6-84
7. 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.
6. The Preamps have bypassed and decoupled voltage supplies. Check that the bypassing com-
ponents 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.
5. The Preamplifier operating mode is set by a serial data word sent from the System /-LP.The CD
input, pin 22, is the serial data input. A TTL-level logic swing should be present on this line when-
ever the Attenuators, Preamps, or AlB Trigger Generator operating modes are being set up. If
there are no FAIL flags under these major test categories, the CD circuitry is most likely function-
ing properly. The CC input, pin 23, is the control clock input, and it should have a TTL-level logic
swing on it only when the particular hybrid, in this case the Preamp hybrid, is being set up. Check
that this line is high initially and pulses LO eight times for each Preamp load cycle. (The eight
pulses may be separated into several groups of pulses.)
Use either the Front Panel Controls or the Force DAC test to check the VARIABLE GAIN and
VERTICAL POSITION. Balance may be varied only using the Force DAC function. Varying the
channel balance should appear as a dc offset change to the vertical signal level. Prior to making
any adjustments after a COLD START (either with the Front Panel controls or the Force DAC
function), the signal at the output of the Peak Detectors should have a +8.7 V dc level with an ac
signal (replica of the input signal) of approximately 0.5 V peak-to-peak.
The check is set up by first doing a COLD START (to again set the calibration constants to known
values), and then applying a 4-division signal to the CH 1 and CH 2 vertical inputs (or to the bad
channel if only one is bad). Observe the signal on the crt, if possible; otherwise, use a test scope
to probe the vertical signal path to check for correct response. Since the Preamp outputs are not
accessible, use the output of the Peak Detectors to verify the signal through the Preamp. The side
3 Peak Detector output for CH 1 may be checked on R441 and R540; and for the CH 2 side 3
output, check at R244 and R341.
CH1 Band CH2B to pin 2 for balance.
CH1G and CH2G to pin 18 for gain control.
1POS and 2POS to pin 17 for position input.
4. Check that the Preamp is responding to the DAC control voltage being changed. These are
respectively for CH 1 and CH 2:
3. After doing a COLD START, use the FORCE DAC test to determine if the DAC system can con-
trol the Preamp DAC voltages correctly for the tests that are flagged FAIL. Troubleshoot the DAC
system for those DAC outputs showing no or improper control.
2. If the 8000 level is flagged FAIL, there will be failure at one or more of the lower level tests. This is
the case because one failure that misbiases the Preamp can cause several SELF CAL tests to fail.
Table 6-6 (cont)
Maintenance-2430 Service
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