How to Repair an Amateur Radio MFJ-949C Antenna Tuner

​What is an antenna Tuner?

An Antenna Tuner is a device that is connected between a radio transmitter's (or transceiver) antenna output and the antenna. Its job is to provide efficient power transfer between radio transmitter and the antenna. The Antenna Tuner achieves efficient power transfer by matching the impedance (resistance to alternating current flow) of the antenna to that of the radio transmitter. In most modern transmitters, the antenna output impedance is 50 ohms.
Most Antenna Tuners have a SWR (Standing Wave Ratio) meter which measures forward and reflected power. You use this meter to correctly adjust the dials and switches until the SWR meter indicates a ratio of close or equal to 1:1. This is when the most efficient power transfer occurs.
An antenna that is not properly impedance matched to the transmitter antenna output could cause the transmission line to heat up, or the reflected power could cause damage to your transmitter!

---

When do I need a Antenna Tuner?

If an antenna's impedance is perfectly matched to the band of frequencies the transmitter is to operate, then a Antenna Tuner is not required. For example, I have a Yaesu FT-1900R 2M FM Transceiver connected to an Arrow J-Pole Antenna in the attic. I do not need an Antenna Tuner as the antenna is impedance matched for the 2M band.
Typically, an Antenna Tuner is required when using a multi-band transceiver to drive a multi-band antenna. A mutli-band antenna typically cannot be perfectly impedance matched for all the bands of frequencies it is intended to be used on. The Antenna Tuner is used to compensate for the compromises made to the antenna in order to cover multiple bands of frequencies.

---

​What's inside the MFJ-949C Antenna Tuner?

The MFJ-949C is a manual Antenna Tuner that is composed of an T Network consisting of two air insulated variable capacitors in series between the Antenna Input and Antenna Output SO-239 coax connectors. There is also a mutli-tapped air wound inductor between the capacitors which connects to ground. Impedance match is obtained by changing the capacitance and inductance of the T Network using the front knobs until a SWR meter reading equal or close to 1:1 is obtained.
A tapped torrid transformer on the input is first rectified then attenuated in order to drive the forward and reflected power meter movements. The MFJ-949C also has a built in 50ohm dummy load that can be switched in and can handle 300 Watts of transmitter power for short durations.
A 1:4 Balun is also installed in the enclosure to allow the MFJ-949C to drive a balanced line instead of coax.
In addition, the SWR meter has a jack on the back so the back light can be powered by a 12 Volt source.
​

---

​The tough life of my MFJ-949C Antenna Tuner

My MFJ-949C has been riden hard and put away wet! It is the first and only Antenna Tuner I ever owned since I started my Amateur Radio hobby. As a "newbie", my poor Antenna Tuner had to bear the brunt of my inexperience. More then once I did not have it set correctly an internal arcing has occurred. In addition, I have "pegged" the SWR meters by having the switch on the front set to the 30 Watt instead of the 300 Watt range. Now it is time to assess the damage done.

Keep things organized during the dis-assembly process!

I keep all of the nuts, bolts, screws, knobs, and washers I remove during dis-assembly in a disposable salad bowl so I don't lose anything!

Step 1 Cover Removal

There are three screws on each side of the MFJ-949C that need to be removed. See areas circled in red.

In addition, two screws on the top of the cabinet need to be removed as well. See areas circled in red below​.

​Step 2 Examine the inside of MFJ-949C to look for problems

The first thing I noticed was carbon on the printed circuit board around the connection to the inductor (circled in red). I found that this connection was loose, probably where the arcing occured.

I also found a carbon resistor, circled in red, with a piece of the external coating missing. Could have been a defect in the resistor or over heating.

Step 3 Circuit Board Removal

De-solder the wires from the circuit board to the connectors on the back of the chassis. See areas circled in red in the picture.

De-solder the wire from the circuit board to the Transmitter Matching variable air capacitor. See area circled in red in the picture.

Disconnect the coupler that connects to the Mode Switch knob on the front of the Antenna Tuner. See area circled in red in the picture.

Disconnect the tapped air wound Inductor from the base of the Antenna Tuner, it is held in place by two screws. See areas circled in red in the picture.

Remove the knobs, then unbolt the Transmitter Matching, Inductor Selector, and Antenna Matching shafts from the front of the Antenna Tuner.

