[ICOM] Icom 706 Dead on Tx and Rx (Again)

[Al Waschka]

Seveal months ago I posted on here about a problem with my IC-706 (Mk I).  After finding and replacing a damaged cap, the radio got well and stayed well for several months, but the problem has now returned.  The replaced cap is fine.
 
Symptoms are no transmit output and severely degraded (40 dB) receiver operation on HF.  Based on output Rx noise, the attenuator and preamp appear to be working fine.  I can hear the noise at three distinct levels depending on whether the preamp or attenuator are selected.  On strong injected signals, the relative levels of the three options appear correct.  VHF works fine.
 
I have instrumented the radio with test points and traced signals through to the main board.  It appears that the filter unit is working normally.  Signals injected on all HF bands pass through the filter unit and appear unattenuated at the input to the main board.  After the receive signal is routed to the main board from the filter unit, it passes through a high pass filter consisting of C3, C5, C7 and L4, L5.  The output of this filter does not look right.  Either some of the diode switching is malfunctioning or components in the filter have changed values.  Instead of the 3.9 MHz signal I'm injecting, the filter is passing only high order harmonics, and those at attenuated levels.
 
Has anyone seen anything like this, and if so, was a cure discovered?  I'll continued to troubleshoot, but I thought I'd poll the group to see if there might be a shortcut.
 
Thanks,
 
Al K5TAN