by Jeff Liebermann AE6KS
2011-09-05
MFJ-269 antenna analyzer arrived with the usual symtoms. Bizarre readings, weird numbers, strange reading, and no obvious cause. The problem is usually the diodes in the broadband detector, which are easily destroyed by RF, ESD, and transmitting into the antenna connector. On the schematic, the usual problem is that diodes D15 and D16 are blown. However, to maintain some semblance of calibration, I decided to also replace D14 and D16. The diodes are marked with red circles in the photo below. All 4 diodes are Avago HSMS-2820 zero bias shottky diodes available from Digikey as 516-1817-1-ND or Mouser as 630-HSMS-2820-BLKG for about $0.60/ea.
RF Section
RF Section Schematic
Partly disassembled and ready for new diodes
Disassembly was somewhat awkward but not difficult. Remove batteries, and battery holder. Remove knobs (1.5mm hex wrench). Unsolder green wire from the freq counter BNC connector. Remove 4 large silver colored corner screws supporting main board. Remove two hex standoffs that support the main board, but also block access to the screws holding the RF board in place. Remove 4 large silver colored screws holding RF section. Note that these are shorter than the screws holding the main board. Wiggle both boards out of the case until it looks like the photo above. If you feel some resistance when removing the boards, check the red/black battery wire, which gets hung on the hole in the PCB.
Remove old diodes with stainless tweezers and a sharp tip soldering iron. Replace with new parts. Work fast as holding the iron in place too long will delaminate the solder pads. When done, put it all back together.
Of course, when I got it back together (for the 2nd time), I found the reading to be erratic and intermittent. It took a while to discover that the owner had somehow spread the gold center contact leafs in the type-N connector and my adapter wasn't making contact. My initial tests also were with random 50 ohm loads scattered around the bench, most of which were nowhere near 50 ohms. Have a known good 50 ohm load handy.
Full schematic of MFJ-269 (borrowed from mods.dk).
I tested the diodes that were removed. Three of them were open, and one was ok. Looks like replacing all the diodes was a good idea.
by Jeff Liebermann AE6KS http://802.11junk.com
