Tektronix 2236, Instruction Manual

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Maintenance— 2236 Service
nal source current, such as the R X 1 kfi range. The diode
resistance should be very high in one direction and very low
when the meter leads are reversed.
When conducting, silicon diodes should have 0.6 to 0.8 V
across their junctions, and Schottky diodes should have 0.2
to 0.4 V across their junctions. Higher readings indicate that
they are either reverse biased or defective, depending 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 con­
tinuity with an ohmmeter. Shorted or partially shorted induc­
tors can usually be found by checking the waveform
response when high-frequency signals are passed through
the circuit.
CAPACITORS. A leaky or shorted capacitor can best be
detected by checking resistance with an ohmmeter 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 ohm­
meter. An open capacitor can be detected with a capaci­
tance meter or by checking whether the capacitor passes ac
signals.
10. Repair and Adjust the Circuit
If any defective parts are located, follow the replacement
procedures given under “Corrective Maintenance” in this
section. After any electrical component has been replaced,
the performance for that particular circuit should be
checked, as well as the performance of other closely related
circuits. Since the power supplies affect all circuits, perfor­
mance of the entire instrument should be checked if work
has been done in any of the power supplies or if the power
transformer has been replaced. Readjustment of the af­
fected circuitry may be necessary. Refer to the “Perfor­
mance Check Procedure” and “Adjustment Procedure”
(Sections 4 and 5) and to Table 5-1 (Adjustment
Interactions).
CTM TROUBLESHOOTING PROCEDURE
The following information is intended to aid in the diagno­
sis and repair of a malfunctioning CTM. The troubleshooting
procedure consists of two parts: “Kernel and Display Sys­
tem” and “Power-UP Checks and Diagnostic Routines”. Ob­
serve the following symptom on the readout to identify
which procedure to use:
A blank or erroneous message on the readout will indi­
cate a circuit failure in either the kernel or the display sys­
tem. Proceed to the “Kernel and Display System” procedure
to identify the problem.
The readout displays error message. This indicates that
the kernel and display system are working correctly but the
Power-Up Checks found a fault. Proceed to “Diagnostic
Routines” for definitions of error messages on the readout.
Kernel and Display System
The CTM kernel consists of the 6802 microprocessor
(CPU), CPU reset circuit, CPU clock and interrupt genera­
tor, CPU address decode logic, counter data (CD) bus
switch, buffered data (BD) bus latch, and two ROMs. The
entire kernel must be operating correctly before proceeding
to the diagnostic routines. Unless there is an obvious dis­
play system problem (bad readout, disconnected cables
etc.), it is assumed that the malfunction is in the kernel.
KERNEL CHECK. Ensure that both the microprocessor
and ROMs are plugged in correctly before performing the
following kernel troubleshooting checks.
1. Check that the voltage to the ICs in the kernel is 5 V
±
5
% .
2. Check the output of the CPU reset circuit for a HI at
U1400 pin 40.
3. Check the CPU clock signal at U1400 pin 39 for a
2.5 MHz square wave.
4. Check the CPU interrupt generator signal at U1400
pin 4 for a negative-going pulse, approximately 31 us wide
at approximately 600 Hz. If this signal is present, then the
CPU and ROMs are working correctly. The problem may
either be in the CPU address decode logic, the BD bus latch,
or the display system. If the interrupt signal is a 305 Hz
square wave, the firmware is not executing correctly; pro­
ceed to step 7 in this procedure.
5. Check address bus for shorts or opens by placing the
microprocessor into a nop loop. This is accomplished by
moving jumper PI 404 from its Norm position to the Test
position and removing PI 402 from its pins. Attach a 40-pin
1C test clip to the microprocessor. Connect 1C test clip pins
26 through 32 to pin 21 of the 1C test clip. This will force the
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