[AMPS] AL-1500 oscillation?

[W7XU Arliss]

I noticed a problem with my Ameritron AL-1500 amp
tonight.  When the amp is keyed in the CW position
with no drive (but with the exciter connected to the
input of the amp), all seems fine, with no RF out
and no grid current.

But if I then switch the mode position of the amp
from CW to SSB, with no other changes, the grid current
begins to flow (slowly at first, then more rapidly) 
and the watt meter indicates about 50 W output.  If the
feedline is removed from the exciter, the problem 
persists.  However, if I then short the input, the grid
current and power out drop to zero.  When the exciter is
reconnected to the input, the amp takes off again.  This
happens with the band switch in any position, but becomes
progressively worse as you go lower in frequency.

Since it's the weekend I haven't had an opportunity to
contact Ameritron, but I'm wondering if others have 
experienced this problem and what was needed to correct it.

73,  Arliss  W7XU

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