[ICOM] IC-7800 main rx trouble

[Bill Gode, Sr.]

John - For possible future reference...

----- Original Message ----- 
From: <kk2ed at comcast.net>
To: <icom at mailman.qth.net>
Sent: Saturday, February 09, 2008 10:54 PM
Subject: [ICOM] IC-7800 main rx trouble


> 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 36KH
> z, 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
>
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