SB401 Problem

[Dan Pividal N4VET]

Have an SB401 that is unstable on transmit.
I tune it up and I sometimes don't get full, only 10 watts, output in tune
mode.
If while in tune mode I turn the drive control all the way up the unit snaps
into full output.
Also, if I tap on the inside of the rig while in tune it snaps into full
output.
If I turn the mode control in and out of tune position it sometimes snaps
into full output without turning up the drive.
The problem is worse if unit is cold or has not been turned on for a few days.
If I use the rig often the problem only takes place once or twice.
When it is not working right it will show about 10 watts output.
I think a bad solder joint is the problem but where should I start to look?
Has anyone had this problem?
Dan

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