On Thu, 18 Feb 2010 15:00:15 -0800 (PST), klem kedidelhopper
wrote:
I have an old MFJ SWR meter with a built in counter.
Which one of these?
http://www.mfjenterprises.com/Catergories.php?sec=13
The only one that has both an LCD and meter is the MFJ-259B:
http://www.mfjenterprises.com/Product.php?productid=MFJ-259B
Any particular reason why you find it necessary to hide the model
number? You generally get much better answers if you disclose such
information.
It has an LCD
display to read frequency and it uses a small 2 inch korean made
plastic analog meter to read SWR. I keep this unit in my service truck
wrapped up, however it may have gotten jarred because the unit still
works fine but all of a sudden mechanical zero is up about ten percent
of full scale.
See if you can find a similar meter. Note that the jeweled bearings
at the ends of the meter needle shaft are thin brass cantilevered
arrangements, which are easily bent by a high G shock load. It's not
unusual for the meter pivot point to literally fall out of the jeweled
bearing. More commonly, the arm gets bent, causing the meter to move
erratically. It's also possible to "tangle" the two springs that zero
the meter. That results in a zero drift, similar to what you're
seeing. Swing the meter back and forth sideways and see if the needle
moves smoothly.
There is no zero adjustment accessable through the front of the
instrument. This is a nice little unit however to adjust mechanical
zero you have remove the analog meter and open the case. This requires
complete disassembly of the entire instrument and it is a real pain.
Yep. MFJ probably saved about 5 cents on the cost of the case by
removing the meter adjust holes. However, meters should not require
zero adjustment unless affected by a nearby magnetic field. With two
meters that close together, there's going to be some magnetic
interaction. Perhaps MFJ adjusts it at the factory, and doesn't want
anyone changing it as the procedure might be a bit convoluted. Dunno
and I'm guessing.
That brings about another possibility that all the jarring around in
the service truck cause the meter magnet to break loose. Shake it.
Hear anything rattling?
I did this and opened the small meter and adjusted mechanical zero.
This is a simple procedure however the problem is static electricity.
Depending on how I hold this meter, touch it, rub my finger or even a
piece of cardboard near it, mechanical zero may shift. At times I
think that I have it zeroed and I close the case up only to find that
zero has shifted. Then I touch it wirth my finger and zero will
sometimes shift again.
So the problem is that I don't know if after I touch this thing if It
is totally discharged and it is really at zero or if static
electricity has influenced the position of the needle. This simple job
is really driving me nuts. It is very dry in the house and I'm sure
that this may have something to do with it, but I just don't know how
to compensate for this.
I don't think it's static electricity. If it were, BOTH meters would
be affected. Since you didn't mention that there were two meters or
that the other meter was affected, my guess is that only the SWR meter
is giving you problems. Also, the meter plastic front is not large
enough to collect a very large static charge.
Schematic:
http://www.dk3red.homepage.t-online.de/en/s8a.htm
Hmmm.... SWR meter has a 100K pot in series to the output of the OP
amp. Not exactly low impedance. So, the meter is essentially running
with no dampening. Eliminating static electricity and a broken shunt,
that leaves mechanical damage.
Obviously it is important that my adjustment is correct as there is no
access to it once the unit is re assembled. Can anyone offer any
advice? Thanks, Lenny.
No advice. Just two questions:
1. What's the model number?
2. Are both meters exhibiting the same zeroing problem?
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
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