It is very important that an NMR spectrometer operate within a fixed temperature range. To control the temperature, many units within the console are equipped with cooling fans which must be kept in good working order. Failure to do so will shorten the life of the spectrometer and cause instability or malfunctions. In the Bruker AVANCE series of spectrometers, the SGU (Signal Generation Unit) boards are particularly sensitive to temperature. Each SGU houses a DDS (Direct Digital Synthesizer) containing numerically controlled oscillators which regulate rf frequencies and amplitudes. The DDS is normally temperature regulated at 55° C to insure stability. Both the SGU board temperature and DDS temperature can be measured either with the "UniTool" (AVANCE and AVANCE II) or web based tools (AVANCE III).
After years of service, two of our Bruker AVANCE spectrometers were running very warm. Using "UniTool" to check the temperature of the SGUs revealed that the board temperatures averaged 58° C! The DDS temperatures were all >55° C and not regulated. The air filters in the console doors were removed and cleaned and all eight fans of the AQS unit, housing the SGU's, were replaced on each spectrometer. After a day for the instruments to come to a thermal steady state, the SGU and DDS temperatures were again measured. The SGU board temperatures averaged 49°C. The DDS units in three of the five SGU's were now regulated properly at 55°C. The other two were still > 55°C and unregulated. In an attempt to lower the temperature further, the backs of the spectrometers were removed and again the instruments were allowed to reach a thermal steady state over a 24 hour period. The SGU and DDS temperatures were measured again. This time the SGU board temperatures averaged 38°C and all five DDS units were regulated properly at 55°C. The SGU temperatures in our AVANCE II and III spectrometers (with backs on) did not exceed 40°C and the DDS temperatures were regulated properly.
Although this may not be recommended by Bruker, I now run our AVANCE spectrometers with the back panels removed.