HP 212A - Page 19

The greatest duty cycle for the stop thyratron, Vlb, occurs at maximum
pulse rate and minimum pulse length.
At that time it is conducting for
the longest period of time.
Under these conditions the stop thyratren
will be most subject to jitter and slowness.
-- Microphonic s usually appear as a small jump in pulse
ultivibrator tubes VlOl and V105 are particularly subject
to microphonica. When changing VlOl and V105, it may be necessary
to try several new tubes to obtain one which is not microphonic and
which gives the required advance and delay. A small jump in the
trailing edge only would indicate that VlO8 is microphonic. .
A quick operational check for L2AU7 tubes can be made by exchanging
with a 12AU7 not in a critical circuit, any position other than Vl Ol or
V105.
Interference Through Power Line -- Interference through the power line
can occur when instruments on the line are not filtered or if transformers
connecting to the line do not have shields between the primary and sec-
ondary windings. The interference may appear in various forms. A
visual indication may be seen when viewing very low amplitude pulses
on a very high sensitivity oscilloscope. The interference will cause
~inging at the leading and trailing edges of the pulse.
Zn some models of the 212A Pulse Generator the filament transformer
was constructed without an internal shield, Some of these models con-
tained a line filter which is not shown in the schematic diagrams.
If
an instrument is known to cause interference through the power line,
the
line filter circuit shown at the rear of the manual should be added.
2 ADJUSTMENTS
Pulse Length Calibration -- The PULSE LENGTH dial is hand calibrat-
e d to the potentiometer in the pulse length multivibrator circuit. Com-
position potentiometers vary greatly in taper and can change taper after
use.
The calibration of the PULSE LENGTH dial will be onlyapproxi-
mately correct alter the potentiometer has been replaced or has aged.
An external time standard is necessary where high accuracy is required.
Following replacement of a component within the circuits V108 the
calibration of the PUZSE LENGTH dial should be checked,
If neces-
sary, the dial calibration may be adjusted for greater accuracy as
fallows:
1. With pulse generator connected to a calibrated oscilloscope, set
the
PULSE LENGTH dial to 1Q.
2.
If necessary, adjust C128 to obtain a pulse length of exactly 10
microseconds.