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[Yaesu] FT-920 Antenna Tuner Problems

To: <yaesu@contesting.com>
Subject: [Yaesu] FT-920 Antenna Tuner Problems
From: adb1x1@yahoo.com (Anthony Bowyer)
Date: Tue, 10 Oct 2000 17:11:22 -0700 (PDT)

   Nearly a year ago I traded for a Yaesu FT-920 in
excellent condition, except for the antenna tuner.  I
went to each band and started the matching process by
holding the ?TUNER? button in, and the tuner arrived
at a good match on all the bands my antenna covered
(40-6 meters). Then, the next day, the match was gone
on some bands. Trying to tune those bands ended in a
match failure, even with a dummy load! 
   I first suspected a relay or some mechanical
failure, so I opened the transceiver and tried to
determine the problem. There seemed to be no
mechanical problems; everything was secure. But to be
sure I went through the Antenna Tuner Adjustment
procedure in the Service Manual. Everything seemed to
work for a couple of days, and then the problem
started again. I dug into it once again and tapped on
all the relays as it was tuning, hoping to find one
sticking or making erratic contact-None found! 
    I decided to watch the tuner as I went through all
the bands, and noticed that one of the stepper motors
had turned its Variable Cap beyond the fully unmeshed
position. Normally the home position for these guys is
fully meshed. There is a mechanical stop at this
position. Looking from the front of the radio, the
stepper motors turn the caps clock-wise, to, at most,
the fully unmeshed position. There is no stop at this
position, and there is no feedback to the controller
to indicate whether this position, or any for that
matter, has been reached. This time one was beyond the
unmeshed position and I thought that one of the
stepper motors had a problem and replaced it. I should
have watched it more closely, because that wasn?t the
case. The problem returned, and with it, continued
slipping of my sanity. I had about decided that this
was a return to Yaesu problem, and connected a manual
tuner. It was driving me crazy, however, knowing that
the auto-tuner was there, but not working. 
   I opened it up again, started the tuner on each
band and noticed this time that BOTH of the stepper
motors had gone beyond the unmeshed position! Normally
when you first turn the transceiver on, the Caps go to
the home position (fully meshed) and then go to the
saved position for that particular freq. But after one
or both of the caps have gone beyond this unmeshed
position, it seems the home position reference is
lost. Then when you move to another band, the
controller reads the stepper motor positions and relay
sequences from memory, and acts accordingly. However,
the motor position information is referenced from the
home position, which has now changed, and since there
is no positional feedback, the cap settings are
completely wrong. And if you try to tune again, the
controller thinks the caps are somewhere within that
180 degree arc from meshed to unmeshed, but instead
one or both of them is beyond 180 degrees. Therefore
the caps will be turning in an opposite direction.
That?s why no match could be achieved, even on a dummy
load. If you turn the radio off and back on, the
motors will turn for the length of time they are
programmed to turn, but will not go fully home, if
they?re beyond 180 degrees. If you do this several
times, they will eventually get there.  Then the
positional information for most bands is correct
again. You go to one of those bands and everything
works again.  
   I eventually discovered that doing the warm reset
(pressing GEN and ENT on the keypad while turn the
radio on) would clear up the problem for a while, but
it kept returning, usually every few days. I knew I
shouldn?t have to reset it that often. 
   Then one afternoon it was doing this wonderful
thing again. Everything was fine on all bands until I
went to 6 meters. The match was fine there, but when I
went back to 10 meters, I heard a dip in the signal
level and then it came back up a little. I knew the
caps where in the wrong place. I opened it up, and
that was indeed the case. I kept it on 10; turned the
radio off and on enough to get the home position
correct, and started the tuner on 10. It did its thing
and a match was achieved. I went to 6 meters, the caps
moved to the new location. Then I went to 10. Both
caps went beyond 180 degrees. I went through the
off-on-tune thing several times, and each time, when I
went to 6 and back to 10, the caps went passed 180
degrees. The only way to clear it was to do a warm
reset, and retune on each band.
   It then dawned on me what the problem could be. The
tuner uses a serial eeprom to store values. This
eeprom, as well as the micro controller itself are
powered from 5 vdc coming from a regulator on the
Tuner Control Board. This 5v line also goes to the
Main Tuner Board where the relays, coils, caps, etc
are.  Having had some experience with these devices, I
surmised that noise/rf could be getting into them via
this 5v line and causing erroneous values to be
written into memory. There are a couple of 1 mf
electrolytics and .01 mf disc caps along this line,
but with as much rf as could be running around in
there, I thought it needed more. I took the board out,
fired up the soldering iron, and added a couple of
parts. With these additions, I?ve had NO problems with
the tuner at all over a 3-week period. As a matter of
fact, it seems to tune faster and smoother than
before. Here?s what I added:

1. 330 mf  16V Electrolytic. This could probably be
much smaller and work as well, but this is what I had
on hand.

2. .047 mf ceramic disc. I wanted a .1 but this is
also what I had on hand. 

A.  Very carefully unplug all cables from the Tuner
Controlled board. Don?t try to jerk them, you?ll pull
a wire out or break it. Gently rock them from side to
side till they come loose.

B. There?s a white (at least on mine) flat cable that
comes up from underneath the transceiver and slides
into a connector on the Tuner Control Board. Don?t
force this one out. There are small clips on each side
that you have to pull up to release the pressure on
the cable before it will come out correctly.   

C. Remove the 4 screws that hold the board in place.

D. There is a screened position on the board marked 
?C5547?. Nothing is in that position on the board
(mine), nor is there a part on the schematic with this
designation. This is where I placed the 330 mf
Electrolytic. Please note that the Negative connection
is marked with a dot on the board, at least on mine,
please verify that on yours! 

E. The .047 mf cap needs to be soldered as close to
pin 8 on JP5004 as possible and ground. I traced the
5v line and found a spot on the board where I was able
to get to a ground connection that already had solder
on it. 

   I also dress-up the leads going to this board some.
Esp. the leads going to the stepper motors. They are
at least 3 times longer than necessary. I bundled
them with a wire tie.

   That?s it! Put the board back in place and replace
all the connectors. Be sure to get the right connector
to the right socket for each stepper motor.

I know this is fairly lengthy, but I wanted to give
you some of the history in case you have similar
problems. I hope this is of some help.   73 -----
Anthony NT4ZO


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