[Yaesu] Yaesu Digest, Vol 101, Issue 4

[Dr. Gerald N. Johnson]

Sometimes an intermittent open in a SWR bridge can cause it to indicate 
high reflected power and the PA protection circuit uses that indication 
to cut way back on drive. I'd look over that circuit carefully, heating 
each solder connection, then I'd test with an external SWR/power meter 
to see if the power was going away before that indication showed up. I'd 
also try heating and cooling the circuit parts with a hot air gun (or 
hair dryer) and a circuit cooling spray to find the temperature 
sensitive component. In the worst case of not being able to find a 
cause, heat and cool about half the radio. One half at a time. You will 
be able to cause the symptom then cut the area heated and cooled in 
half. One half of that will contain the problem, keep cutting in half 
until you are to the individual part. Replace that part after checking 
the solder connections by reheating them. Sometimes an IC can develop an 
intermittent open inside the package, sometimes its the connection to 
the external circuits.

73, Jerry, K0CQ

On 9/25/2012 3:24 PM, David Thompson wrote:
> I still have two working FT-980's and my newest one has developed a problem
> with transmit.  When first turned on transmit at 100 watts (any mode except
> AM) works fine.  But eventually in the middle of transmitting the SWR goes
> off the top and the transceiver stops transmitting.  It comes back quickly
> but after a few seconds again stops.
>
> I found this only occurs with over 50 or 60 watts.  The cut off occurs with
> both an antenna or into a dummy load.  I have
> looked all over for signs of a problem or a bad connection.  I have had
> FT-980's since 1983 and this is a new problem for me.
>
> Any ideas?
>
> 73 Dave K4JRB
>
>