[1000mp] Monitor roughness

[Pete Smith]

Ever since my Mark 5 was a pup, it has had a problem with the monitor note 
(on CW) becoming rough on 15 meters after the transmitter has been in use 
for a while.  There does not appear to be any corresponding roughness on 
the transmitted note, and it never happens when the radio has not been 
transmitting, however long the radio has been on in receive.

The radio has been back to Yaesu once with the problem, but they were 
unable to find anything.  Since then, it seems to be getting gradually 
worse.  It is definitely NOT RFI getting into the sub-RX -- I can turn the 
RF output down essentially to zero and the problem continues, and even 
disconnect every external connection except the key jack, and it still happens.

Anyone got any ideas?

73, Pete N4ZR