Using the LFI3751, we fail to get any output voltage. On start-up its temperature display is flashing although no other LED is flashing simultaneously (none of the four red error LEDs). While flashing it appears to show the temperature correctly. When we try to enable the output, the instrument gives a long tone, but fails to reset and to output a voltage.
If the ACT T and SET T temperature display is flashing, the A, B, and C sensor values are most likely set too close together. For example, if you are using a 10 kΩ thermistor, if the C value is set to within about 2.5 to 3K of the B value, the display will flash, you will hear a long beep (the tone you mentioned) and the output will be disabled.
Using a 10 kΩ thermistor, first try the values on the top left of page 35 of the LFI3751 User’s Manual for a standard set-up (on pages 35-38) to stop the display from flashing and re-enable the output. Once you have the instrument set up for standard operation and have it functioning correctly, you can tune the thermistor calibration to whatever range you want to use as long as the C value is not set to within 2.5 to 3K of the B value.
If you are setting these values too close together in order to reach a particular temperature, you need to use a different (higher resistance value) thermistor to get to that range.