[ICOM] 756ProIII PA Failure

[Frank Haas KB4T]

This past weekend the output power on my 756ProIII dropped to 3 watts
while I was calling CQ on CW. Upon investigation I found that both of
the 2SC1972 driver transistors were shorted base to collector. One of
the two drivers had gotten so hot it charred the circuit board holding
D1 in place over it. I don't see any other problems. 

There is no obvious cause for this failure. No power line surge or
lightning at the time of the failure. The PS-125 power supply seems to
be delivering a stable 14V to the radio. Nearly one year ago to the
day my home took a lightning strike which damaged other gear but
didn't seem to affect the ProIII. I suppose it's possible that this is
a long delayed stress failure from that incident. The ProIII was
running 100 watts output to a tuner and a ladder line fed 80M full
wave loop that has no obvious problems. 

With the ProIII down, I moved my backup ProII into the primary
operating position and I'm still on the air without any problems now.

It appears that 2SC1972 transistors are hard to find now. I read on
Adam's web site that there is a later version of the PA available
using newer style components. 

My question is: Should I try to locate a pair of 2SC1972s and complete
the repair for about $150 or replace the PA? 

I was surprised to find that power is applied to the PA all the time
even with the rig turned off. I wonder if this is a good practice?

73,
 
Frank N. Haas KB4T
Florida