On 9/5/2011 6:47 PM, Owen Duffy wrote:
Jeff wrote in
:

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.

But the reality is that people do damage these things... and so the
method you suggested earlier is not likely to be sufficient to protect
them.

I am careful to avoid connecting an instrument of this type to an
antenna system unless I have drained any static charge first, and avoid
other transmitters on air nearby.

I think you are correct. I do think that more care needs to be taken
with these devices than with a lot of other electronic items Hams use.
I liken them to RF attenuator blocks. You see a lot of them in company
repair shops, because people are often trying to put too much power
through them.

After getting the new replacement analyzer, and using the dummy load,
and I do the static drain too, plus I keep it away from other people,
it's lasted FB.

- 73 de Mike N3LI -

On Mon, 05 Sep 2011 19:46:16 -0400, Mike Coslo wrote:

I liken them to RF attenuator blocks. ... people are often trying to put too much power
through them.

This leaves me wondering about intentions:

"Must - get - more - power - out - of this attenuator!"

73's
Richard Clark, KB7QHC

On Mon, 05 Sep 2011 11:47:21 -0400, Mike Coslo wrote:

On 9/5/2011 2:58 AM, Jeff Liebermann wrote:

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've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.

I used to have a Sony Mavica MVC-FD73 camera. Nice camera for 640x480
closeups. I gave it as a present to a friend, who bashed in the LCD
display. I repaired it, and she bashed it in again. I gave up.

Per one of my previous postings, none of the failures coincided with
any high power RF nearby. One was connected to an antenna when it
probably failed, but there were no transmitters active. I've tried
interrogating the owners and none claim that they did anything
potentially destructive. I've been assuming it was ESD, not RF that
is killing the diodes, but I'm not sure.

I just tested the diodes. 3 diodes were open, one was ok.

I haven't had any problem with the analyzer since installing my dummy
load on it.

Probably a good idea.

But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?

Owen answered that question. L12 (100 uH) is in series with the 50
bridge "load" and therefore allows measurements to be made without
padding the antenna with an additional 50 ohms.

I threw together a web page on the MFJ-269 repair:
http://802.11junk.com/jeffl/MFJ-269-repair/
Hopefully, this will help anyone following my succession of mistakes
and screwups. I'm not really done as I want to verify that it's
working correctly and hopefully not require recalibration.

Incidentally, no lock washers on any of the screws inside.


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

Mike Coslo wrote in news:j43n06$ds52$1
@tr22n12.aset.psu.edu:

;... plus I keep it away from other people,
it's lasted FB.

That is probably key, don't trust other people... and be pretty wary of
one's self!

It is interesting you mention the Other Persons thing... I am in the
process of drafting a web article commenting on the advice that is so
common these days on online fora, "can you borrow an analyser?..."

This is almost always offered to some newbie who is having trouble
"tuning up his antenna".

Most don't understand complex numbers, impedance, transmission lines,
etc etc and someone tells them get an analyser and tune for resonance. I
can only guess that the advisers are equally lacking in knowledge and
experience, but I will concede that the analyser dumbed down to a VSWR
meter with self contained RF source than can be tuned outside band
limits, can be a pretty handy thing.

But despite the fact that adjusting many, if not most antennas for
minimum VSWR is the sensible objective, the resonance brigade chimes in
with make X zero looking into some arbitrary length of feed line, a
condition which isn't always associated with minimum VSWR and may
actually not be achievable. Some newbies have clipped their mobile whip
away to make that discovery, but perhaps without understanding that it
was a likely outcome from that strategy.

So Mike, it is likely that someone borowing an analyser to fix a problem
wont obtain real benefit, but may damage it in the process.

Owen

On Mon, 05 Sep 2011 17:21:56 -0700, Jeff Liebermann
wrote:

One was connected to an antenna when it
probably failed, but there were no transmitters active.

Perhaps not intentional Ham transmissions.

Roy often counseled about how his experience included many interfering
commercial sources. And this in a rather remote reach outside
Portland, I believe.

"No one ever expects the Spanish Inquisition!"

73's
Richard Clark, KB7QHC

On Tue, 06 Sep 2011 11:23:53 -0700, Richard Clark
wrote:

On Mon, 05 Sep 2011 17:21:56 -0700, Jeff Liebermann
wrote:

One was connected to an antenna when it
probably failed, but there were no transmitters active.

Perhaps not intentional Ham transmissions.

The first step to solving a problem is to blame someone. I prefer not
to blame hams.

Actually, I lied. Two of those that I've repaired were attached to
antennas. However, the locations are not conducive to having nearby
transmitters. One is in the deep dark forest, where the nearest
neighbor, much less nearest possible RF source, is about 1/2 mile
away. The other lives in a nearby rural area. No forest, but also no
nearby transmitters. However, it's possible that the transmitter in
question may have been one owned by the hams in question. Keying a
nearby HT or mobile might have been the culprit. Dunno.

Roy often counseled about how his experience included many interfering
commercial sources. And this in a rather remote reach outside
Portland, I believe.

Attach a wattmeter to any VHF antenna on a mountain top with FM/TV
transmitters, and you'll get a watt or more of RF indicated. Nice way
to blow up the instrument.

