Tektronix 585 - Page 59

Maintenance — Type 585
make sure that the line voltage is set near the design center
for your instrument (117 or 234 v) and check the rms voltage
across the secondary winding for each supply; the nominal
value of each secondary voltage is indicated on the circuit
diagram. If the secondary voltages are all correct, check the
operation of the bridge rectifiers. This can be done by
measuring the rectified voltage at the input to each regula 
tor. These values are also indicated on the circuit diagram.
Then check for off-value resistors, especially in the refer 
ence dividers, and for open or leaky capacitors.
The material that follows may be used as a quick index
for troubleshooting the regulator circuits:
If the output voltage is high with excessive ripple, check:
a. For high line voltage.
b. For open voltage-regulator tube, V609.
c. The amplifier tubes in the regulator circuits, V664, ect.
d. For insufficient loading.
If the output voltage is high with normal ripple, check for
proper resistance values in reference dividers, refer to the
diagram to determine the location of the resistors involved.
Since these are generally precision resistors (±1% toler 
ance] the use of a good bridge is recommended in check 
ing the value.
If the output is low with excessive ripple, check:
a. For low line voltage.
b. For shorted voltage-regulator tube, V609.
c. The amplifier tubes in the regulator circuits, V664, etc.
d. For excessive loading.
e. Open or leaky filter capacitors.
f. Defective rectifiers.
If the output is low with normal ripple, check:
a. The resistance values in the reference dividers.
b. The capacitors across the dividers.
IMPORTANT: If any components in the — 150-volt supply
are changed, or if the setting of the — 150 Adj. control is
changed, it will be necessary to check the calibration of
the instrument.
TROUBLESHOOTING THE AMPLITUDE
CALIBRATOR
If the output square wave is not nearly symmetrical (the
positive portion has a duration considerably different from
that of the negative portion or vice versa), the two tubes
in the Multivibrator circuit are not being held cutoff for
equal periods. This will normally be caused by a defective
tube. If tube replacement does not correct the waveform
the circuit components must be checked. The pentode in the
Multivibrator is held cutoff for an interval determined by
the discharge of C871 and the triode is held cutoff for an
interval determined by the discharge of C874. A change
in the value of either capacitor, or in the value of the re 
sistors through which they discharge, could produce an
asymmetrical waveform.
In addition, the time needed for these capacitors to dis 
charge a given amount is affected by the potential toward
which they discharge; this would be the voltage at the plate
of the triode in the case of C871 , and the voltage at the
screen of the pentode in the case of C874. Since these
voltages are affected by the value of R870 and R875, these
resistors should be checked. The resistors in the plate cir 
cuit of the pentode should also be checked, since they will
affect the plate-to-screen ratio of the pentode.
Incorrect Output Voltage
The amplitude of the output square wave is determined
almost entirely by the plate circuit of V875. The accuracy
of square wave voltages less than 100 volts is determined
by the resistance values in the divider in the cathode-follower
stage. A quick check of the + 100-volt maximum output
value can be made by turning off the AMPLITUDE CALI 
BRATOR and measuring the voltage at the CAL. TEST PT.
(see Figs. 5-9 and 5-11). If the test point does not measure
exactly + 1 00 volts, the CAL. OUT voltages will not be
correct.
The CAL. ADJ. control R879 will vary the voltage at the
test point over about a 5-volt range. If this voltage cannot be
set to exactly +100 volts, and if the tubes have been re 
placed, then check to be sure V875 is completely cut-off.
TROUBLESHOOTING THE CRT CIRCUIT
The intensity, focus, geometry and calibration of the crt
display depend on proper operation of the three high-
voltage supplies in the crt circuit.
No Spot or Trace
If the low-voltage power supply is operating normally, but
no spot or trace is visible on the crt, the trouble could be a
defective crt, a defect in the crt cathode circuit including
the — 1350-volt supply, or an unbalanced d c condition in
either or both of the deflection amplifiers. In the latter
case the d c unbalance is producing improper positioning
voltages and the beam is deflected off the crt screen.
To determine which circuit is at fault, turn the ASTIGMA 
TISM control fully counterclockwise, then turn the INTENSITY
control clockwise. If a flare (dimly lit diffused area of the
crt face) is observed on the crt, one of the deflection ampli 
fiers is probably at fault; the procedure for troubleshooting
these circuits precedes in this section. If no flare is ob 
served with the INTENSITY control turned fully clockwise,
the trouble will either be due to a defective crt or to an
inoperative cathode supply circuit (V820 rectifier circuit).
The cathode supply can be checked by measuring the volt 
age at the HV. Adj. Test Point. (Fig. 5-16). This test point
is accessible through a hole in the shield at the top rear
of the instrument. The voltage at this point should be — 1350
volts, although it will vary with the setting of the HV Adj.
control R81 1. If a voltage reading near — 1350 volts is
obtained, turn the instrument off and remove the shield
covering the test point. Then measure the resistance of
R847, the 27-k resistor connected to the test point. If this
resistor is not open a defective crt is indicated.
5-13