Tektronix WFM8200, Service Manual

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General Maintenance
Detailed Troubleshooting Procedures
The following tests should be run as indicated in the Symptom and Causes table.
(See Table 4-4 on page 4-7.). The procedures check for speci fi c problems or
will help you isolate a problem to a board. You can run them at any time for
informational purposes but if you do not run the procedures in the correct context,
then the
fi nal recommendation identifying a root cause might be suspect.
General Checks 1. Verify that the power cord is connected.
2. Check that all internal cables are correctly connected and seated.
3. Check for any discolored or burned components.
Fan Failures
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 on the Main board lights.
Fan 1 fails: LED DS12 lights
Fan 2 fails: LED DS11 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 temperature and the options installed on the instrument.
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 main board.
If both fans are spinning, but either LED DS11 or DS12 is lit, the problem
is probably the tachometer feedback line on pin 3 of either fan. Inspect the
wiring and resistors R637, R635, R638, and R636 on the main board, and use
the oscilloscope to look for a 3.3 V square wave on the tachometer feedback
line. Replace the affected fan if resistors are intact but there is no signal on the
tachometer line.
4–10 WFM8200 and WFM8300 Waveform Monitors Service Manual