"No one ever expects the Spanish Inquisition!"

The Spanish Inquisition had quite a bit of support from those outside
the clergy. Much of it was about confiscating the property of alleged
heretics. When it grew in size to become a source of regular revenue,
it became an institution (much like our war on drugs). It fizzled out
when they ran out of victims.

At this point, I don't know what is causing the failures. However, I
do know what to do. Add a dummy load and buy some more diodes.

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

Jeff Liebermann wrote:


Actually, I lied. Two of those that I've repaired were attached to
antennas. However, the locations are not conducive to having nearby
transmitters. One is in the deep dark forest, where the nearest
neighbor, much less nearest possible RF source, is about 1/2 mile
away. The other lives in a nearby rural area. No forest, but also no
nearby transmitters. However, it's possible that the transmitter in
question may have been one owned by the hams in question. Keying a
nearby HT or mobile might have been the culprit. Dunno.

Attach a wattmeter to any VHF antenna on a mountain top with FM/TV
transmitters, and you'll get a watt or more of RF indicated. Nice way
to blow up the instrument.



At this point, I don't know what is causing the failures. However, I
do know what to do. Add a dummy load and buy some more diodes.


If you do not short the center conductor (on the DUT) to ground
immediately before connecting to the analyzer you risk discharging a
large "capacitor" directly into the instrument. If I am at an antenna
farm I am grounding all my test gear prior to putting it into service.
There is a Ground lug on the 259B right next to the SO239. They
provide a Type N adaptor with a new meter, too, BTW.

MFJ isn't the only ones that can suffer at the hands of others.

I was trying to locate some interference using a UHF Yagi and
a Motorola R2018 service monitor. Do not key up a hand held in
front of the antenna. At least Motorola anticipated that, and
it took out the picofuse behind the antenna connector instead
of the front end mixer.

Jeff-1.0


--
"Everything from Crackers to Coffins"

On 07 Sep 2011 13:27:08 GMT, dave wrote:

If you do not short the center conductor (on the DUT) to ground
immediately before connecting to the analyzer you risk discharging a
large "capacitor" directly into the instrument.

Good point except that none of the 3 analyzers that failed were
(allegedly) attached to equipment. They were attached to antennas.
Antennas normally do not have BFC's (big fat capacitors) attached, but
do build up static charges. With the humidity currently at 60% or
more, I don't think that's likely. Also, any antenna design, that
would fry an analyzer, might also fry a radio front end. It's
possible, but unlikely.

If I am at an antenna
farm I am grounding all my test gear prior to putting it into service.
There is a Ground lug on the 259B right next to the SO239. They
provide a Type N adaptor with a new meter, too, BTW.

I carry a neon lamp line tester in my tool box. It's useful for
checking AC power, but also for detecting high voltages on antenna
terminals. It's amazing what I find at broadcast sites.

The plot thickens. I checked the accuracy of the MFJ-269 and is was
off on the real part of anything it was measuring about +10%. A good
50 ohm load would read about 56 ohms. It didn't take long to find the
culprit. Instead of 51.1 ohms (1%), R88 showed 54 ohms, while R85 and
R86 read 52 ohms (or there about within the limits of my cheapo
ohms-guesser). It therefore seems possible that the owner had
transmitted into the instrument (even though he denies it). That also
might explain how 3 out of 4 diodes were found fused open. Most
diodes I've seen end up shorted when the junction is punctured. I'll
be replacing all the 51.1 ohm resistors.

Meanwhile, I added a grounded load cap, returned the instrument to its
owner, extorted a free lunch, applied the requisite instrument safety
lecture, and warned him of the accuracy problem. The dip in the VSWR
curve is all he needs at this time, which is unaffected by bridge
imbalance.

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

On Wed, 07 Sep 2011 08:41:34 -0500, Jeffrey Angus
wrote:

MFJ isn't the only ones that can suffer at the hands of others.

I was trying to locate some interference using a UHF Yagi and
a Motorola R2018 service monitor. Do not key up a hand held in
front of the antenna. At least Motorola anticipated that, and
it took out the picofuse behind the antenna connector instead
of the front end mixer.

Well, do the math. The front end of the service monitor is 50 ohms.
The fuse is probably 0.1A. Power to blow the fuse is:
P = I^2 * R = 0.1^2 * 50 = 0.5 watts = +27dBm
I'll guess a 10dBi yagi and a 3 watt (+35dBm) HT into a 0dBi rubber
ducky.

[Q]: How close to the antenna can you use the HT before you blow the
fuse?

Go thee unto:
http://www.terabeam.com/support/calculations/som.php
and inscribe the tx power (in dBm) and the antenna gains into the
boxes. Use 0dB for coax losses and 440MHz for the frequency. Don't
worry about RX sensitivity. Plug in different values of distance
(miles) until you get an RX signal level of +27dBm. I get:
0.0003 miles = 1.6 ft

So, stay at least 1.6 ft away from your antenna and you won't blow the
fuse. Please change my guesswork to match reality and recalculate.

Drivel: Yes, I know we're close to the near field and the numbers
aren't very accurate.

Jeff-1.0

Jeff 2.0 (the upgraded version).

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