Maintenance—2246 1Y and 2246 Mod A Service
Table 6-9, (cont)
Measurement Error Troubleshooting Hints
Circuit
Problem
Symptoms
B Trig
(U431C)
Gross value problems with Types 2, 3, and 4 measurements.
Gross value problems with B TL.
“ SEARCH RETURNED BAD VALUE AT . . . ’’ on tim e measurements.
B TV TRIG EN line stuck
high (U1103-4)
Gross value problems with Type 2 measurements.
Test: Run CH 1/CH 2 VOLTMETER DC measurement and check that
the B TV TRIG EN signal is at TTL low.
VERT COMP EN line stuck
high (U1103-7)
See notes on “ VERT COMP” (schematic Diagram 2).
Test: VERT COMP EN
should be at CMOS high in normal use. Run
“ SELF CAL” from the CH 1/CH 2 VOLTMETER menu and check
that VERT COMP EN
goes to a CMOS low.
LINE/TIME BASE CAL
signal Mux stuck in
LINE position (U1106A)
“ RETURNED BAD SEARCH VALUE AT . . . ” from “ SELF CAL” for
time measurements.
Test: Run “ EXERCISE TIME REF” diagnostic and check the output of
U1106A (pin 1) for changing signal.
DC Average circuit
(U1101B
Gross value problems for Type 2 measurements (minor value
problems with low frequencies if RC values in the filter are incorrect).
Test: Display only CH 1, run DC measurement. Apply to 50 Hz sine-
wave signal with a DC offset to the CH 1 input. Check that only the
dc value appears at output of the DC Average circuit.
DISPLAY AND TRIGGER LOGIC AND PROCESSOR INTERFACE
(schematic Diagram 4)
MB RETURN line
(U502-12)
See notes on VERT COMP (schematic Diagram 2).
MP DLY SEL line
(U503-13 or
MP DLY SEL Interface
(U602)
“ SEARCH RETURNED BAD VALUE AT 5 |j.s” on time measurements.
Test: Turn all measurements off. Use the A Horizontal mode. The
MP DLY SEL signal should be at a TTL high.
SR DATA line
(U606C-6)
Effects are the same as those caused by malfunctions in SR 0 and
SR 1.
Test: The A Sweep rate changes when SEC/DIV knob is rotated in the
A Horizontal Display Mode.
SR 0 CLK line
(U606F-12)
Effects are the same as those caused by malfunctions in SR 0.
Test: Channel 1 sensitivity changes when CH 1 VOLTS/DIV knob is
rotated.
6-29
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