Tektronix 2440, User Manual

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Maintenance— 2440 Service
Table 6-6 (cont)
4. Check U850 pin 13 that the first 8-bits of the 16-bit pattern comes out as the second is shifted
into U850 at pins 1 and 2. (A Sec/Div setting of 0.5 ms on the test scope is good for viewing the
data pattern, and the latched data on pin 13 is much easier to view than the input data pulses). If
the data is not shifting through U850, then replace U850.
5. If the data is coming through U850, check U851 pins 1 and 2 to verify that it is ok there. Check
pin 13 of U851 for a data pattern of 1010010110100101. (Each bit is approximately 0.2 ms wide,
so a 0.4 ms wide pulse is two bits.)
6. Replace U851 if not shifting the signal through.
2600 Holdoff Register A11U762 (schematic diagram 13):
SIDE U761/U762
Testing Method:
From this level, all four tests are selected in turn. Individual test may be called by selecting test
numbers 2610 to 2640. The test involves writing 4 unique patterns to U762 and reading them back
from U 761. The four patterns test for all stuck-at(s) and for lines shorted to other lines. By knowing
which test FAILS and the bit pattern, one may easily determine a bus problem by observing which bits
are the same in the failed tests.
The HOREG register is two integrated circuits; U762 is a latch and the read back is U761. If all tests
pass, the test result is set to PASS; any failure sets it to FAIL.
N O T E
B it 3 o f th e te s t p a tte rn s is n o t a llo w e d to b e s e t L O a s it w o u ld re s e t th e G P IB c h ip a n d
w e c a n n o t re s ta rt it fro m th e d ia g n o s tic ro u tin e s .
Test 1. 10101101 is sent to U762 and read back via U761. Test result is set to FAIL if not a match.
Test 2. 01001011 is sent to U762 and read back via U761. Test result is set to FAIL if not a match.
Test 3. 10011110 is sent to U762 and read back via U761. Test result is set to FAIL if not a match.
Test 4. 00101101 is sent to U762 and read back via U761. Test result is set to FAIL if not a match.
Troubleshooting Procedure:
If the failure occurs for all tests:
Set up the test scope as in Step 1 of the 2110 troubleshooting procedure.
Now using CH2 probe:
1. Check that U762 pin 1 HOREG is LO about 12
ns
after the trigger strobe. If HOREG is absent,
test the inputs of U781. Replace U781 if the inputs are ok; if not ok, troubleshoot that problem.
2. Check that U762 pin 9 (WR clock) has a LO-to-HI transition during the enable time. (Save enable
in REF1 and display it while looking at the clock.) Clock line is the write line; if missing, suspect
open run or connection.
3. Check the outputs U762 (pins 15, 12, 10, 7, and 5) for the proper levels for the pattern that is
being looped on. Replace U762 if incorrect.
4. Check U761 pin 1 to be enabled after the clock to U762 pin 9. If present, then the problem is pos­
sibly U762.
6 -7 3