Internal Testing Capabilities - Tektronix 2246 1Y Service Manual

Maintenance—2246 1Y and 2246 Mod A Service
I CAUTION |
When checking a diode, do not use an ohm-
meter scale that has a high internal current.
High current may damage a diode. Checks
on diodes can be performed in much the
same manner as those on transistor em itter-
to-base junctions.
DIODES. A diode can be checked for either an open
or a shorted condition by measuring the resistance
between terminals with an ohmmeter set to a range
having a low internal source current, such as the R X
1 k fl range. The diode resistance should be very
high in one direction and much lower when the
meter leads are reversed.
Silicon diodes should have 0.6 V to 0.8 V across
their junctions when conducting; Schottky diodes
about 0.2 V to 0.4 V. Higher readings indicate that
they are either reverse biased or defective, de­
pending on polarity.
RESISTORS. Check resistors with an ohmmeter.
Refer to the Replaceable Electrical Parts list for the
tolerances of resistors used in this instrument. A
resistor normally does not require replacement
unless its measured value varies widely from its
specified value and tolerance.
INDUCTORS. Check for open inductors by checking
continuity with an ohmmeter. Shorted or partially
shorted inductors can usually be found by checking
the waveform response when high-frequency sig­
nals are passed through the circuit.
CAPACITORS. A leaky or shorted capacitor can best
be detected by checking resistance with an ohm­
meter set to one of the highest ranges. Do not
exceed the voltage rating of the capacitor. The
resistance reading should be high after the capacitor
is charged to the output voltage of the ohmmeter.
An open capacitor can be detected with a capaci­
tance meter or by checking whether the capacitor
passes ac signals.
12. Repair and Adjust the Circuit
If any defective parts are located, follow the replace­
ment procedures given under Corrective Mainte­
nance in this section. After any electrical component
has been replaced, the performance of that circuit
and any other closely related circuit should be
checked. Since the power supplies affect all circuits,
6-10
performance of the entire instrument should be
checked if work has been done on the power sup­
plies. Readjustment of the affected circuitry may be
necessary. Refer to the Performance Check Pro­
cedure and the Adjustment Procedure, (sections 4
and 5) and to Table 5-1 (Adjustment Interactions).

INTERNAL TESTING CAPABILITIES

The diagnostics built into the 2246 1Y and 2246
Mod A permit the technician to test much of the
digital circuitry and the digital-to-analog interface.
The following text describes the testing capabilities
of the Measurement Processor and the firmware
controlled circuitry.
Power-Up Testing
The systems shown in Figure 6-1 are tested at
power-on. Failure codes appear in the Trigger
MODE LEDs, with ON being shown as " x " and OFF
as " o " in the figure. In the event of a display failure
where error message cannot be displayed on the
crt, the codes indicate a failure area to begin
troubleshooting.
Tests Run
System RAM
System ROM
Readout Interface
Figure 6-1. Power-on test failure codes.
Power-up tests performed are:
1. RAM diagnostics—failures indicated by flashing
AUTO LEVEL lamp. Three diagnostics are run on
all locations in the RAM:
a. Store and read 00.
b. Store and read FF.
c. Store and read pseudo-random pattern.
Failure Code
(seen on the
Trigger LEDS)
X 0 0 0 0 0
0 X 0 0 0 0
0 0 X 0 0 0
6557-95