[ICOM] IC-7800 main rx trouble

[kk2ed at comcast.net]

Good Evening all;
I have an IC-7800 that gave me some trouble yesterday, and was wondering if any other 7800 owners have experienced similar events.
The radio was working fine all day. Worked some DX, then left the radio on and left the shack for dinner.  Came back to the shack 2 hours later and found that the main receiver was dead. It was acting like the sub receiver does when dual watch or split is turned off.
If I put the radio into split mode, RX-B moved over to the main dial position, and receive worked fine. Turned off split, back to RX-A, and no good. Turning on dual watch, which enables RX-B, worked fine. Meanwhile, the whole time the band scope displayed receiver activity perfectly as if all was OK. 
First thing I did was try a CPU reset as per the manual. Same results/troubles.  Once depression settled in, I decided to leave the rig for the evening until I calmed down. Not a warm feeling when your 10k rig dies! 
This morning I decided to dive in to the radio with service manual at hand. I have 20 years experience with radio repairs, so while I've repaired hundreds of HF rigs over the years, this was the first time in a 10k 7800, which of course you can imaging has a different feeling to it!
Since the band scope was still functioning, this led me to believe that the RX-A-PLL unit was working fine. Next in the chain after the PLLIF is the MAIN and DSP units. Therefore, the first place I started was at the MAIN unit. Since dual watch and split mode RX worked fine, that told me that RX-B's AGC and detected AF audio was driving the main and sub audio amplification/volume/squelch/meter detect circuits just fine. Being both RX-A and RX-B are identical, this allowed some A/B swapping to aid in troubleshooting. With the scope, it appeared that DSP-A unit was dead. No AF output from DSP-A. Since both RX-DSP-A and RX-DSP-B modules are identical, and un-plug off the MAIN unit, I swapped the A and B DSP units around. Same results. RX-A dead, RX-B alive. So this ruled out a bad DSP module. I then suspected that the 36KHz IF that feeds the DSP unit was inactive. With a 14.200 100uV signal injected into ANT-A, I plugged a scope probe into my spectrum analyzer, dialed it to 36KHz, and
 probed the IF input to the DSP-A and DSP-B units. Once I saw how the RX-B 36KHz IF signal looked and sounded, I probed the RX-B 36Khz IF signal.  I soon discovered that the RX-A 36KHz IF signal was in fact present and function properly at the input to the DSP-A module. 
After staring at multiple schematic pages and scratching my head for a few minutes, I discovered that the only thing left is the possibility that the DSP-A unit was not being controlled properly by the radio's CPUs.  I didn’t have a logic analyzer handy, so I had no way of analyzing the data and clock lines from the LOGIC board which controls the MAIN and DSP units. So I decided to at least perform a visual check of all connections and voltages on the LOGIC board. 
Once I removed the 7800's front panel to gain access to the vertically mounted LOGIC unit, which is located on the front vertical panel of the main chassis, I first checked all of the connection. All looked fine. Then all of a sudden, I discovered what appeared to be a reset/initialization switch on the LOGIC unit! 
I couldn't find any reference to this switch at all in any of the manuals (service and owners). However, studying the board layout and schematic, it appeared to reset the CPU which controls the MAIN and DSP units. Having nothing to loose at that point, I proceeded to reset the board.
I re-applied power to the radio, and BAMM!  RX-A came alive with an S-9+ signal and full audio. A quick check of all functions relating to RX-A and RX-B proved everything to be back to normal.
Anyway, I don't know who to be more mad at - me for not finding the switch before 3 long hours of exhaustive troubleshooting, or ICOM for not fully documenting certain functions of the radio. 
Anyway, I guess one good thing after all this is I can consider myself trained on the innards of 7800s!
Oh, and by the way, I also had a Power Supply unit fail on this 7800 at 6 months. It is now 30 months old already. Seems like I got it just yesterday. How time flies!
Anyone else have experiences with 7800 failures or troubleshooting?  I'm curious as to what else may lurk down the road. 
73 for now,
Eric
KE2D