Tektronix 3A74 Instruction Manual page 47

TABLE 5-5 (Coni'd}
Maintenance — Type 3A74
Symptoms
Checks to Make
Some Possible Causes
Probable Area at Fault
35. Improper multi-trace operation.
Only two channels operate in alter-
nate mode and they-can be switched
to another two by operating the
MODE switch. In chopped mode there
are twice as many segments in chan. 1
and 4 traces as there are in chan. 2
and 3. Time duration of each seg-
ment is about 1.1 jsec indicating that
the Blocking Oscillator rate is about
900 ke.
Check for shorted D581 diode. Check the Blocking Oscillator stage.
Refer to PART Il.
36. Improper chopped-mode multi-
trace operation. Channel 2, 3, or 4
has twice as many segments as the
others. Time duration of each seg-
ment is normal (about one psec).
Check for an open diode using this
pattern as an example: if chan. 2 has
twice as many segments, check for an
open D547 diode; if chan. 3 has twice
as many segments, check for an open
Check the Ring Counter circuitry. Refer
to PART Il.
D557 diode.
37. Incorrect chopped-mode rate. Rate
is higher than normal. Segments are
1.5 psec in duration indicating that the
Blocking Oscillator free-running rate is
about 670 ke.
Check for an open D580 diode. Check the Blocking Oscillator stage
and Ring Counter circuitry. Refer to
PART II.
38. Switching transient pulses are
present on trace when only one chan-
nel is on.
Pulses are about 2 usec apart. As
trace is positioned to top of graticule,
pulses increase in amplitude and are
negative going; when trace is posi-
tioned to bottom of graticule, pulses
reach a maximum amplitude of about
one major div. at top or bottom of
graticule.
D581 diode.
Pulses indicate that the Blocking Oscil-
lator is free running when only one
channel is on—-check for an open
Check Blocking Oscillator stage. Refer
to PART Il.
39. No chopped-mode blanking dur-
ing chopped-mode multi-trace opera-
tion,
Check for defective D595 diode. If
shorted, check Q590.
Check Blanking Pulse Regenerator
V533B stage.
Check Q583.
40. No alternate-mode = multi-trace
operation.
Check the Syne Pulse In EF Q583
stage.
Part I—Troubleshooting the Amplifier Stages
A. Isolating DC Imbalance
For free-running traces to appear within the useable view-
ing area of the crt screen, the de voltage as measured be-
tween pins 17 and 21 of the interconnecting plug must be
less than -£72 volts (based on a crt vertical deflection
factor of 18 volts/div). A voltage difference which exceeds
-+72 volts between these two points may position the trace
above or below the range of visibility.
The de voltages at pins 17 and 21 of the interconnecting
plug depend on the de balance of each stage in the Output
Amplifier as well as all the stages between input and output
of any channel that is turned on (MODE switch set to NORM.
or INV). Since all the amplifier stages are de coupled, any
excessive imbalance condition anywhere between input
®
and output can unbalance the output and cause the trace to
deflect out of the viewing area.
Fig. 5-10 shows the voltage difference limits between each
stage. If the voltage limits are exceeded in one stage, the
limits will be exceeded in the following stages (looking to-
ward the output) and the trace will deflect off the screen.
For example, if the voltage between the collectors of Q444
and Q454 reads -+6 volts, the voltage between emitters of
Q463 and Q474 will also be about +6 volts, and the volt-
age between the plates (pin 6 of V464 and V473 will be
about -+110 volts, causing the trace to be deflected off the
. screen.
One quick method for isolating de imbalance either to one
of the Switched Amplifier or to the common Output Ampli-
fier is to turn one channel on at a time to see if its trace
can be normally positioned onto the screen. If the trace for
one channel cannot be positioned onto the screen, then the
5-15