driver
transistors was probably not caused by the lightning strike....but
I'll never know. The max base to collector voltage of the 2SC1972 is
only 35 volts so the part is probably more sensitive than it should be
to momentary voltage spikes. This may be part of the reason why the
part is no longer made. Since both parts suffered identical failures
and one is hotter now than the other, I'm guessing that one suffered
the base to collector short first which put excessive voltage on the
base of the other which then suffered a junction failure. 

I haven't seen much commentary about PA failures in the ProII or
ProIII so it's not been a major issue. I will chalk this one up to
component failure possibly affected by lightning stress and see if the
replacements can last longer than the 2 years the first set worked. I
will be shutting down the power supply along with the rig from now on
in the hope that this will lengthen the lifetime of these parts.

Does anyone know if there is a schematic and/or board layout pix of
the new PA for the 756Pro series? I'm surprised that it's not an
upgrade option since the parts used in current PA's are no longer
being made (to my knowledge.) Perhaps all 756ProII & ProIII users will
be expected to replace failed units with the IC-7600, 7700 or 7800???

73,
 
Frank N. Haas KB4T