Kenwood TK-80, Service Manual

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TK-80
16
4-2. Power control circuit
Power is controlled by lowering the base voltage of Q61 in
the TX-RX unit. As the base voltage of Q61 decreases, the
emitter voltage of Q60 and Q61 decreases. This activates
ALC and reduces the power even if the base voltage (VSF) of
Q60 is low. The power change is shifted by D/A converter
IC13 for changing base voltage of Q61 (Fig. 7).
4-3. Protection circuit
When the reflected wave voltage (VSR) detected by the
filter unit rises, Q63 in TX-RX unit turns on to reduce the volt-
age of the ALC time constant line. The drive is decreased
and the power is reduced to protect the final transistor.
8F
+
Q64
Q60 Q61
Q62
Q65
D69
D68
D67
D71
D72
VR14
+
+
+
VSF
VSR
Ext ALC
D70
+
Q63
VR12
ALC out
–6V
TXB
Q69
Q67 Q68
AMB
IC13
D/A
3
POC
Fig. 7 ALC and power control circuits
5. Digital Control Circuit
The TK-80’s digital control circuit comprises a 16-bit micro-
computer CPU IC5, a reset IC3, an EEPROM IC6. Expander I/
O IC7 since there are many control signals for TX-RX unit and
filter unit, they are output to the shift register (serial to parallel
converter) in series (Fig. 8).
5-1. Power switch
With this transceiver, the power is turned on and off by
the microcomputer. When the power switch is pressed, the
microcomputer detects it and energizes, the power relay to
supply 14V to the transceiver. When the power switch is
pressed to turn the transceiver off, the microcomputer
checks it a little longer than when turning the power on and
de-energizes the power relay.
CIRCUIT DESCRIPTION
4-4. Temperature protection
If the final heat sink temperature rises, Q9 in the final unit
turns on and the fan starts running at low speed and during
transmission Q10 turns on and starts running a high speed if
the final heat sink temperature rises further.
If this is not enough to prevent the temperature from con-
tinuing to rise, the control unit CPU IC5 temperature detec-
tion port THP becomes “H”, forcibly lowering the RF output.
Also, if there are any fan troubles or if something happens to
get entangled and prevents the fan from turning, the RF out-
put is similarly forced down to prevent overheating.