Tektronix 535A, Instruction Manual

Page: 52 / 156

Maintenance— Type 535A
TROUBLESHOOTING THE VERTICAL AMPLIFIER
No Spot or Trace on CRT
If all power supply voltages are normal, and the crt is
known to be good, failure to display a spot or trace on the
screen will be due to improper positioning voltages. This
condition is caused by dc unbalance in either or both of the
deflection amplifier circuits.
To determine which circuit is at fault, adjust the Time-
Base controls for a free-running sweep at 1 millisec/cm
(STABILITY control full right). Set the INTENSITY control to
midscale. Using a screwdriver with an insulated handle,
short the vertical deflection plates together at the neck pins
on the crt. These are the pins marked BLUE (UPPER) and
BROWN (LOWER). Be careful not to short either pin to the
metal shield around the crt, or to the pin marked ORANGE
(GEOM). If d c unbalance is being produced in the vertical
deflection circuit, the trace will appear at or near the center
of the crt. If the trace does not appear, the trouble does
not lie in the vertical circuit. The dc balance of the hori­
zontal circuit can be checked in a similar manner, by short­
ing the horizontal deflection plates together.
If it is determined that the vertical deflection circuit is un­
balanced, the next step is to check the Delay Line. For this
check, it is convenient to have a short lead (about 6 to 10
inches) with a rubber-covered alligator clip on each end.
Connect this lead across the input to the Delay Line. If the
trace appears on the crt, the Delay Line is not at fault. If the
trace does not appear, check for an open line by turning
off the oscilloscope, and checking the continuity of both
sides of the line with an ohmmeter.
If the Delay Line is not at fault, connect the shorting strap
between the grids (pin 2) of the output amplifiers, V554 and
V564. If the trace does not appear, check for an open plate
inductor (L553 or L563).
If the trace does appear when the grids of the Output
Amplifiers are shorted together, the trouble is occurring in
one of the input stages. If tube replacement does not cor­
rect the trouble, then look for open peaking coils, defective
resistors, and shorted or leaky capacitors.
Insufficient or No Vertical Deflection
Insufficient deflection indicates a change in the gain
characteristics of the Vertical Amplifier. If only a slight
change in deflection is apparent, the circuit can normally be
recalibrated for gain. In this event, refer to the Calibration
Procedure section of this manual.
If the change in deflection is more pronounced, or if there
is no deflection at all, the tubes should first be checked.
Then check for components which affect the gain of the cir­
cuit but not the dc balance. Such components would be the
common plate-load resistors R505 and R528, the common
cathode resistors R513, R514, R558, R568, and the GAIN
ADJ. control R570.
Waveform Distortion
Any waveform distortion that may be produced by the
Type 535A will generally be of high-frequency nature. There
will be no low-frequency distortion since the deflection cir­
cuit is dc-coupled from input to output (unless one or more
of the tubes enter into heavy grid current, a condition that
will produce other types of distortion as well).
A distortion-free positive step function, having a risetime
of 10 millicroseconds or less, may be used to observe the
high-frequency characteristics of the oscilloscope/plug-in
combination. This is illustrated in the photographs of Fig.
5-12.
insufficient high-frequency peaking, which limits the rise­
time and consequently the band-width, will produce the
'rolloff ' or undershoot condition illustrated in Fig. 5-12a.
Rolloff is the result of insufficient high-frequency compensa­
tion. A small amount of rolloff is normally due to a change,
with age, in the characteristics of the circuit components,
and can usually be compensated by adjusting the variable
peaking coils. If the rolloff is more pronounced the tubes
should be checked. If a tube cannot deliver current, instan­
taneously on demand, the transients in the signal will not
be produced.
Shorted or partially shorted peaking coils will result in a
rolloff. Be especially careful when soldering around the
peaking circuits as hot solder dropping on a coil may burn
through the insulation and short the turns.
Excessive high-frequency peaking will produce the "over­
shoot'' condition illustrated in Fig. 5-12b. This is generally
caused by misadjusting the peaking coils. In these cases the
distortion can generally be eliminated by readjusting the
variable peaking coils L506 and L523 for the Input Amplifiers,
and L553 and L563 for the Output Amplifiers.
Fig. 5-12. Three types of high-frequency distortion.
5-8