Three screws, circled in red, attach the printed circuit board to the base of the Antenna Tuner. They will need to be removed.

​Picture of T Network removed from Chassis!

​Move Evidence of Arcing!

Circled in red is more evidence of arcing between the printed circuit board and the chassis, most likely due to an error in on my part when adjusting the Antenna Tuner's settings.

​Step 4 Look for damage on the trace side of the printed circuit board

The trace side of the circuit board is where the discrete components are joined electrically. Circled in red in the picture is where arcing occurred between the printed circuit board and the metal chassis. Examine all traces closely for other damage.

Step 5 Clean Electrical Contacts

The electrical contacts of the Inductor Selector switch can be cleaned by spraying contact cleaner at the spots electrical contact is made then rotating the switch to all positions. Repeat this process until the contact points look shiny and clean.

The wafer switch connected to the Antenna Selector can be cleaned by spraying contact cleaner in the holes, then rotating through all positions.

Step 6 Replacing Compromised Resistor

It is important to find a replacement resistor as close as possible to the value of the original resistor, otherwise, it could affect the calibration of the SWR meters. De-solder then measure the resistance of the original compromised resistor.

Resistors usually come in multiples of 10, 20, 50 or 100, all of the same value. Due to tolerance, a resistor's value can be slightly off. Choose a replacement resistor that most closely matches the resistance of the original.

Insert replacement resistor through the appropriate holes in the printed circuit board then solder in place. Clip the excess leads.

​Step 7 Re-solder the Inductor connection to the printed circuit board

Insert the Inductor connection through the appropriate hole in the printed circuit board then re-solder in place. This is where the arcing occurred so make sure you scrap off the carbon from around this connection as is could be conductive.

​Step 8 Re-mount printed circuit board to chassis

The circuit board is held in place by three screws and standoffs. Make sure the 30/300 Watt Selector button is properly protruding out of the front of the enclosure.

​Step 9 Re-mount the T Network

Place the T Network components that are wired together back in the enclosure. See area outlined in red in the picture.

The shafts to the Transmitter Matching, Inductor Selector, and Antenna Matching controls are to be insulated from the chassis, make sure you re-install the fiber washers!

Install the metal washer and nut onto Transmitter Matching, Inductor Selector, and Antenna Matching shafts, carefully tighten.

Use multimeter to check for continuity between nut on Transmitter Matching shaft and chassis. There should be no continuity.

Use multimeter to check for continuity between nut on Inductor Selector shaft and chassis. There should be no continuity.

Use multimeter to check for continuity between nut on Antenna Matching shaft and chassis. THERE SHOULD BE CONTINUITY

Re-mount Air Inductor to chassis, you will need to use needle nose pliers to hold the nut in place while you thread the screw in from under the chassis.

Step 10 Re-solder connections to Printed Circuit Board

Re-solder the connection between the printed circuit board and the Transmitter Matching variable capacitor.

Re-solder the connections between the printed circuit board and the Twin Lead, Antenna, Coax1, and Coax2 connectors located on the back of the chassis, see areas circled in red in the picture.

Use a multimeter to check for continuity between the center pin of the Antenna connector and the chassis. There should be no continuity with the Antenna Selector switch in the Coax1 position.

Use a multimeter to check for continuity between the center pin of the Coax1 connector and the chassis. There should be no continuity with the Antenna Selector switch in the Coax1 position.

Use a multimeter to check for continuity between the center pin of the Coax2 connector and the chassis. There should be no continuity with the Antenna Selector switch in the Coax1 position.

​Step 11 Re-install Knobs

It is important to re-install the knobs so that they point to the proper position.

I made sure I clicked though all positions to check alignment once the Antenna Selector knob was installed.

The Transmitter Matching and Antenna Matching Knobs are properly installed if the air capacitor's plates are fully meshed when at position 6.

Picture of air capacitor's plates fully meshed. See areas circled in red.

The Inductor Selector knob should be pointed to A on the front of the Antenna Tuner when the center ring is touching the contacts to right of the bottom bolt that holds the rotary switch together.

Step 12 Cover Install

There are three screws on each side of the MFJ-949C that need to be installed.

In addition, two screws on the top of the cabinet need to be installed as well.

​Conclusion

Manual Antenna Tuners, such as the MFJ Industries MFJ-949C Versa Tuner II, are pretty simple to service as long as you keep organized and don't loose any parts during the process!