General Maintenance
Front Panel
Troubleshooting
If the instrume
nt has a reasonable display on the screen, but the front panel does
not work, follow this procedure:
1. Do the LEDs tur
n on for 2 to 3 seconds at power up? If not, check the 5
V fuse, F130, on the m ain board and check the cable from the main board to
the front panel. If there is power to the front panel and the LEDs do not turn
on at power up, then replace the front panel board.
2. After power up, do most of the LEDs turn on and stay on? If not, proceed
to next step. Otherwise probe J390, pin 6 and pin 7 on the main board. Pin
6 should be TTL high (about 3.3 V) and pin 7 should be low (RS232 level,
about – 7 V). If not, s uspect the main board. Otherwise probe pin 6 and 7 of
J670 on th
e front panel board. If they are high and low, respectively, suspect
the front panel, otherwise replace the cable.
3. When you
press buttons on the front panel, do LEDs DS291 and DS292 on
the main board flash? These LEDs m onitor the transmit and receive lines on
the main board side of the RS232 transceiver. If they flash,thengotostep5.
4. If DS291 is not flashing, then check pin 3 of J390 on the main board. When
you p ress a key, you should see pulses about a 15 V high and 100 to 300 µs
long. If not, suspect the front panel board.
5. If pin 3 of J390 is toggling and DS291 is not, then the RS232 receiver on the
main board is not working. Replace the main board.
6. If both DS291 and DS292 are flashing, the n check p in 5 on m ain board
connector J390. When a front panel button is pressed, the front panel sends a
message to the main board via pin 3. The CPU on the main board responds
via pin 5 and this instructs the front panel to change which LEDs on the
fr
ont panel are illuminated. Pin 5 should have a burst of pulses with about
15 V signal swing and a basic pulse width of about 100 µs. If there is no
response from the CPU on pin 5 of main board connector J390, then the
RS232 transmitter is b ad; replace the main board.
7. If both the DS291 and DS292 LEDs are flashing and the signals at the main
board connector are also good but the operation of the front panel is not
correct, check the cable between the two boards. This can be done by probing
pin 5 on J670 of the front panel board. As in step 5, it should have a burst of
pulses about 15 V high and 100 µs long after a button press. If the signal is
bad, suspect the cable. If the signal is good, replace the front panel board.
8. If some buttons work, but others do not, then the front panel or the elastomeric
key pads are bad. Disassemble the front panel assembly and use a small piece
of wire or foil to emulate the carbon pad on the s witch mat. If this registers
as a switch closure then replace the elastomeric key pad. Otherwise replace
the front panel board.
9. If some front panel button LEDs w ork a nd others do not, then the problem
must be in the front panel board. These LEDs have the anodes connected to
+5 V and the cathodes connected to a resistor driven by a logic gate. Probe
4–16 WVR6020, WVR7020, and WVR7120 Waveform Rasterizers Service Manual
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