Tektronix 2246 1Y Service Manual page 159

Maintenance—2246 1Y and 2246 Mod A Service
3. Check the outputs of the relay driver transistor
arrays (U174 and U175). When a transistor is
blown in one of the arrays, the usual symptom is
8 V on its output.
4. Go to a known setup and check the outputs for
correct levels (see
Section 3). The MAKE
selection under INTERNAL SETTINGS of the
SERVICE MENU provides known control states.
5. Check relay contacts.
6. Follow the signal path and check for correct sig­
nal and gains. Put in a known signal for each
attenuator setting and check at the Vertical
Preamplifier inputs to determine if the signal
path is ok. The front panel boards and the
attenuator shield have to be removed to gain
access to the solder side of the Main Board.
7. Check the channel input buffer amplifier (U112
or U122) output if the vertical deflection of either
channel 1 or channel 2 is defective. If the buffer
amplifier output is held at -6 V or a strange saw­
tooth signal is present,
amplifier.
8. Check gains and offsets of the CFI 3 or CH 4
input buffers (Q131 or Q151).
Schematic Diagram 2—Vertical PREAMPLIFIERS,
DELAY LINE DRIVERS and OUTPUT
Perform the following troubleshooting checks with no
signal input.
1. Check both inputs of the delay line. If offset on
either side, troubleshoot the offsetting side.
Inputs to the bases of Q250 and Q251 should be
at +7.5 V.
2. Differential voltage across the delay line should
be 0 V ±0.5 V.
3. Check signal gain through the Preamplifier ICs
(U210, U220, U230,
10 mV/division of input signal.
4. Check INVERT operation.
5. Check the operation of U260 if the inputs to de­
lay line driver are not at 7.5 V. This operational
amplifier is the bias stabilization circuit that com ­
pares the average dc level to +7.5 and moves
the emitters (and therefore the bases) of Q250
and Q251 to return the inputs to 7.5 V.
6-20
Circuit Description in
FACTORY SETTINGS
replace that buffer
and U240). Gain is
"
w a r n i n g
Vertical output transistors Q701 and Q702
run extremely hot (in excess of 100°C). Use
care when probing in those areas to not touch
the heat sinks or cases with bare fingers.
W A R N IN G
The vertical output amplifier runs hot. DO
NOT touch it with bare fingers.
A / W
£ CAUTION |
C w v w w v r S
The metal tab on top of the vertical output
amplifier 1C (U701) is NOT ground. Do not
connect a ground lead to it. Doing so may
cause the 1C to fail and usually causes R733
from pin 14 of U701 to the -5 V supply to
open.
6. Avoid moving the package or flexing the leads of
R708. If the resistor pack is moved excessively,
its leads may break. The resistor pack will then
have to be replaced.
NOTE
The heat sinks on Q701 and Q702 may be
removed for short periods of time to permit
access for a test probe around the close-in
circuitry. DO NOT leave them off for extended
periods. Check that they are on all the way
when replaced.
7. If the heat sinks on the output transistors shake
loose, the plastic grommet inserted in the top of
the heat sink prevents the sink from touching the
metal cabinet. If the grommet is left out, then
the metal cabinet may come in contact with the
heat sink; and the transistor, the vertical output
amplifier 1C, and R733 will usually fail. If the heat
sinks are removed during maintenance, they
must fit tightly when replaced and the grommet
must be checked.
NOTE
The cases of Q701 and Q702 are the base
leads of the transistors, not the collector as is
usual for a T05 case. Also, the tab marks the
collector lead, not the emitter.
^!
V W U l