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QRZ! Ham Radio 21
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QRZ Ham Radio Callsign Database - Volume 21.iso
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MFJ1279.TXT
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2002-12-30
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81 lines
I don't know if you'd care to publish this, but I'd like to disseminate it
to the ham community at large:
REPAIR of a "NUKED" MFJ259 SWR ANALYZER
(Do NOT attempt unless you have good eyes or bifocals or both and steady
hands!)
I managed to damage my 2-year old MFJ-259B Antenna Analyzer a couple weeks
ago by transmitting into a non resonant mobile antenna with a new IC706 at
full power while the MFJ was attached to the VHF 2 meter antenna just a few
feet away, on the vehicle roof.
I noticed that the Analyzer suddenly maintained about a 6:1 SWR reading,
whether the antenna was attached or not, and whether a dummy load was
connected or not!
I emailed MFJ about possible repair strategies and costs. They responded
with a minimum of about $70 for in-shop repair and offered nothing in the
way of information on what to look for myself. $70 is not an unreasonable
shop repair price; my biggest fear was having to wait a month or more to
get the instrument BACK.
I took the cover off and found the "Bridge" of diodes that is the heart of
the analyzer. The diodes looked like surface-mount 3-legged transistors,
but only 2 legs on each device showed any diode action, and ONE of the
diodes was dead shorted. I emailed MFJ again, asking for part numbers and
prices and availability, and several days later I have still not heard from
them. Rather than sit on the phone trying to get through on the MFJ tech
support number, I decided to just try installing some similar devices.
I rationalized that, if I could install 4 similar devices, the bridge should
work acceptably. I am not interested in minute accuracy and the MFJ is not
marketed as a commercial-grade calibrated instrument anyway.
I found some defunct RF receiver boards that contained schottky diodes that
were surface-mount and strongly resembling the MFJ devices, which are marked
"CO" only.
The MFJ originals test at 23 ohms forward bias, and my junkbox substitutes
tested at 12 ohms, but I decided to try them anyway, theorizing that if I
replaced all 4 devices in the bridge circuit, it should still work. My
junk-box diodes were also "backwards" in their pinout, so I had to install
them upside down to get the polarity right.
I'm happy to report that the analyzer now shows >25 SWR with nothing
attached to the test antenna port, and 1.0:1 when a 50 ohm load is attached.
My antennas now test normally as they used to with this device.
First, the aluminum outer case is removed by releasing the 8 screws
attaching it. Then, the plastic AA battery tray must be removed. It has 2
small phillips head screws holding it to the main circuit board. Remove the
exposed 4-40 screw and nut holding the upper half of the SO-239 antenna
connector. Unsolder the grounded solder lug on the RIGHT side of the SO-239
connector as you view it with the connector pointing up; the analyzer being
on its face and you viewing it from the back. Bend this lug up so it will
allow the main circuit board to be lifted past it to expose its underside.
Remove the 4 screws on the outer corners of the main circuit board so the
board can be lifted. Use a small (1/16th inch, I believe) Allen wrench to
remove the 2 knobs from the front of the analyzer so the entire board can be
removed.
Remove the 4 small Phillips head screws holding the digital display to the
front of the analyzer. This will be removed along with the main circuit
board. On mine, some of the hot-gun glue holding this display and cable
together was melted on the inside of the front face, requiring gentle
probing with a knife blade or small screwdriver to loosen the glue so the
display would release from the panel and come out with the main circuit
board, to which it is attached by a short ribbon cable.
Unsolder the center pin of the SO-239 connector, lifting the entire main
circuit board up about 1/8 inch. Unsolder the short wire from the circuit
board to the frequency counter BNC input connector. Lifting gently,
watching the interference as the tuning capacitor shaft and band select
switch shaft come out of their sleeves, and bringing the circuit board out
past the BNC connector on the top.
Carefully turning this main circuit board over, you will see 4 small
surface-mount diodes surrounding the now empty slot that was just unsoldered
from the SO-239 antenna connector. One on the left, one just the opposite
direction of the antenna solder-hole, one on the right, and one a bit to the
right and upward, as you view the board upside down. Replace these diodes
with your desired substitutes. I'm sure MFJ will sell replacements to you,
if you are patient and bother them enough. But I suspect that as long as
the polarity is correct, and they are surface mount devices rated into the
VHF region (most are), the device will measure SWR acceptably well even
though the sensitivity may be changed somewhat.
You can test the analyzer with:
Nothing connected to the antenna port - Should read very high SWR
50 ohm load connected - Should read very close to 1.1 to 1
100 ohm load (if you have one) - Should read very close to 2:0 to 1
Thus ends my "Fix it without a Service Manual" essay.
Jim Devenport W5AOX