Icom IC-22S PLL Synthesized 2-Meter Transceiver
Instruction Manual and Service Notes
14
Power supply
Reverse connection protection circuit
If power with the wrong polarity is applied, D28 is forward biased and there is, therefore, a large
current flow which blows the fuse provided on the external lead, preventing damage to circuit
elements.
Power supply circuit, stand-by circuit
The constant 9 V supply appears as regulated voltage at the anode of D20 due to the action of the
clamp circuit of R142, D20, and zener diode D21. This voltage is sent by the emitter-follower circuit
Q3i and supplied to the PLL, IDC circuit, reception AF circuit, and the low pass filter group.
Similarly, 9 V for reception is taken from the clamp circuit of R147, D27, and D21 by Q34 in an
emitter-follower configuration. This voltage is supplied to the RF, IF, 2nd LO and noise amplification
circuits.
The 9 V for transmit is similarly taken from emitter-follower circuit Q32 from the clamp circuit
constituted by Ri43, D22, D21, and is supplied to the IDC, 10.7 MHz oscillator, transmission mixer,
inter-stage amplifier, and bias circuits.
The 13.8 V supply is supplied to the ALC DC amplifier, exciting amplifier, power amplifier, and IC2.
In the stand-by mode, when the PTT switch is switched off, D24 and D26 are both non-conductive
and +9 V for reception is obtained. Since D25 also is switched off, Q33 conducts due to bias
established by R145. The base of Q32 is connected to ground through D3, and transmit voltage
ceases.
When the PU switch is switched on, the base of Q34 is connected to ground through D26 and the 9
V receive supply dies. D24 connected to the emitter of Q34 is used for effecting rapid discharge of
the electrolytic condenser connected to the 9 V receive supply line. Q33 becomes non-conductive
since its base is connected to ground through D2 5, and 9 V for transmit is obtained.
