Repairing Bird Wattmeter Slugs

Amateur radio operators use in-line Bird wattmeters to tune transmitters and linear amplifiers for optimum performance by measuring the output of radio frequency energy in the coaxial cable leading to the antenna. The wattmeter is set up for a particular frequency range and transmitter wattage (power) by the use of a replaceable plug-in element known as a "slug" which installs into the front panel of the unit. An assortment of slugs to cover the various amateur frequency ranges and output powers represents a substantial investment above the cost of the wattmeter itself. Though not all defects in a slug are user serviceable, the option to repair certain simple faults such as a dead diode or blown resistor may save considerable money over replacement.

Things You'll Need

    Small slot screwdriver
    Multimeter

    Soldering iron and solder

Instructions

1.Remove the metal label affixed to the front of the slug to expose the cap screw. If two rivets securing the label are visible, they may be pried out with a small flat-blade screwdriver. If the label is glued on to the slug, remove it by inverting the slug and submerging just the label in a commercially available solvent for label and gum removal. Alternatively, the metal label may be ground away using a hand-held rotary saw with a grinding wheel.

2.Locate the 1/8-inch access hole for the trimpot adjustment screw visible when the metal label is removed. Plug the slug into the Bird wattmeter and attempt to adjust the trimpot for proper reading on the wattmeter. If results are unstable, replacement of the trimpot may be necessary by unsoldering the original trimpot and installing a new unit of equal ohm values. If simple adjustment of the trimpot appears to rectify the problem and produce stable results, reassemble the unit.

3.Unscrew the large center cap screw and take out the cover plate. Bend the leads of the trimpot to move it out of the way or unsolder it. Remove the piece of insulating paper under the trimpot.

4.Remove the two small screws under the insulating paper which secure the black insulated plastic cover.

5.Remove the black insulated plastic cover which mounts the electronic module inside the slug body.

6.Look for any loose wires or broken components shorting the slug electronics.

7.Check the diode using an ohm meter. Replace with an exact replacement if necessary. Examine the single installed resistor and capacitor for burning and replace if necessary. Reheat the various soldered joints to repair any intermittent connections.

8.Reassemble the slug in the reverse order of disassembly. Observe the correct alignment of the orienting pin when replacing the electronics module – it must be installed into the slug body in a particular orientation. Make sure to replace the piece of insulating paper under the leads of the trimpot. Do not reattach the metal label to the head of the slug until testing is completed.

9.Test the slug in the wattmeter and attempt to calibrate it by turning the trimpot adjustment screw. If results are satisfactory, install the metal label and press fit the two rivets into place or re-glue the label if it was glued.

10.Purchase a new slug if the repaired slug still cannot be calibrated or is unstable.

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