Tektronix 2440, User Manual

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2440 Service
CHECK GATE DRIVE
WAVEFORMS AT PULSE
WIDTH MODULATOR FET
GATES BY ATTACHING
SCOPE GNO TO CATHOOE
OF CR426 AND PROBING
GATES DIRECTLY.
DRIVE >- 7V.
K45X DUTY CYCLE.
PER 100 « 40M
s
.
CHECK T620 (FOR
CRACKED/BROKEN
CORE), Q421 AND
Q423. CHECK U233
PWM IC OPERATION
CHECK VOLTAGES AND
COMPONENTS (Q240 , Q148 ,
Q836 , VR144, VR929, CR239,
C244, C245) IN CONTROL
POWER SUPPLY FOR DEFECTS.
REPLACE DEFECTIVE COMPONENTS
WAVEFORMS
CORRECT?
YES
CHECK GATE DRIVE WAVEFORMS
AT CHOPPER FET GATES.
DRIVE >- 10V.
50X DUTY CYCLE.
(SEE NOTE FOR CORRECT WAVEFORMS)
CHECK U 2 3 3 - PWM (P U L S E W ID TH MOOULATOR) I C O PER ATIO N
PIN 15 - SAME VOLTAGE AS Q836 EMITTER.
PIN 12-0 VOLTS AND PIN 16 - +5.IV.
IF NOT, CHECK FOR EXCESSIVE LOADING
(S.C. CURRENT - 100mA MAX)
(NOMINAL LOAD <20mA);
CHECK C238 FOR SHORTS)
CHECK U233.
PIN 4 - K10X DUTY CYCLE PULSE AT «20ps PERIOD.
IF NOT, CHECK OSCILLATOR COMPONENTS
(R227 ,
C227 , R22fe).
PIN 10 - TTL LOW LEVEL.
IF NOT, CHECK CHIRP TIMER.
PIN 8 - OC LEVEL HIGHER THAN PEAK RAMP
VOLTAGE AT U233-7.
IF NOT, CHECK Cl28, CR930 AND U233.
PIN 13 - NOT LESS THAN 2V BELOW PIN 15.
IF NOT, CHECK R238 AND Cl38!
CHECK FOR SHORTED C525)
CHECK T620 FOR CRACKED OR BROKEN CORE)
CHECK R323 AND R137.
PINS 11 AND 14 -S545X DUTY CYCLE PULSES
AT «40ps PERIOD.
E X E R C IS E S / D (SHUTDOW N)- ENSURE CLOCK KEEPS R U N N IN G .
CONNECT U233-10 TO 5.1V REFERENCE (U233-16).
OBSERVE THAT U233-4 PULSES ARE STILL OCCURRING.
OBSERVE THAT U233-11 , 14 WAVEFORMS STOP AND A LOW
VOLTAGE IS PRESENT ON THESE PINS.
EXER CISE E /A (ERROR A M P L IF IE R ): TO V E R IF Y FU N C T IO N A L IT Y .
OBSERVE U233-11 , 14 OUTPUT DRIVER WAVEFORMS.
CONNECT U233-I TO U233-15.
OBSERVE THAT WAVEFORM BECOMES A STEADY LOW VOLTAGE.
WAVEFORMS
CORRECT?
YES
CHECK OSCILLATOR OUT (U233-11) SIGNAL
PATH THROUGH U840-I0, 11 , 13 TO U829-11.
CHECK LEVEL AT U840-10.
CHECK U829 FOR DEFECTIVE FLIP-FLOP.
CHECK FETS.
CHECK THE CURRENT-LIMIT COMPARATOR, OVER-VOLTAGE COMPARATOR,
CLOCK BUFFER, ANO MONOSTABLE ONESHOT. (U840 , U829A)
1. CHECK LEVEL AT U840-9 FOR 0.38V.
2. CONNECT "- LINE REFERENCED" SCOPE PROBE TO U233-10.
MOMENTARILY CONNECT U840-8 TO CONTROL ELECTRONICS
SUPPLY VOLTAGE, AND OBSERVE THAT U233-10 GOES HIGH
AND REMAINS HIGH FOR 200 TO 300ms AFTER CONNECTION
IS REMOVED.
3. VOLTAGE ON PIN 7 IS V (SUB)REF.
4. MOMENTARILY CONNECT U840-6 TO CONTROL ELECTRONICS SUPPLY
VOLTAGE AND OBSERVE THAT U233-10 GOES HIGH AND REMAINS
HIGH FOR 200 TO 300ms AFTER THE CONNECTION IS REMOVED.
CONTROL ELECTRONICS
TROUBLESHOOTING
(VOLTAGES WITH RESPECT TO
- LINE REFERENCE)
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