[1000mp] Unexplained Final failures in Yaesu and Icom and Kenwood HF transceivers

[Russell Sherry]

Unexplained Final failures in Yaesu and Icom and Kenwood HF transceivers
After 10 plus years of operation my FT 1000MP MK5 (200 Watt) had a final failure. After diagnosis and the determination that the failure was not explained some six months later there was a second failure. At this point additional thought was given as to the actual cause and future prevention of this type of event. Output power at the time of both events was 70 watts operating in class AB1.
Typical HF solid state transmitter topology uses a transistor amplifier followed by a bank of relay witched low pass filters that is followed by a VSWR detector. That detector is also used to control forward power. VSWR shutdown is done through the ALC circuit in the transmit IF amplifier.
When a lowpass filter selection relay is open and there is no load or antenna connected to the power amplifier (PA) and drive is applied to the PA high voltages are generated at the drain of the PA output transistors. This is especially true if the output selection relay in the two relay lowpass filter selection circuit is the one that is open. The reactance of the filter components generates high drain voltages. Since there is no RF going to the VSWR  and forward power detection circuit because of the open relay the ALC signal is not reduced and maximum drive is applied to the gates of the PA transistors. The high drain voltage along with the high gate voltage from maximum drive causes a drain to gate short in one or both of the output FETs.
This is what in my opinion causes many unexplained final transistor failures.
Prevention of final failure is done by placing a DC voltage and current  through the low pass filter relay selection contacts in such a manner that when a relay or other component such as a connector or cable connection is not present transmission is prevented. This is done through using the VSWR shut down of the ALC input to the transmitter IF chain. This DC supervision is done automatically and happens before any RF is applied to the PA transistors preventing them from being damaged.
To find out about how this is very simply done contact me at rsherry46 at hotmail.com or my present QRZ email address and I will provide you with a PDF file including pictures and schematic for the  installation of this modification in an FT 1000MP MK5.
This solution is also applicable with modification to Icom and Kenwood HF transceivers.
Russ Sherry, NM6DX.