[Fourlanders] FT-920 problem

[Jim Worsham]

When you TX when in transverter mode what does the ALC do?  Does it stay at 
or near zero?  Go full scale?  The ALC rectifies some of the TX power and 
feeds the resulting DC voltage back to some previous stage to reduce gain 
(negative feedback).  Every transceiver I have ever seen provides a way to 
monitor that DC voltage so you can set it (usually using the mic gain) to a 
value that results in optimum TX linearity.  Since you have diasabled the PA 
there shouldn't be any ALC voltage generated so I am as confused as you 
about what is reducing the power.  That is why I want you to look at the ALC 
voltage.  If there isn't any then something else is causing this to happen. 
SWR foldback would be the next suspect.  Again with the PA disabled I don't 
see how that could be an issue either.

73
Jim, W4KXY

----- Original Message ----- 
From: "Bob Lear" <w4zst at windstream.net>
To: <fourlanders at contesting.com>
Sent: Wednesday, August 15, 2012 10:28 PM
Subject: [Fourlanders] FT-920 problem


>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
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