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Troubleshooting and Diode Repair
of the MFJ-4712 Remote Antenna Switch!

by Jimmy, WX9DX

After having problems with my MFJ-4712 Remote Antenna switch, I found little if any repair information for it on the internet, so here are some repair tips for you to enjoy!

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.
Another problem was that the unit in the house did blow the fuse in the 12 volt power cord.

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
 "Tower Side".
This I'd like to share with the Ham radio Community, so that they know what is up on their tower. I also put back heavier diodes in the unit during the repair!

Included in these repair hints is the drawing of the board in the remote switch along with other helpful photos seen below!

The MFJ-4712 consists of two parts.
1. The remote switch inside the house
2. The Relay box outside usually mounted at the antennas feed points location.

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.

For some of us Geeks:

Antenna #2 has a ground on it's center conductor when Antenna #1 is in use,
 " no DC voltage on coax". This happens inside the antenna relay box on the tower. This happens because of connection from Com " GND" to
NC " Ant #2" of RLY #2.

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.

MFJ-4712 Remote Control Switch Box

Internal components of MFJ-4712 Remote Switch

Schematic of MFJ-4712 Remote Control Switch

MFJ-4712 Remote Antenna Switch Main Circuit Board (outdoor unit)
(Notice the 2 Diodes in the center of the board just above C1 and C2)

Parts list

2 each 1n914 diodes
1 each 1.5 amp 1000piv diode
(availiable at any good electronic components supplier)

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!

(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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