Maintenance-2246 1Y and 2246 Mod A Service
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Table 6-7
Horizontal Display State Logic
DO
NOT short the horizontal output leads
together or to ground. This will cause the out
put amplifier FETs to fail.
2. The MOSFET output transistors (Q801 and Q802,
left plate; Q805 and Q806, right plate) run hot. If
either side is cold, it is defective.
3. If output is railed to one side or the other, check
U801A and the com m on-m ode feedback. This
circuit is supposed to keep the outputs at about
70 V average to ground.
HDO
HD1
Display
0
0
Readout
0
1
A Sweep
1
0
B Sweep
1
1
X-Y
Schematic Diagram 7—Z-AXIS, CRT, PROBE
ADJUST, AND CONTROL MUX
NOTE
Pins 12 and 13 of U802 may be shorted to
gether to determine if the unbalance is before
or after the horizontal preamplifier (U802).
DO NOT short to ground.
4. Check the A RAMP and B RAMP input signals (A
Horizontal mode for A RAMP and B Horizontal
mode for B RAMP). They start at -2 V and ramp
up to about +2.5 V.
5. Check the RO HORIZ input for correct waveform.
6. Check for the X AXIS input signal on pin 7 of
U802 in X -Y mode (a signal must be applied to
the CH 1 input).
7. Check at the junction of R855 and R854 (the
com m on-m ode bias point of Q810 and Q809)
for 9.5 V.
8. Check at the junction of R846 and R852 (the
com m on-m ode source voltage of Q802 and
Q805) for 15 V.
9. Check at the junction of R845 and R847 (the
com m on-m ode collector voltage of emitter fol
lowers Q803 and Q807) for 24 V.
10. Check the HDO and HD1 signals to U802 (see
Table 6-7 for display states).
11. The horizontal preamplifier, U802, runs warm to
the touch, but not hot.
1. Turn off the Readout (READOUT control fully
CCW) and test the node between CR2703 and
C2711 for correct Z-Axis waveform. Vary INTEN
to check operation. (Readout signals add con
fusion to the waveforms.)
2. Check for correct auto-focus operation at the
junction of CR2715 and the collector of Q2712.
Circuit action is exactly opposite of the Z-Axis to
obtain focus tracking.
3. Parts replaced in the CRT High Voltage circuit
and Z-Axis are safety controlled parts. Replace
ments need to be exact. The power supply is
capable of delivering more than 15 watts at high
voltages.
Schematic Diagram 8—MEASUREMENT
PROCESSOR
1. Check U2501, pin 57 for a RESET condition.
Processor will be in permanent reset condition if
RESET is high. Check that RESET goes high then
low again at power on.
2. Check SYS RESET at U2506C pin 8.
3. Check that U2502 pin 5 is low when RESET is
high. (This signal prevents random RAM writes
on power up and power down when the pro
cessor is being reset.)
4. Check the 8 MHz clock (CLK 8M) at U2501
pin 56.
8. Check address decoding. Using a data analyzer
or word recognizer probe, set up to recognize
the address that produces a selected enabling
strobe from the address decoding circuitry.
Observe that the strobe is produced when the
correct address is output by the Measurement
Processor. The easiest way to generate most
addresses during normal operation is to change
a front-panel setting. See Table 6-8 for the
addresses.
6-22
Summary of Contents for 2246 1Y
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