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Dia nasties
Gain Off
There is a GainOff diagnostic for each channel. What these diagnostics test is the ability of each channel's
display multiplexing circuitry to turn its output OFF. The channel's noninverting input is grounded, and its
internal Offset ACVS resource is used to generate a positive and negative stimulus at the channel's inverting
input. This stimulus is of sufficient amplitude that,
if
the channel's display output was ON and had enough
dynamic range, deflections of plus and minus ten divisions would result.
The test is to see what the effective gain of the channel is with its display turned OFF; the difference in
deflections measured by the mainframe for each of the two test offset voltages is divided by the 20 division
figure that, theoretically, the channel would produce
if
ON. The resulting normalized gain is required to be
less than± 0.002.
Test Failure
If
the normalized gain exceeds the ±0.002, the test has failed indicating a problem with the output amplifier
(M377) ICs or the M382's channel switching.
Gain On
The GainOn diagnostics test their associated channel's ability to pass a signal from attenuator input through to
its display output, at roughly the correct gain. The tested channel's attenuator is connected to the calibration
reference voltage from the mainframe, and placed in its lOX range. The other channel is also connected to the
reference, but at lOOX attenuation. Both channels' variable gain controls are set at default values intended to
result in design-center path gains.
The mainframe is asked to generate+/- 300mV test signals, and to make a measurement of the amplifier's
display output for each applied voltage. The apparent normalized gain of the tested channel is then
calculated by dividing the difference of these two deflections by the theoretical ideal difference of 6 divisions.
This normalized gain is displayed in the diagnostic's "Actual" field against limits of 0.9 and 1.1 (+/- 10%).
Test Failure
If
the normalized gain exceeds the limits, the test has failed indicating problems with the M377s, the M382's
channel switching, the channel's attenuator or the calibration signal path, including the oscilloscope's
calibration voltage reference (CVR).
Unreported Amplifier Calibration Failures
There are situations in which the oscilloscope will report that calibration failed even though no failures are
reported in the Extended Diagnostics mode. The most common of these is a failure in the trigger path.
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11A52 Extended Service Manual
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Troubleshooting
