On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

Apparently you missed my previous rant on the topic. See the
schematic extract of the RF section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
Notice the directly connected diodes. The diodes in question are
Avago HSMS-2820 zero bias shottky diodes.
http://www.avagotech.com/docs/AV02-1320EN
15V Max PIV is rather low. It won't take much voltage at the antenna
go exceed 15V. The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.
An important clue is that BOTH D3 and D4 appear to be blown each time,
which implies an external failure, not a component failure.

Argh. I just started working on the analyzer, and found a few errors,
all of which are my mistakes.

1. The antenna analyzer that arrived (3 days late) today is not the
expected MFJ-259B but an MFJ-269. The difference is that the MFJ-269
goes up to UHF frequencies. The front end is similar, but not
identical.

2. The MFJ-269 has a type-N connector, while the MFJ-259 has a UHF
SO-239 connector. So much for the idea of substituting a type-N
connector.

3. Despite my idea of installing a bleeder resistor on the antenna
connector to drain off the static charge, the antenna connector
already shows 50 ohms resistance to ground. I missed this path
because of the rather difficult to read schematic of the MFJ-259 RF
section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
The path is from the antenna connector, through R24, L11, and then to
ground.

The MFJ-269 schematic is easier to read at:
http://802.11junk.com/jeffl/crud/MFJ-269-RF-section.jpg
which goes through R88, L12, and then to ground.

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I haven't finished working on the MFJ-269 quite yet. A report and
photos when I'm done.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558