Tektronix 585 - Page 56

Maintenance — Type 585
one or more of the amplifier tubes. The output dual-pentode
amplifier V1284 is particularly prone to develop cathode
interface. The 6939 tube used in early instruments very often
developed cathode interface after just a few hours of opera 
tion. A superior replacement is an Amperex 7699 . The 7699
can also develop cathode interface, but they are much less
likely to do so than the 6939.
To check for cathode interface, display a fast-rise square
wave. It will be necessary to use a variable line voltage
autotransformer between the power line and the Type 585.
Adjust the line voltage to a value between 120 and 125
volts and observe the waveform carefully. Then reduce the
line voltage to a value between 105 and 110 volts. After
a short period of time the amplifier tube's cathode tempera 
tures will lower, increasing any cathode interface effect. If
cathode interface is present, it will show up as increased
overshoot at the leading edge of the square wave at low
line voltages. If interface is present, first replace the output
amplifier V 1 284. If this does not correct the overshoot, a
general search for the cathode interface effected tubes must
follow. Replace vertical amplifier tubes one at a time with
new tubes until the bad ones are found. After finding the
defective tube(s), it may be necessary to readjust the high-
frequency compensations of either the Output Vertical Am 
plifier or the P80 Probe and Type 80 Plug-In Unit. See the
Calibration Procedure step 45a.
A. 60 cycles as measured at TRIGGER SLOPE switch
side of R27. 1 V/CM, 5 M1LLISEC/CM.
B. 60 cycles, including switching of tunnel diode D47.
Measured at pin 5, V34. 5 V/CM, 5 MILLISEC/CM.
TROUBLESHOOTING THE TIME-BASE A
TRIGGER CIRCUIT
Unstable Triggering
If the display of a repetitive waveform cannot be made
stable, the sweep generator may not be receiving proper
triggering signals. If the trace can be turned off and on with
STABILITY control, the sweep generator is capable of being
triggered. This indicates the trigger circuitry is not func 
tioning properly.
Trouble can be anywhere from the Trigger Pickoff circuit
on the Delay-Line Driver chassis to the grid circuit of the
Sweep Gating Multivibrator.
To determine if the trouble is within the Time-Base Trigger
circuit, and not the Trigger Pickoff circuit, the internal LINE
triggering signal can be used. Figure 5-15 illustrates three
waveforms within the tunnel diode Time-Base Trigger circuit
when the triggering signal is the line frequency. The test
oscilloscope used to take the waveforms of Figure 5-15 was a
Tektronix Type 543A with Type K Plug-In Unit and P6000
10X Probe.
Should the Time-Base Trigger circuit not be func 
tioning properly, the trouble probably lies within the Trigger
Pickoff amplifier and cathode follower system. A visual
inspection will aid to find burned components. Tube sub 
stitution should be tried first when no burned parts are
evident. A possible cause of unstable triggering is heater 
cathode leakage in a Trigger Pickoff tube. Also volt 
age measurements can aid in finding the problem. Voltages
at important points in the Trigger Pickoff circuit are part of
the Vertical Amplifier Delay-Line Driver stage diagram.
C. Circled portion of waveform B expanded. .5 V/CM,
1  SEC/CM.
Fig. 5-15. Trigger Circuit waveforms when using LINE triggering.
TRIGGER SLOPE switch set to 4-.
Refer to the circuit description of the Trigger Regenerator,
Page 4-4, for additional voltage-current relationships within
the Time-Base Trigger circuit.
TROUBLESHOOTING THE TIME-BASE A
GENERATOR
No Horizontal Sweep
If the Time-Base Generator is not producing a sawtooth
sweep voltage when the STABILITY control is adjusted for
a free-running sweep, some defect in the generator is hold 
ing the Miller circuit. Depending on the on-off states of the
Disconnect Diodes V152, the Miller circuit may be held
at either the high end or the low end of the sawtooth. The
5-10