[evlatests] bistable 26D, dead 21

[Rick Perley]

    Some free dynamic time came up, I stuffed in a 60 minute observation 
at X-band.  Continuum, 0.4 sec averaging. 

    Antenna 26D has bistable amplitudes, (similar to 24C's old problem, 
but with only two states).  The ratio of the visibility amplitudes is 
about 1.6:1.  

    Antenna 21, all IFs, is essentially dead (very weak fringes only). 

    All others look good to cursory inspection.