Tektronix 3A74 Instruction Manual page 45

TABLE 5-5 (Cont'd)
Maintenance — Type 3A74
Symptoms
Checks to Make
Some Possible Causes Probable Area at Fault
11. Same as Symptom No. 10 but trac-
es of three channels cannot be posi-
tioned to top of graticule.
Check for an open D441 diode in
channel that seems to operate norm-
ally.
Check switching diodes in apparently
normal-operating channel.
12. Limited positioning range (about 4
div.} for three channels. POSITION
control of remaining channel affects
trace position of other three channels.
if signals are applied to all channels,
14% of remaining channel signal!
feeds into other three channels; amp-
litude of display in other three chan-
nels decreases by 92%.
Check for shorted diode in collector
circuit of Ring Counter transistor that
controls the remaining channel using
this example as a pattern: channel 1
—check for shorted D548 diode; chan-
nel 2-—check D558 diode.
Check Ring Counter circuitry.
13. Microphonics in one channel ex-
ceeds one major division in amplitude
when front panel of Type 3A74 is tap-
ped lightly with one hand. AC-GND-
DC switch is set to GND.
Check for microphonic V423 or V433
in channel at fault. NOTE: H AC-
GND-DC switch for channel 1 or 3
is set to AC or DC and if VOLTS/DIV.
switch is set to .02, C401 input coup-
ling capacitor can cause a micro-
phonic symptom if it is not rigidly
mounted.
Check Input CF V423 and DC BAL CF
V433 stages.
14. Trace of one channel is noisy.
Check C428 in channel at fault.
Check Input CF V423 and DC BAL CF
V433 stages.
15. Trace of one channel shifts verti-
cally more than one minor division
when AC-GND-DC switch of that
channel is set from AC or DC setting
to GND or vice versa.
Excessive V423 grid current in channel
at fault.
Check Input CF V423 stage.
16. No waveform display when using
one of the channels, Positioning is
normal.
Check VAR. GAIN, GAIN ADJ., or
R437 for open circuit. Check for short
or open circuit between Input connec-
tor of channel at fault and grid of
V423.
Check the channel at fault.
Refer to PART 1B.
17. Low gain in one channel.
VOLTS/DIV. switch set to any position.
Check Q424 or Q434.
Check for trouble in the channel at
fault. Refer to PART 1B.
18. Incorrect deflection factor when
using some of the VOLTS/DIV. switch
settings of one channel.
Check for faulty attenuator compon-
ents used in those switch positions.
For example: if the .05, .6 and 5
VOLTS/DIV. deflection factor settings
are incorrect, the X2.5 attenuator is at
fault.
VOLTS/DIV. switch SW410 in chan-
nel at fault.
19. Low gain in all channels and gain
varies with rotation of POSITION con-
trol (s}.
Check for an open arm of POSITION
RANGE control.
Check Output Amplitier stages. Refer
to PART 1B.
20. Gain is normal in all channels at
graticule center. POSITION controls
off center. Gain decreases up to 50%
if display is positioned to top or bot-
tom of graticule.
Check for open R448 or R458.
Check Output Amplifier stages. Refer
to PART 1B.
21. Low gain in all channels. Gain
decreased about 14%.
Check for shorted C478.
Check for open R478.
Check Output Amplifier stages. Refer
to PART 1B.
22. Gain increased by 14% in all
channels.
Check for shorted D468 or D469
diode.
Check Output Amplifier stages. Refer
to PART 1B.
23. Display of any channel compres-
ses slightly (about Y. minor division)
as the display is positioned to the bot-
tom of screen.
Check for open D468.
Check for trouble in the Output Amp-
lifier stages. Refer to PART 1B.
5-13