On 11/9/2015 11:18 AM, amdx wrote:
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/


That's how I got my MFJ-259B, about 15 years ago, Someone at the local
ham club had a defective one, gave it to a fellow ham to fix, he
returned it saying that he couldn't fix it.
I told the owner, let me have a shot at it, if I can fix it I'll
buy it from you. It was "the usual blown diode problem:" I shocked the
owner when I gave him $60.00 for the repaired unit. I don't think he
expected me to contact him again.
Mikek

Good for you Mike,

I once came across an Ameritron ALS-600 with analogue power supply and a
couple of vhf-uhf transceivers in a junk box at a garage sale. I asked
how much for the whole box. I was told 60 bucks. I assumed the stuff was
probably defective. However, the person selling it was a woman who had
lost her ham husband.

I wrote her a check for $500. That is what I figured a defective ALS-600
might be worth. She was shocked. I did not do this to be generous. I did
it because I knew that if I really screwed her on that deal, I would
feel guilty every time I used that amplifier. It turned out that the
amplifier actually worked but introduced a bit of distortion. I sent it
to MFJ and they rebalanced the output transistors. It has very good
fidelity now. The trip to MFJ cost about 200 dollars. I had to buy one
of their shipping boxes. Those are not cheap. Still, 700 dollars for an
ALS-600 is a bargain.

I liked that design so well that I bought an ALS-1300 which is just two
ALS-600s in a box. So, I have the 600 as a backup amp. That 1300 has
been such a good work horse. Tom at MFJ and others who worked on that
amp design did a fine job. Originally there was a defect in the
switching supply that they bought for it. Mine has been updated to
protect the 12 volt supply.