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On 11/7/2015 8:25 PM, Jeff Liebermann wrote:
I just repaired an old MFJ-269 antenna analyzer. This time, the 4 diodes around the RF connector were NOT blown. Instead, it was crappy SMT soldering for the components surrounding the diodes. The solder joints looked ok before I resoldered them, but were obviously a bad connection because several resistors just fell off the board when I touched one end with the soldering iron, indicating the other end was not properly soldered. With the resistor removed, the PCB pad looked like it had never been soldered. If you don't have good soldering tools, a steady hand, and a good microscope, DON'T try this as you'll probably make a mess, as I did before I realized what was happening. Because the various parts on the RF board were not originally making good contact to the PCB pads, the calibration is off. The display reads about 12% too high for impedance and dead on for reactance. I'm debating if it's worth calibrating. http://www.w8ji.com/mfj-259b_calibration.htm Now working on an MFJ-259A, which looks like the usual blown diode problem: http://802.11junk.com/jeffl/MFJ-269-repair/ I've generally found MFJ stuff has decent design and rather good components, but terrible wave soldering on the PC boards (and sometimes hand soldering on components off the board like chassis mounted connectors). On my 259B, it worked well for almost a year, and then became erratic. When I saw how many soldered joints weren't, I decided not to try to figure out which ones were bad: I just went around and resoldered everything on the board. Since then it has worked perfectly. Wave soldering is a marginal thing, I think it is amazing how many times it does work. Think about how it goes: Basically the PC board is suspended just above a vat of molten solder, and a wave is caused to run across the vat. The wave, we hope, just reaches up to the connections on the board and solders them. Too high and it makes solder bridges. Too low and it either misses connections or at least does not stay on them long enough. And it is not long on a connection at best, so does it get it hot enough? It is not just fairly inexpensive things like my MFJ analyzer that have had problems. I did the "resolder the whole board" thing on my Phase Linear preamp many years ago also, and that was by no means low end! Bob Wilson |
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