Looks like IF AGC overload. I looked at manual & the schematic is missing.
Probably on page 46 which is BLANK !
Anyhows check all the voltages around IC101 first.
There is also a "noise" control that is probably the RF AGC control.
Try turning that.
If you post the schematic separately I will look at it to.
73 Zeno
LFOD !