Troubleshooting Measurement Errors - Tektronix 2246 1Y Service Manual

2. Check Readout Request pulse (RO REQ, U2410
pin 14).
3. Check Readout Blanking signal
U2410 pin 16).
4. Check activity of Readout Processor (U2400).
5. Check outputs of
Readout DACs
respectively).
6. Check outputs of
Readout Mixers (U2416A
spectively) and multiplexers (U2414 and U2415,
respectively).
Schematic Diagram 10—SWITCH BOARD AND
INTERFACE
1. Run the EXERCISE SWITCHES exerciser and
check each of the front panel switches for cor­
rect operation. The circuit number of the latest
switch pressed is displayed in the readout.
Pressing END exits the
CLEAR MEAS' MT
Program.
2. Run the EXERCISE LEDS exerciser and check
that each of the front panel LEDS may be turned
on. The circuit number of the lighted LED is dis­
played in the readout. Circuit numbers DS2016,
DS2017 ,' DS2018, DS2019, DS2024, DS2040,
and DS2049 do not correspond to any LEDs on
the front panel.
excerciser.
Schematic Diagram 11—ADC, DAC SYSTEM
1. Run EXERCISE DACS (under EXERCISER, PROC
BOARD, and A TO D menus) and probe the
demultiplexed outputs of U2303 (Diagram 11)
and each of the sam ple-and-hold circuits for
proper operation.
2. Run EXERCISE PORTS (under EXERCISER, PROC
BOARD, and A TO D menus) and trace the signal
path of any problems
Converter.
( RO BLANK,
Vertical and Horizontal
(U2412 and U2413,
Vertical and Horizontal
and U2416B,
exerciser; pressing
terminates the Service
Pressing END exits the
with the A -to -D
Maintenance—2246 1Y and 2246 Mod A Service
Schematic Diagram 12-POW ER SUPPLY
For safety reasons, an isolation transformer
must be connected whenever troubleshooting
is done in the Preregulator and Inverter Power
Supply sections of the instrument.
1. If the fuse blows, check that Q2201 is not
shorted. (If a variac is available, slowly increase
the line voltage from 0 V until the voltage across
C2202 is about 40 V. If the same voltage is
re­
across C2203, Q2201 is probably shorted.)
2. If the Preregulator fails to come up (44 V not
present across C2203):
a. Check +DC at the output of the line rectifier
bridge (across C2202) for approximately
( VacRMS X 1.414).
b. Check the Start-up circuit.
across C2204 should ramp up to about 20 V,
at which point Q2204 and Q2211 turn on to
supply voltage to pin 10 of U2201.
c. Check the
pulses with a repetition rate of about 25 >xs
should be present on pin 8 of U2201 when­
ever supply voltage is present on pin 10.
3. If the power supply is in the chirp mode (con­
tinually restarting and shutting down), excessive
loading of the +44 V supply is probable.
TROUBLESHOOTING MEASUREMENT
When certain measurement malfunctions occur, the
symptoms usually indicate the circuit components
that may be causing the problems.
following text to become familiar with the terms used
in describing a measurement failure problem and
the setup conditions needed to determine the
symptoms. Then use Table 6-9
measurement malfunction symptoms and probable
causes.
1. Verify all the following conditions and read the
definitions before attempting to use Table 6-9
for locating the source of measurement-error
problems.
W A R N IN G I
The voltage
Preregulator circuit. Voltage
ERRORS
Read the
to check for
6-25