Cpu Troubleshooting Hints - Sanyo AVM-2550S Training Manual

Described in this section are some suggested techniques for discovering
defects in the frequency synthesizer and CPU controlled circuits. An isolation
transformer should always be used when servicing the TV to prevent possible
electrical shock (Hot Chassis) and equipment damage. When connecting any
equipment to a circuit, start by connecting the negative side. This will reduce
the possibility of damage to semiconductors and IC chips.
When determining the cause of a trouble by checking a waveform, start
tracing it down from the output or input end of the signal system to facilitate
this work. This will help you discover in which circuit the cause of trouble is
located.
The troubleshooting hints included here do not include all possible defects
that may be encountered. This section is only intended to be a guide. Always
refer to the service literature for the specifications, parts lists, and safety
related items. DO NOT defeat any safety items or features.
Dead
1. Check the 5 volt power supply (IC801, pins 14 and 22).
A. If no 5 volt supply, check IC681 and D624.
2. Check reset terminal (IC801, pin 25) for 5 volts.
A. If no 5 volts, check Q831.
3. Check operation of clock oscillator (IC801, pins 19-20).
A. Check X801.
B. Check IC801.
4. Check for a LOW to HIGH state change (IC801, pin 27) with operation of
the power key.
A. If no change, check all keys for stuck-closed condition.
B. Check remote operation.
C. Check IC801.
5. If pin 27 of IC801 changes from a LOW to a HIGH state, check +12V
Switch Drive Transistor Q681 and +12V Switch Transistor Q627.

CPU TROUBLESHOOTING HINTS

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No Remote Operation (Manual Operation OK)
1. Check for 5 volts on pin 2 of RC Pre-Amp (A1901).
2. With an oscilloscope, check for output pulses on pin 1 of A1901 with
operation of a known good remote.
3. Check IC801.
No Manual Operation (Remote Operation OK)
1. Check for correct voltage changes at pin 9 of IC801.
pressing keys).
2. Check IC801.
Display
1. If incorrect position or no display, check Q881 and Q882 for proper sync
pulses.
2. If no display, check IC801, pin 17 for proper sync pulses in the composite
video signal.
3. Check IC801, pins 39-42 for 5Vp-p pulses.
4. Check IC101.
Audio
1. With an oscilloscope, check for audio output signal on pins 3 and 4 of
IC3401 while pressing volume keys.
A. If signal is normal, check IC001 and/or associated circuitry.
B. Check IC801.
AFT
1. Check PLL Tuning alignment per service literature.
2. Check for correct operation of TB circuitry (Q371, Q372).
3. Check IC801.
Tuning (Cannot Receive Stations)
1. Check IC801, pins 32 and 34 for 5Vp-p pulses.
2. Check Tuner voltages and Tuner.
A. If tuning voltage (TU) changes when selecting different channels,
problem is with tuner and/or associated circuitry.
(Check while