Tektronix WFM5250, Service Manual

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Troubleshooting
LCD Troubleshooting Perform this test if there is no display on the internal LCD, but an external monitor
connected to the DVI output displays the usual instrument display starting with
Power-up Diagnostics.
1. Check the cable from the internal display to J7 on the FPGA board. This
carries the XGA data to the display.
2. Check to see if the backlight is on. Look into the vent holes behind the
upper right corner of the LCD. You should see the backlight come on about
20 seconds after power up.
3. If the backlight is on, replace the front panel assembly.
4. If the backlight is not on, check the backlight power cable from the LCD
assembly to the CPU board J5. Also check connector J18 itself for any solder
issues.
5. If the backlight is still bad, replace the front panel assembly (although there is
still a small chance the problem is in the main board).
Fan Checks This instrument contains two fans: they are controlled by both hardware and
software. Hardware controls the fans to a certain speed, but software sets the
speed target as a function of the temperature measured on a variety of temperature
sensors.
If a fan fails, its corresponding LED lights up on the CPU board:
Fan 1 fails: LED DS3 lights
Fan 2 fails: LED DS2 lights
A fan failure is sensed through tachometer feedback; a fault will be asserted if
the fan is not connected, is stalled, or if the tachometer feedback line is not
working correctly.
If one fan fails, the control circuit will increase the voltage to the 13.5 V max
level, causing the remaining fan to run at maximum speed.
The normal voltage to drive the fans (pin 1) varies from about 6 V to
13.5 V depending on internal temperature.
If a fan is not spinning, measure the voltage on pin 1 of the connector on that
fan. If the voltage is near 13 V, then replace the fan. If the voltage is not above
10 V, then replace the CPU board.
If both fans are spinning, but either LED DS2 or DS3 is lit, the problem is probably
the tachometer feedback line on pin 3 of either fan. Inspect the wiring and
resistors R201, R203, R202, and R204 on the CPU board, and use the oscilloscope
to look for a 3.3 V square wave on the tachometer feedback line. If resistors are
intact but there is no signal on the tachometer line, replace the affected fan.
WFM5250 Waveform Monitor Service Manual 23