American Magnetics 420 Power Supply Programmer - Manual (Page 120)

102 Rev. 7
Service
Troubleshooting Hints
5.2.13 There is excessive LHe boil-off during operation.
Excessive LHe consumption is usually attributable to two things: thermal
energy being conducted into the cryostat or electrical energy being
converted into thermal energy within the cryostat. Analyzing the
circumstances under which the high boil-off occurs will help determine
what is causing the problem.
1. For magnets equipped with switches for persistent operation, verify
that the persistent switch heater power supply is operating at the
proper current for the installed switch. Excessive currents cause
excessive boiloffs. The typical switch requires approximately 45 mA
to function correctly. Refer to the documentation provided with the
magnet for proper operating current. See Figure 3-5 on page 50.
2. Verify that the protective diodes on the magnet are not turning on.
Damaged diodes may short causing current to flow through them
whenever magnet current flows and cause excessive heating. This
can be identified by observing a change in the apparent field-to-
current ratio since some of the current is bypassing the coil. If the
boil off rate returns to normal with the magnet de-energized, this
may indicate a defective diode.
3. Ensure that there are no inadvertent thermal paths between the
cryogenic environment and the 300K environment. Ensure all
transfer lines are removed from the cryostat; check the position of
break-away vapor-cooled current leads.
4. Ensure the LHe level sensor is not continuously energized if
continuous level indication is not necessary.
5. Ensure the vacuum in vacuum-jacketed dewars is of sufficiently
low pressure.
5.2.14 The Model 420 will not display the magnetic field strength,
only magnet current
1. Enter a coil constant in accordance with paragraph 3.2.2.2 on
page 44.
Note
Setup menu limits are always required in terms of current.
5.2.15 The Model 420 is not responding to remote communica-
tions commands.
1. Verify your communications cable integrity and wiring. See the
Appendix for DB-25 to DB-9 translation for RS-232 cables.