Mitsubishi Electric HITACHI VS-50111 Troubleshooting Manual page 28

V20 Chassis Technical Training
and Troubleshooting Guide
Two Clock oscillators supply signals to time opera-
tions:
1) A 27 MHZ ceramic filter controlled oscillator
at pins 17 and 18.
2) A 10MHZ crystal controlled oscillator at pin
13 and 15.
The Reset action is provided by IC7C10, a Watch Dog
Reset IC. When power is initially applied to the TV,
IC7C10 outputs a delayed pulse to reset the µPC to
its' nominal starting point, preventing lock up.
IC7C10 also monitors the µPC's operation after initial
power has been applied to the TV. When the µPC is
functioning properly, pulses are output from pin 11 of
the µPC and are directed to pin 4 of IC7C10. The
pulses constantly reset the internal counter in IC7C10
so a reset pulse is not generated.
If lock up should occur:
• No pulses are generated from pin 11 of the
µPC.
• The watch dog counter is not reset.
• The counter reaches its' maximum count.
• A Reset pulse is generated.
• The µPC resets to clear the lock up.
Input Commands
The Input Command circuitry is also similar to that in
previous models, as shown in Figure 2. The front panel
buttons are connected in a two-column resistive ladder
configuration. Each column is connected to a Key Scan
(KSC) input on the µPC. Pressing a button changes
the voltage at that column's KSC input. The change in
voltage at the Key Scan input tells the µPC what the
command is.
Commands may also originate from the Remote Con-
trol. The Remote signal is amplified in the Remote
Preamp and applied to pin 19 of the µPC. The µPC
determines what the command is from the remote se-
rial data code.
In V20C+ models a second Remote Preamp (Re-
peater) is added. This is a wideband amplifier and
accepts and amplifies infra-red signals from most other
brand remote controls. The signals are amplified and
output from IR Output Jacks on the rear of the TV.
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