Service manual

Checking the waveform across U242 pins 4-6 involves circuits that are connected to the ac mains. To lessen

the danger of personal injury, the mainframe should be connected to the ac mains through an

isolation transformer when checking the fan supply. When checking all other waveforms, connect the

oscilloscope common to one of the common points shown in the figure.

If the fans do not change speed, use the following chart to troubleshoot the fan speed circuit. The individual inputs for U245

and U233 can be controlled using the fan command from the front panel keypad by pressing the

key, then , and

then entering a value that corresponds to the bit you want to toggle. Press

to enter the value.

For example, pressing

will toggle the Fan 2 OFF bit (256). Although there are fifteen

discrete speeds for each fan, distinguishing a speed change between two adjacent numbers is difficult. The chart only

provides three examples to program the fans off, slow, or fast.

Check if the bit indicated on the chart is high (1) or low (0) at the following pins:

Fan2_Speed 0 = U233-4,5

Fan2_Speed 1 = U245-3

Fan2_Speed 2 = U245-5

Fan2_Speed 3 = U245-11

Fan1_Speed 0 = U233-1,2

Fan1_Speed 1 = U245-13

Fan1_Speed 2 = U245-9

Fan1_Speed 3 = U245-1

Fan2_OFF = U209-16

Note: The automatic fan speed control circuit varies the speed of the fans based on the temperature of the

module heatsinks. Programming the fans from the front panel disables the automatic fan speed control.

You must cycle ac power to re-enable the automatic fan speed control.

50/60 Fan2 Fan2 Fan2 Result

Hz OFF Speed 3, 2, 1, 0 Speed 3, 2, 1, 0 Fan 1 Fan 2

Decimal

Value

512 256 128643216 8421

768 1 1 0 0 0 0 0000slow off

512 1 0 0 0 0 0 0000slow slow

767 1 0 1 1 1 1 1111fast fast