After reverting MARS/CAP mod everything works as expected. I don't know whether the problem was caused by the mod per se or maybe just one of FCC cables was a little bit misplaced when the mod was made.
That's an oddball receiver, employing a so-called Polyakov mixer. Its local oscillator is supposed to run at half the received frequency. For 80M, the local oscillator runs at about 1.8 MHz.
The RF amplifier ahead of the mixer should be very linear...this one is biased weakly, and could easily distort when presented with strong input signals.
This type ...