Motorola GM328 Service Manual page 110

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If the voltage is between 3.3 and 6.4V, the device will be active but has its input (U0273-1/9) off. This
is a mute condition which is used to prevent an audio pop when the PA is enabled.
The SPK+ and SPK- outputs of the audio PA have a DC bias which varies proportionately with FLT
A+ (U0271-7). FLT A+ of 11V yields a DC offset of 5V, and FLT A+ of 17V yields a DC offset of 8.5V.
If either of these lines is shorted to ground, it is possible that the audio PA will be damaged. SPK+
and SPK- are routed to the accessory connector (J0501-16 and 1) and to the controlhead
(connector J0401-2 and 3).
2.4.4 Handset Audio
Certain hand held accessories have a speaker within them which require a different voltage level
than that provided by U0271. For those devices HANDSET AUDIO is available at controlhead
connector J0401-7.
The received audio from the output of the ASFIC CMP's digital volume attenuator and buffered by
U0211-1 is also routed to U0211-3 pin 9 where it is amplified 20 dB; this is set by the 10k/100k
combination of R0261 and R0262. This signal is routed from the output of the op amp U0211-3 pin 8
to J0401-7. The controlhead sends this signal directly out to the microphone jack. The maximum
value of this output is 6.6Vp-p.
2.4.5 Filtered Audio and Flat Audio
The ASFIC CMP has an audio whose output at U0221-39 has been filtered and de-emphasized, but
has not gone through the digital volume attenuator. From ASFIC CMP U0221-39 the signal is routed
via R0251 through gate U0251-12 and AC coupled to U0211-2. The gate controlled by ASFIC CMP
port GCB3 (U0221-35) selects between the filtered audio signal from the ASFIC CMP pin 39
(URXOUT) or the unfiltered (flat) audio signal from the ASFIC CMP pin 10 (UIO). R0251 and R0253
determine the gain of op-amp U0211-2 for the filtered audio while R0252 and R0253 determine the
gain for the flat Audio. The output of U0253-7 is then routed to J0501-11 via DC blocking capacitor
C0542 and R0531. Note that any volume adjustment of the signal on this path must be done by the
accessory
2.4.6 RX Secure Audio (optional)
Discriminator audio, which is now encrypted audio, follows the normal receive audio processing until
it emerges from the ASFIC CMP UIO pin (U0221-10), which is fed to the Secure board residing at
option connector J0551-35. On the Secure board, the encrypted signal is converted back to normal
audio format, and then fed back through (J0551-34) to AUX RX of the ASFIC CMP (U0221-43).
From then on it follows a path identical to conventional receive audio, where it is filtered (0.3 - 3kHz)
and de-emphasized. The signal URX SND from the ASFIC CMP (U0221-39), also routed to option
connector J0551-28, is not used for the Secure board but for other option boards.
2.4.7 Option Board Receive Audio
Unfiltered audio from the ASFIC CMP pin UIO (U0221-10) enters the option board at connector
J0551-35. Filtered audio from the ASFIC CMP pin URXOUT (U0221-39) enters the option board at
connector J0551-28. On the option board, the signal may be processed, and then fed back through
J0551-34 to AUX RX of the ASFIC CMP (U0221-43). From then on it follows a path identical to
conventional receive audio, where it may be filtered (0.3 - 3kHz) and de-emphasized.
Controller Board Audio and Signalling Circuits