This can be caused by a blown relay Driver transistor on the the FILTER board. There are a set of band-pass filters switched by input/output relay pairs.
If one of the relays stays closed, then two filters are effectively in parallel, so the filter tuning is stuffed, and the PA doesn't see the correct load.
Check by measuring the relay drive power as the receiver is tuned. (Set the dial tuning steps to 100 KC for ease of tuning.)
Measure the drive change on each relay coil at the appropriate frequency transisiton.
The switch points for changeover of the filters are:
Freq MHz Drive Line Relay pair
0 - 2 L1 RL14 and 15
2 - 4 L2 RL12 and 13
4 - 8 L3 RL 4 and 5
8 - 15 L4 RL 8 and 9
15 - 22 L5 RL 10 and 11
22 - 30 L6 RL 6 and 7
30 - 60 L5 RL 2 and 3
The problems in both cases had probably been caused by RF leakage on transmit across the PCB due to salt. The rig had been used in the tropics on an island in a past life. There was corrosion evident on some of the tin-plate shields, mild-steel screws, etc.