6-87
See the Troubleshooting Procedure for tests 8400 and 8500.
Troubleshooting Procedure (refer to test 8000 for more information):
In this test, the change in Preamplifier control which will yield an attenuation of 2.75 from the Cali-
brated VOLTS/DIV setting is measured on both channels at 50 mV per division in normal mode. This is
done by re-performing the 50 mV noninverted gain test seeking a value of +2.5 divided by +2.75
(+0.91) division and -2.5 divided by -2.75 (-0.91) division on the output. The difference between
the resulting gain control DAC setting and the gain control DAC calibration constant is the Var Max
value. Inability to achieve an attenuation factor of 2.75 is a test failure.
Testing Method:
8600
PREAMP
VAR MAX
4. Swap Peak Detector hybrids between channels to see if the problem channel reverses. If so,
replace the faulty Peak Detector.
3. 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.
2. 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.
1. COLD START and use the Force DAC test to check that the DAC system is controlling the CH1-
GAIN-CAL and CH2-GAIN-CAL voltages correctly. If not ok, troubleshoot the DAC system.
Troubleshooting Procedure (refer to test 8000 for more information):
During calibration, gain constants are computed by using the Balance control to position +2.5 and
- 2.5 divisions and computing the next gain DAC value until the result is set to be within
specifications. For diagnostics, the swing is reduced to
±
1.5 divisions to allow for thermal drifts that
occur due to temperature changes between power off and power on. The effects of thermal drift are
especially noticeable at high vertical sensitivities. Limits are 1 DL for calibration and 2 DL for diagnos-
tics. Gain is done for all five Preamplifier ranges in both normal and invert modes. Both Preamplifier
channels are tested simultaneously. Since the transfer function of the gain control is non-linear, correc-
tion is done iteratively either until the gain is within specifications or until the maximum number of
acquisitions allowed for the test has been reached. If the result is found prior to a test abort, the test
result is set to PASS; otherwise, it is set to FAIL.
Testing Method:
8400
PREAMP GAIN
and 8500
PREAMP
INVERT GAIN
Table
6-6
(cont)
Maintenance-2430 Service
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