HP 120A - Trigger Generator

Sect. IV Page 3
However, do not overlook the possibility of a fault
in another circuit affecting the faulty circuit, such as,
low power-supply voltages affecting other circuits.
In case of difficulty, the Model 120 A can be trouble -
traced by following the alignment procedure in its
entirety. Directions are given for the cabinet model
but are the same for the rack-mount model except for
location.
4-7 TURN ON
a. Measure resistances from the power supplies to
ground with a ($) Model 410B Voltmeter. They
should be within 25% of the following:
+ 380 volt supply 50K ohms
+100 50K ohms
-150 12K ohms
+ 24 5. 5K (remove 12AU7)
Measure resistances to ground into the vertical and
horizontal input terminals for all switch positions,
place AC-DC switches in DC position. As measured
with a (§») Model 410B Voltmeter these resistances
should read:
VERT. SENSITIVITY = 1 Megohm
OFF = oo
CAL = oo
SWEEP TIME HOR. SENS = 1 Megohm
b. Turn on 120A and set -150 (115v input) with R344.
This voltage point is any of the purple wires.
Check voltage at the other power supplies. Set the
high voltage to -2250 volts with R308. This voltage
can be measured conveniently at the orange lead to
the FOCUS control.
CAUTION
When turning the instrument on for the first time after
repair work in any power supply circuit, turn the in 
tensity, the H. V. adjust control and the H. V. limit
control full counter-clockwise before applying power.
Failure to follow this precaution may result in a per 
manently damaged cathode-ray tube.
c. Adjust FOCUS and ASTIGMATISM, R316, con 
trols at low intensity to obtain a small, round,
and sharply focused spot.
d. Turn Sweep Stability control R228 full counter 
clockwise so sweep free-runs. Set SWEEP TIME
switch to 5 MILLISECONDS/CM. Align CRT so that
the trace and graticule are parallel. Be sure that the
CRT face is positioned just behind the filter in the
bezel, but not touching as Newton rings will occur
4-8 POWER SUPPLIES
a. Measure -150v bus (any purple wire) with a
($)410B voltmeter. Set this voltage to -150 volts
with R344. Measure hum on this same bus with a
<$)400D voltmeter. Check regulation from 102 to 128
volts by observing the ripple change. Turn the sweep
off while measuring ripple. The following are the
specifications for the various supplies:
Wire Allowable
Max. Ripple 102v H5v 128v Color Variation(115v in)
-150 supply 3mv 3mv 3mv Purple Set to -150v
+ 380 60mv20mv20mv Red ± lOv
+ 100 4mv Yellow ± 3v
+ 24 60mv40mv40mV/White x ± Iv
' Vert, amp.'
b. Measure the cathode voltage of the cathode-ray
tube. This voltage may be measured on the or 
ange wire going to the FOCUS Potentiometer. DAN 
GER HIGH VOLTAGE I Measure this voltage with a
$)450A High Voltage Probe connected to a <^410B
Voltmeter. Set this voltage to -2250 with R308. Using
. 0068 /zfd 5kv voltage rating condenser for blocking,
measure the ripple on the cathode and grid (green
wire), pin 3-can be measured through hole in handle
of the cathode-ray tube with a ($)400D. Cathode ripple
should be less than 350 millivolts and grid ripple less
than 50 millivolts rms.
c. Check orientation of INTENSITY knob. Set knob
to 9 o ’ clock. Set Intensity Limit, R322, so that
the sweep on the screen of the cathode-ray tube is
just extinguished. Check for best focusing in center
third of FOCUS control and set ASTIGMATISM con 
trol, R316, for a small round spot.
d. With sweep at 1 MILLISECONDS/CM, turn
SWEEP TIME VERNIER down about 3:1. Feed
a Ike signal into the Vertical Input. Tune VERNIER
to observe horizontal hum modulation on the sweep.
Switch EXPAND switch to X5 and adjust hum balance
potentiometer, R360, to minimize modulation.
The CRT shields should not be subjected to shock as
they will then lose their shielding properties. They
will then have to be demagnetized which is most easily
done by annealing.
4-9 TRIGGER GENERATOR
The Trigger Generator must be working in order to
adjust the Vertical and Horizontal Amplifiers. Also,
these amplifiers must be adjusted before the Saw 
tooth Generator can be adjusted. To check the Trig 
ger Generator proceed as follows:
a. Check -150 volts and reset, if necessary.