[Fourlanders] FT-920 problem

[Bob Lear]

I have a problem with my Yaesu FT-920.

About 10 years ago, I added transverter ports to my 920.  All I did was 
put a DPDT relay inside to switch the low-level signal lines for RX and 
TX that go from the main board where the ~1mW TX signal originates 
before going on to the PA module and the received RX signal comes to the 
main board after coming from the BandPass Filters.  These lines are fed 
with mini coax to TMP connectors.  I also broke the T12 line to the PA 
board to shut it off completely when using a transverter.  I had 
previously done the same thing to an Icom IC-736.

All was well with both radios and were used extensively with 
transverters for many years.  Then a problem appeared with the 920.  I 
wasn't getting any TX output to a transverter, but RX was working fine.

I at first blamed the problem on a faulty relay, but after replacing it, 
found that I still had the problem.  At the same time, I modified 
another 920 for a friend in exactly the same way, and it also would RX 
but not TX to a transverter.  When the relay is switched to normal 
operation mode the radios (920's) both operate on HF/6 as normal with 
full 100W output.

Now I have narrowed the problem down to something like the ALC folding 
back the output power when in transverter mode.  I checked the output 
power at the TMP connector going to the PA board using a Boonton 
microwattmeter.  In normal mode the power is just over 1 mW driving the 
PA to 100 W.  In transverter mode, the steady state power out is folded 
back to just a few microwatts.  I can see on the analog meter on the 
microwattmeter that on first key down, the meter starts to jump up, but 
then it drops back, indicating that the ALC/SWR/Temp power reduction 
circuitry is doing something to reduce the power.

I really don't think I understand ALC circuitry and would like to have 
some help to figure this problem out.

The strangest part is that my 920 worked fine with transverters for so 
many years and then changed.  The second 920 had reduced output 
immediately after I put in the relay/modifications.

I have a three page pdf schematic set for the 920.  Can't attach it on 
the reflector, but if anyone is interested in helping me out, I can send 
it separately.  On page two of the schematics (main board), all the 
modifications are on the left side, the TX out in upper left and RX in 
below are where the relay is inserted.  Lower down, on J1014 is where 
the T12 line goes out to the PA unit.  That had a switch inserted there 
at the connector to open the T12 line to the PA board for transverter 
operation.

Thanks for any help you can be on this.  Bob W4ZST