MFJ 989D Intermittant SWR readings above 20 meters
 

I have an MFJ 989D that I picked up used. I changed out the flimsy
rotary Inductor with a well insulated ceramic insulated one from an
old military tuning unit measuring 31 uh. It works great.It was
physicially the same size and only took an L bracket made from
Aluminum and the gears line up perfactly for the turns counter. What I
see now and BEFORE the inductor change is above 20 meters, i.e. 15
and 10 meters, while applying 30 watts to tune the antenna, the power
and swr meter pulsates, and then shuts down. I put another swr bridge
into the input of the 989D and it acts normal and I can use it to tune
the antenna with out the pulsating.

Anyone ever seen this problem? The 989D works great as far a tuning
but the SWR indicator circuit seems to have some kind of problem above
20 meters.

Any help would be appreciated..

thanks, John aka W3ZDF

MFJ 989D Intermittant SWR readings above 20 meters
 

On Fri, 07 Aug 2009 15:36:36 -0400, John wrote:

I have an MFJ 989D that I picked up used. I changed out the flimsy
rotary Inductor with a well insulated ceramic insulated one from an old
military tuning unit measuring 31 uh. It works great.It was physicially
the same size and only took an L bracket made from Aluminum and the
gears line up perfactly for the turns counter. What I see now and
BEFORE the inductor change is above 20 meters, i.e. 15 and 10 meters,
while applying 30 watts to tune the antenna, the power and swr meter
pulsates, and then shuts down. I put another swr bridge into the input
of the 989D and it acts normal and I can use it to tune the antenna with
out the pulsating.

Anyone ever seen this problem? The 989D works great as far a tuning but
the SWR indicator circuit seems to have some kind of problem above 20
meters.


I have the same unit but it does not do this.

However, what I think I'm reading is that the meter or the bi-directional
coupler is having problems and that the matching network is fine?

If the matching network is the issue, I've had the fingers that feed the
plates of the variable caps get dirty. If the silver plating burns
through, you will have to replace the variable caps as a greater failure
is emanate.