The Y arm had not locked since the new 18bit DACs were installed.  One problem was a bad DAC.  DAC 0 was not giving linear outputs.  I have replaced this DAC.  The other problem was channel 0 on DAC4 which would not output any signal.  After a reboot/IO chassis power down the problem with DAC4 was gone.  We then had to replace DAC0 which seems to have fixed the problem.