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

Tue Feb 21 14:13:51 EST 2012

I use a tuner tuner noise bridge  to tune tuner without putting any power into finals-also shows antenna short or open--really saves those finals!
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-----Original Message-----
From: Roy Westra <rpwestra at gmail.com>
Sender: 1000mp-bounces at mailman.qth.net
Date: Tue, 21 Feb 2012 10:53:49 
To: All about Yaesu 1000mp<1000mp at mailman.qth.net>
Reply-To: All about Yaesu 1000mp <1000mp at mailman.qth.net>
Subject: Re: [1000mp] Unexplained Final failures in Yaesu and Icom and
 Kenwood HF transceivers

I have had the finals in my Mark V blow twice, we believe the culprit was
my Palstar AT2K antenna tuner, since I had the shunt board replaced in the
Palstar tuner I have had no problems. However I am gun shy every time I
tune, do you have a way to protect those expensive 200 watt finals ?

Roy
AA6QQ

On Tue, Feb 21, 2012 at 10:48 AM, Russell Sherry <rsherry46 at hotmail.com>wrote:

>
>
> 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.
>
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