[Moon] DL0SHF 10GHz beacon status and request for help

Charles g3wdg at moon-net.eu
Thu Jul 8 07:09:07 CEST 2021


Hi All

Per noted a couple of days ago that the beacon is off the air at the 
moment. and has returned the failed PA to me to look at.

The active device  seems (on the basis of DC checks) to have survived.  
It is possible that the primary cause of the failure was a shorted 1uF 
0805  decoupling capacitor on one of the drain bias feeds.  There is a 
lot of charring in that region, and a melted PCB trace leading from the 
point where the incoming bias lead was soldered to the capacitor, 
suggesting the capacitor was the source of the failure.

However, there is also evidence of black sooty looking deposits in the 
region where the RF output lead connects to the output 50ohm line. 
Measuring DC resistance from RF output to ground shows about 77 ohms, 
whereas it should be an open circuit. This suggests the deposits are 
partially conducting, not a good thing where there is high power RF 
present! It is not clear whether these deposits were a contributing 
factor to the present failure or whether they  would have caused a 
failure in the future.

This is the second such failure of the beacon PA, and we feel that it 
would be beneficial in terms of future long term reliability, to try to 
determine the exact cause of this present failure. We would really 
appreciate help in finding out the cause of the failure.  One avenue 
that is no longer open to me since retirement would be an EDAX analysis 
of the various deposits around the failed and charred areas, to 
determine their chemical composition.  This is big machine stuff 
(electron microscope with EDAX attachment) which is  only likely to be 
found in semiconductor manufacturers' facilities or universities. Maybe 
someone has contacts?  If so, please let us know.

Also, help from anyone who is experienced in  failure analysis would be 
appreciated, in case we are missing something simple. Photos and details 
of how the PA is used, including  its operating  enviroment can be made 
available.

We are holding off doing any invasive investigations until we have 
planned out what to do next, so as not to  contaminate the 'crime scene' 
further. In the meantime, DL0SHF 10GHz is off the air.

73

Charlie G3WDG



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