[Yaesu] FT-990 tuner problem

Sat, 31 May 2003 05:11:15 -0400

  The antenna tuner adjusts until it nulls a voltage from the reverse port of a 
directional coupler, which is the wire going going out to the antenna, that goes 
through a torid coil the size of a nickel or smaller.  The core has a center 
tapped winding on it, one end is FWD the other is REV, the center is fed a bit 
of capacity coupled signal from the wire going out to the antenna. (to be 
measured)  Both end wires need to be loaded or terminated a bit, if you end goes 
open the other end can go wild.  I believe this swr bridge is goofed up in some 
way and the computer in the tuner is being given bogus (false) information, it 
is possibly timing out.  The swr bridge works the same with the tuner on or off 
so it should be no problem to look at these voltages into the test load.
Set the output power to minimum (10 Watts) and remove the load and key down for 
a second at a time to see high reverse and low incident. (FWD)
The voltages at the two ports should reverse from what you had with 10 Watts 
into as good restive load.  I expect the swr bridge is on or near the relay 
board just behind the rear panel near the antenna jack. (my books are in boxes)

   Great radio, I love mine, seems like great tuner, my first.

   Ron

Zoltan Bordas wrote:
> Hello,
> 
> A friend of mine recently picked a used but very mint condition FT-990. No
> scratch, exceptional conditions. Except one problem, discovered after the
> deal was made: the internal antenna tuner seems to work "inverted". I.e.
> when the START button is pressed, the tuning begins and after 5...20 seconds
> a match is arrived at about SWR 2 ... 3.5. Sometimes the HIGH SWR LED comes
> on. Not always, seems when the SWR rises around 3. Without the tuner the SWR
> on a triband Yagi antenna is about 1:1.5. After the internal tuner stops,
> the SWR will be greater than without tuner at all. With a dummy load the
> phenomenon is the same.
> 
> Can somebody give us a hint where and what to search for to remedy the
> problem?
> 
> Thanks!
> 
> 
> 73 de Zoli / YO5CRQ
>  
> ______________________________________________________
>  
> All outgoing&incoming emails scanned with NAV2003 Pro. 
> ______________________________________________________
>  
> 
> 
> 
> 
> 
> 
> Moderator: Ray Brown, KB�STN
> _______________________________________________
> Yaesu mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/yaesu
> 

-- 
        73 (= Best Regards) de: Ron [email protected]
     100% Slack. since July, 1997 (still free!) SENT D&T are UTC
        Visit my HAM Web SITE at: http://www.qsl.net/ka4inm