[ICOM] IC-756PROIII No Transmit

[Lawrence Young]

   I am in communctions frequently with the Icom service facility in SC. I have been told that rarely does the BJT final transistors fail in the Pro line. It usually is driver failure.
   The reason that Icom changed to FET transistors in the Pro series is because Mitsubishi their supplier for the Pa transistors discontinued their BJT power transistor line.
   A search of the ProIII groups archives will confirm this.
Larry K4LXV


--- On Tue, 3/24/09, Frank Haas KB4T <kb4t at arrl.net> wrote:

> From: Frank Haas KB4T <kb4t at arrl.net>
> Subject: Re: [ICOM] IC-756PROIII No Transmit
> To: icom at mailman.qth.net
> Date: Tuesday, March 24, 2009, 10:59 AM
> 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