[RTTY] Yaesu FT-1000 MP Mk 5 power "drift"

[Dr David L Kepkay]

I am using a pair of Mark 5 Fields to an Acom 2000A system in all modes. 
During the last RTTY contest I played at, I used the Fields at 40w drive and 
that produced about 1000w out.  However there were  2-3 occasions when the 
amp went into fault mode because of "overdrive" and indeed the 40w had 
increased to about 60-75w which meant there was well over 2000w out...no 
wonder the amp faulted!

Since that time I have been trying to understand the events better. I have 
found that both of the Fields react identically:

If I set the RTTY output at, say, 50w using the digital, Array Powermaster 
and transmit....in about 3-4 minutes the output will increase to about 70w 
and plateau there. If I stop transmitting the power out may continue to rise 
say 5w . If I leave the rig for 15-30 min and then transmit, the initial 
value might have fallen back to 50w more or less.

If I transmit again, keeping the output to 25w over the next few minutes, 
and then leave the rig alone once more, the output will have fallen to say, 
4-5w!

This does not happen with CW key down,,,

The fans seem to be working fine and the heat sinks are just a bit warm to 
the touch...say, in the 80's deg F.

I wonder if these observations suggest that the heat build up during 
transmission is causing the power transistors  to have a greater output than 
normal? I have never heard of such a phenomena ...but of course, this is not 
my main field of endeavor. Has anyone ever seen this happen when using RTTY 
mode? Could it hve happened to others but remained obscure because we had no 
such digital precision in output meters?

Will my only option be to "warm it up" first to reach plateau output and 
then set drive where I want it....seems unusual....hhmmmm

73 de David MD VE7KET