[Pafgbt] amplifier failures

[Bill Shillue]

NRAO FLAG PAF has had some difficulties with amplifier failures recently.  The NRAO cryo amp uses an NXP Bipolar p/n BFU-725F. The failures appear to be due to cryo-cycling stress related.  This has slowed our progress toward GBT testing.  We are looking into the cause of the failure, and repairing and replacing affected amplifiers to try to progress with the PAF testing.  We have seen roughly ten failures, about fiver per cooldown over two separate cooldowns.  Prior to that we do not have good records of failure rates but we think it was rare, so it may be an issue that shows up after many cycles.  I am posting to PAFGBT because Steve White mentioned that Cornell was using a similar amplifier.