Tektronix 3A74 Instruction Manual page 44

Maintenance — Type 3A74
TABLE 5-5
Trouble Isolation Procedure
Symptoms
Checks to Make
Some Possible Causes
Probable Area at Fault
1. Overheated or burned components
in the Type 3A74.
Adjacent leads carrying different volt-
ages shorted to each other.
A lead (elevated in voltage above
ground) is shorted to ground.
Isolate trouble to a branch circuit by
first checking the interconnecting plug
resistances to ground. Use Table 5-4
as a guide. Continue with resistance
check until trouble is found.
2. Loss of all the channel traces. No
trace appears when all MODE switch-
es are set to OFF.
Q444, Q454, Q463, or Q473 defec-
tive,
L444, L454, 1464 or L474 open.
Interconnecting plug defective.
V464 or V474 heater open.
Check for de imbalance in the Output
Amplifier stages and locate cause of
trouble. Refer to PART 1A.
3. Loss of all the channel traces or
signals but a trace does appear on
the crt that cannot be positioned with
the front-panel controls. MODE switch-
es set to any position.
C596 shorted.
@590 shorted.
NOTE: If @Q590 is defective, check
D595 also. If D595 is shorted, the
diode can damage the replaced Q590
transistor. —12.2 (DEC) buss may be
at —év.
Check for trouble in the Blocking Os-
cillator Q590 and Blanking Pulse Re-
generator V533B stages.
4. Loss of trace from one channel.
Possible causes in channel at fault:
V423 or V433 defective.
V424 or V434 defective.
D424 shorted; trace can be positioned
into view.
D440 or D443 open.
\f chan. 1 is at fault, also check Q503,
Q504 and D502.
Check for de imbalance in channel at
fault somewhere between its input
circuit and emitters of Q444 and
Q454. Refer to PART 1A, that follows
this table.
5. Unable to position the trace of one
channel. No waveform display.
Check for an open POSITION control
in channel at fault. Check for shorted
diode in collector circuit of Ring count-
er controlling channel at fault by us-
ing this pattern as an example: Chan.
1 at fault-check for shorted D547
diode; chan. 2—check D557.
Check collector circuitry of Q424 and
@434 in channel at fault. Also check
Ring Counter circuitry.
6. Trace for one channel cannot be
positioned to upper half of graticule
and waveform decreases in amplitude
as the channel's POSITION control ts
rotated clockwise.
Check for open D443 diode in channel
at fault.
Check switching diodes in channel at
fault.
7. Same as Symptom No. 6 but count:
erclockwise rotation of POSITION con-
trol decreases amplitude of display.
Check for open D440 diode in channel
at fault.
Check switching diodes in channel at
fault.
8. POSITION controls for three chan-
nels must be set at about 60° clock-
wise from center to position the traces
at or near graticule center. Remain-
ing channel seems to operate norm-
ally.
Check for shorted D443 diode in the
apparently normal operating channel.
Check switching diodes in apparenily
normal-operating channel.
9. Same as Symptom No. 8 but POSI-
TION controls of the three channels
must be set at about 60° counterclock-
wise.
Check for shorted D440 diode in the
apparently normal operating channel.
Check switching diodes in apparently
normal-operating channel.
10. Traces for three channels cannot
be positioned below bottom of grati-
cule and waveform amplitudes change
as their respective POSITION controls
are varied. Remaining channel seems
fo operate normally.
Check for an open D442 diode in
channel that seems to operate norm-
ally.
Check switching diodes in apparently
normal-operating channel.
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