Connex 3400HP terrible receive.

The 455 kHz ceramic IF filters are famous for being off frequency. Makes signals that ARE on frequency sound like doo-doo.

73

Hmmm...Pin Diodes...going bad...

Usually means something spiked them, and you're still encountering the problem (after the 455kHz swap) check the caps at U1 (IC1 Quad Op Amp) - may need to see if the PIN didoes and your "punched down" receive is due to the AGC sending more power to the PIN diodes at idle than what they should be getting.

Might want to DVM the Pin Diode line and also if there's too low of a voltage from the RX switch line to power the "tickler" R43 (270K) (have to say that - Thanks to Mike) that pre-sets the AGC "start" voltage and also the feedback resistor R36 (100k) in the loop of it that keeps the slope linear versus being trigger happy on-off stuff when there isn't any tickler voltage, the loop to make that part of the chip linear (R36) or the caps that "dampen" the clamping action of the AGC - by your description of symptoms are going bad.

Check C44 and C32 -

When you get to the "60dB" to make the receive work, What is the PIN diode line doing? Is it even clamping? The voltage should change - but is it going up or down or nothing?

The AGC line (junction of C44 and unbanded side of D16 - any voltage change when the RX seems to work?

I noticed what sounded like heterodyning tones during rx on signals that should be clear. Turned out to be the power supply I was using was a noisy switching supply and using a linear supply removed the issues. I guess the radio may not have good filtering and the ripples were bleeding through. Just passing on what I observed with this radio. YMMV.