Tektronix 2440, User Manual

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Maintenance— 2440 Service
Table 6-6 (cont)
Troubleshooting Procedure (refer to test 8000 for more information):
1. Check that the decoupling network, R420/C423 or R222/C222 is functional.
2. Use the FORCE DAC test to determine if the CH1-PA-POS and/or CH2-PA-POS voltages are
being controlled by the DAC System. If not, troubleshoot the DAC System.
3. Check that the bypassing components in series with the power supplies are not open. Also check
that the dc bias voltages at the Preamp are approximately the levels indicated on the schematic
diagram in the service manual.
4. If the Preamp hybrid itself is suspected to be defective and the other channel is fully functional,
swap Preamp hybrids between channels to see if the problem moves to the other channel. If so,
replace the defective Preamp. If not, check the hybrid mount connections of the defective channel
for corrosion or contamination that may be causing a poor contact.
5. Swap Peak Detector hybrids between channels to see if the problem channel reverses. If so,
replace the faulty Peak Detector.
8300 Preamplifier Balance A10U420 and A10U320 (schematic diagram 9):
PREAMP
BALANCE
Testing Method:
Balance is performed in all five Preamplifier ranges, and on both channels simultaneously. Balance is
calculated by first taking a ground acquisition in non-invert. Then an acquisition is made in INVERT,
and a new balance DAC voltage is calculated that will keep the trace shift between non-invert and
INVERT within limits for calibration or diagnostics. This is done until balance is within specification or
until the maximum number of acquisitions has been reached. If the result is within specification prior to
acquisition abort, the test result is set to PASS: otherwise, it is set to FAIL.
Balance Test Limits:
Range
50 mV 20 mV 10 mV 5 mV 2 mV
Cal limit 1/2 DL 1/2 DL 1 DL 1 DL 2 DL
Diag limit
1 DL 1 DL 2 DL 2 DL 4 DL
Troubleshooting Procedure (refer to test 8000 for more information):
1. Check that the biasing network associated with the balance input, pin 2, and pins 4, 20, and 25 is
functional and that the voltage levels are approximately those indicated on the schematic diagram
in the service manual.
2. Use the FORCE DAC test to determine if the CH1-BAL and/or CH2-BAL voltages are being con­
trolled by the DAC System. If not, troubleshoot the DAC System.
3. Check that the bypassing components in series with the power supplies are not open. Also check
that the dc bias voltages at the Preamp are approximately the levels indicated on the schematic
diagram in the service manual.
4. If the Preamp hybrid itself is suspected of being defective and the other channel is fully functional,
swap Preamp hybrids between channels to see if the problem moves to the other channel. If so,
replace the defective Preamp. If not, check the hybrid mount connections of the defective channel
for corrosion or contamination that may be causing a poor contact.
5. Swap Peak Detector hybrids between channels to see if the problem channel reverses. If so,
replace the faulty Peak Detector.
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