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MFJ-868 SWR/Wattmeter
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 |
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