7.2 VERTIC L B L NCE:
NOTE: It will be necessary (where specified) to use a
"resistive shorting bar", consisting of a ^ -ohm 1/2
watt composition resistor attached to some convenient
insulating handle, for shorting grids together. Use
of clip leads or wires will introduce enough inductance
to cause oscillation, usually at a frequency far beyond
the amplifier bandpass. These oscillations are rectified
in the amplifier and appear on the CRT as DC level
changes, making balance measurements impossible. The
resistor lowers the Q of the inductive circuit below
the point where oscillations can occur.
7.2.1 Overall Balance: Free run the main sweep at 1 msec/cm or faster.
Locate CRT electrical center by shorting the vertical deflection
plate pins together (NOT to ground). Note graticule position of
electrical center and remove jumper. Now position the trace
back to CRT electrical center, using vertical position control.
Now, use the resistive shorting bar (if test load unit is used,
press "zero reference" button) to jumper the main amplier input stage
grids together. Connect the shorting bar between pins 1 and 3 of
the plug-in interconnecting socket. Log trace displacement from
CRT electrical center in Cal Record. If 1 cm or less, skip
7.2.2 for the moment (it may be necessary to come back to this later).
If more, complete 7.2.2.
7.2.2 Vertical Balancing:
7.2.2.1 Distributed mplifier Stage: Use a short cliplead to jumper
the distributed amplifier grid lines together (the two
distributed lines running outside the rows of tube sockets,
connecting with pins 1 of the D tubes).
(C UTION: Do not short to ground).
If trace-shift from electrical center is 2 mm or less, log
in Cal Record and proceed with 7.2.2.4. If more , complete
7.2.2.2-7.2.2.3.
7.2.2.2 D. . Bias: gain jumper D grid lines. Use DC voltmeter to
measure grid-to-cathode bias for each tube pair. For each
pair, measure from jumper to either cathode (pin 2) of each
pair. For S/N 101-9291 (and 9292-11691 with original tubes*)
bias should be O.9-I.6 volts. For later instruments, 1.0 v
minimum to 2 v max. Replace one or both tubes of each out-
of-tolerance pair. For S/N 101—9291> use checked 6CB6's,
Tektronix p/n 157-037. For S/N 9292-up, select "raw" Type 8l3$is
but see* below. Log high and low bias values in Cal Record.
*6DK6 design changes in late 1958 require modification of instruments S/N 9292-11691
to use the new-style tubes. Modification kit 0^0-191 ($1.80) changes cathode
resistors in the 12BY7 stage to provide 1.0 v minimum bias on output stage, and
permits use of old or new 6DK6's or 81361s. To check doubtful instrument, examine Ri
& R1017— if 3k (5w5%) and 10 k (8w 5%), the instrument has already been modified.
For discussion of type 8136 tube, see page 5“0.
(Revised 5-62) C LIBR TION ND M INTEN NCE PROCEDURE 3-21*
