Adjusting HRCam and WFCam Controller Power Supply Voltage

For some unknown reason (probably temperature changes) the voltage level of the 5V supply on the Leach Controller tends to drift. For an equally unknown reason, the Leach controller is very sensitive to any change in the voltage of this power supply. Whenever this voltage is out of specification the A-D converter on the controller does not function properly and the probability of certain A-D codes increases. The effect of this A-D error can be observed in two ways. The one which you will probably notice first is a slight increase in the noise of the image background combined with what appear to be contour rings around a star when you zoom in on the image. The problem is also visible in the intensity histograms of the data. The histograms will show very distinct peaks every 16 A-D levels. In extreme cases the CCD controller may refuse to initialize.

The distortion of the data caused by this A-D error makes the images useless for science so it is a good idea to perform histograms on occasional images to make sure that this problem does not exist. If it does, then you will probably need to adjust the power supply voltage for the controller. BUT FIRST make sure that the air extraction fan for the pier is on. This fan draws cooling air through the HRCam power supply enclosure and if the pier extraction fan is not on, the power supplies will run hot. After you turn on the fan you should give the system about one hour for the temperature to stabilize and check the images again. If the problem still exists, then follow the procedure outlined in the rest of this document.

Before adjusting the voltage, the electronics should be at their normal operating temperature. The instrument power in the control room at the base of the pier and the pier air extraction fan should both be turned on 1-2 hours prior to making any adjustments.

Once the electronics have warmed up you need to gain access to the power supply and the CCD electronics test points by removing their covers. Do not do this until you are ready to make the adjustment because removal of the covers will affect the temperature of the electronics.

First, use the small Phillips screwdriver to remove the cover from the rear of the CCD electronics enclosure. See figure 1.

Figure 1

Second, remove the cover from power supply enclosure below and to the left of the HRCam. First remove the air extraction hose and then remove the cover via the thumbscrews around the base of the enclosure.

Connect the Digital Voltmeter to the test points shown in Figure 2. Set the meter either for autoranging or to a scale appropriate for reading a 5 volt signal.

The voltage should be 4.95 +/- 0.03V for the HRCam and 4.91V +/- 0.03V for the WFCam. If the voltage is outside this range adjust the power supply voltage using the adjusting screw shown in Figure 3. The adjustment is very sensitive and the adjustment will require an extremely small turn of the screw so be gentle.

Once the voltage has been set, remove the Voltmeter leads from the test points and replace the covers.

Figure 2

Figure 3