Troubleshooting Guide - Zenith ZVM-121 Service Manual

Page 6-1
TROUBLESHOOTING GUIDE
This section could not possibly include all of the
problems you might encounter. However, it offers
some hints for the various circuits in the Monitor.
HORIZONTAL DRIVER
Check the collector pulse and base drive to the hori-
zontal drive. If the horizontal output shows poor sat-
uration or the circuit fails to start, suspect diodes
CR502 and/or CR503. If no waveforms are present,
Q501, CR504, or Q502 could be the problems.
1. No horizontal blanking - R523 may be open.
2. Ringing in picture and/or horizontal scan dis-
torted or reduced - C204 may be open or
shorted.
3. No high voltage, no raster - Q501 may be open
or shorted.
HORIZONTAL PROCESSOR
The horizontal processor uses the 221-141 IC. This
integrated circuit is essentially the same as the 221-
86. The 221-141 can be replaced by the 221-86. How-
ever, the 221-86 cannot be replaced by the 221-141.
1. No lock - R503, R504, or R510 may be open.
2. Check the waveform at 15 for bad Q501 or
RX521.
3. Check the B
+
at pin 6. It should be 8-9 volts. If
the regulator is defective, the voltage drops.
VERTICAL SWEEP
1. Small vertical height - R624 value may be too
high.
2. Picture deflected up - C614 may be leaky.
3. Vertical fold-over - C618, C617, or Q607 may
be open.
4. R631 and R632 will overheat if Q618, Q617, or
Q607 are shorted.
5. Top half of picture lost - R622 or R623 may be
open.
6. Increase dissipation in Q603 and R622 - R622
value may be too low.
7. No vertical deflection - Q604 and Q606 fail
due to high dissipation;CR602 and/or CR603
may be shorted.
8. Center horizontal line in picture (crossover
distortion) - CR602 and/or CR603 may be
shorted.
POWER SUPPLY
The power supply has current limiting capabilities. A
lack of B
+
does not mean the problem is in the power
supply. If there is a short or heavy current drain in the
Monitor, the regulator reduces its output voltage to
zero if necessary. When the fault is corrected, the
regulator will return to normal.
If the 12.7 volts B
+
drops at normal line voltage
ranges (108-132 VAG), Q702 or CR705 may be defec-
tive. A quick check of the zener voltage will help you
diagnose this problem. If RX709 has no effect on B+
voltage, suspect Q701 or QX703.