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Troubleshooting and
Diode Repair
Mine quit and the repair was simple
but costly, because a tower climber was involved. There are 2 diodes inside the remote tower unit.
It had D1 shorted which caused the switch to stay on antenna 1.
After getting it down from the
tower, I took the remote switch apart, and then made an electronic drawing
of the main circuit board in the remote switch Included in these repair hints is
the drawing of the board in the remote switch along with other helpful
photos seen below! The "in house side" (the switch box), of the MFJ-4712 is just an on/off slide switch that sends dc power up the feedline. It is isolated from the feedline center conductor by an RF choke, and has a couple of capacitors to ground off the switch side of the choke. The RF from the transmitter is protected from the dc control voltage by several blocking capacitors, ".01 MFD 1kv", that are in parallel with each other to increase the voltage rating. This blocks the DC from back feeding to the transmitter, but allows RF to flow through the in house switch unit of the system to the feedline. This is called a power inserter. Antenna #1 is being used when there
is no DC being fed to the remote antenna relay switch on the tower.
Antenna 2 is selected at the remote relay switch when DC is being fed up the coaxial cable from the Remote Control "RC" box " MFJ-4712RC". Just a note to
the wise only use + Positive
DC
on the center
contact of the coaxial power connector. You can burn this unit up if you use a
reversed polarity DC or AC from a power source/cube! What blows up is one
or both of the diodes across the relays in the remote relay box "D1, D2".
I use larger value diodes (see parts list
below) across the relays in the MFJ-4712 relay switch box on
my tower. In the power cord feeding DC to this unit I use a 1 amp
fuse.
2 each 1n914
diodes I put in 2 each 1n914 diodes in parallel
for each relay. I also added a 1.5 amp 1000piv Radio Shack diode from the
relay side of the choke " L1" to circuit board ground!
I feel that I over did this, but a little over building in this case does not hurt, especially when there is very expensive radio equipment in the radio shack below!!! :) The larger diode will act as reverse polarity and AC protection in case the wrong power cube is hooked to the unit. But, please only fuse the power cord for this unit with the minimum amp rating you can get by with. You might get by with less than I did " 1 amps works for me" ! :) The 1n914 diodes are very fast
acting and is why I used them for the quencher diodes at each relay.
Like I had mentioned, I used two of these 1n914 diodes insted of the
single small diode MFJ used!
MFJ WEB SITE LISTING (Product Description, download manual, etc) I hope this helps you with repairs of your MFJ-4712...GOOD LUCK IT! 73, JIMMY WX9DX (wx9dx AT
mwii.net)
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