WB4IUY's Random Blog

I've been in Amateur Radio since 1974, and still find new and interesting things to do. I like to build, restore, and operate on the air. This is a blog of various info about my Ham Radio operations and activities, projects, and opinions. Visit www.WB4IUY.net for the lowdown at WB4IUY.

Sunday, November 12, 2017

MFJ-259 Antenna Analyzer Repairs...

I pulled out my MFJ-259 antenna analyzer to start tuning my Force-12 antenna, and discovered that it had a problem... It would not operate on the last 2 bands (lowest frequency ranges). Upon inspection of the internals, I discovered a battery had leaked acid on the main PCB and caused some damage. I washed the board with electronics cleaner and windex to remove oils from the cleaner, scrubbed it with a brass brush, and blew out the excess liquid with canned dry air. Acid had leached into the L2 (a 7.8uH inductor) and damaged it, it was electrically open. I poked around in my bins and found an inductor that was close in value, and installed it on the trace side of the board. This brought the lower bands back to life and restored the operation of the antenna analyzer.

The little hole in the PCB in the middle of that exposed copper area (seen in the photo), leads directly into the adjustable 7.8uH coil. I was able to get close in value with a fixed value coil, and the oscillator covers about the same range it did before the damage. At first I feared that the band switch was contaminated, but it was fine. The frequency range is controlled by a string of adjustable coils, all in series. The band switch shunts around them, so for lower frequencies, more coils are in the path and increases the inductance of the string. That one coil was open, so it wouldn't oscillate in the two bands controlled by it and the next series coil to the right. I was lucky that was all the issue was.


The schematic seen in the following pictures is also on my website at http://www.WB4IUY.net , mouse over [WORKSHOP] in the menu at the top of the page, then click on [SCHEMATICS MANUALs] in the drop down that appears. 

The analyzer was dead on the 2 lowest frequency ranges...


The PCB, after cleaning and scrubbing with a brass brush...


The fixed value coil I installed on the back side of the PCB. I slipped a piece of cardboard between the coil and the PCB to prevent possible contact between the coil and PCB traces / solder joints.


Yipee!!! Back up and running...


I connected the output to the bench counter, just to verify the VFO. 


 This schematic is also on my website at http://www.WB4IUY.net , mouse over [WORKSHOP] in the menu at the top of the page, then click on [SCHEMATICS MANUALs] in the drop down that appears. 


Dave WB4IUY
http://www.WB4IUY.net