[R-390] R390A Popping

[Chuck]

Guys,
My R390A has recently developed a problem. 
During ordinary listening conditions in either AM or SSB, it pops at
regular intervals. The signal strength meter jumps when this occurs.
But, the pops are not always the same amplitude on the meter. The
popping seems to respond to a change in agc settings.
At first I thought it was external interference as it quits when the
antenna was disconnected. However, after comparing other receivers in
the shack and hearing nothing, I came to the conclusion it is internal.
Before tearing into the radio I thought I would ask the group for ideas
and suggestions
TIA
Chuck
ka6uup@pacbell.net