[evlatests] EVLA Antenna power issues

[Jim Jackson]

We did some more tests on Antenna 16 to try to isolate the strange power 
issues we have been seeing.  Rob and I finally managed to duplicate the 
symptoms by interrupting AC power to the fire alarm panel. At that point, 
Bob, Steve and Ken came out from the servo shop and determined that the 
fire alarm system was not behaving the way they expected it to when AC 
power was interrupted. Apparently, there is a short dropout (maybe about 
1/2 second) that occurs in the panel's power supply when it switches from 
AC power to backup batteries. This glitch is disrupting the time delay 
relays in both the main AC contactor panel and the 48VDC shunt trip 
circuit.  The servo guys came up with a straight forward modification to 
the fire alarm panel to correct the problem and will be installing it shortly.

The second part of this was the mystery of why the DTS modules were failing 
when this occurred.  Tests have now confirmed that the 5 volt regulator 
powering the MIB and PSC board in that module shuts down when a short 
glitch occurs on it's input.  After a conversation with Doug and Kerry, we 
believe that adding a capacitor across the 48 volt input to this regulator 
will solve the problem.  This simple modification will be performed during 
the next round of upgrades already planned for these modules sometime in 
the next couple of months.

Cheers,
Jim